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2451 lines
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ReStructuredText
2451 lines
106 KiB
ReStructuredText
==========================================
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The LLVM Target-Independent Code Generator
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==========================================
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.. role:: raw-html(raw)
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:format: html
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.. raw:: html
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<style>
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.unknown { background-color: #C0C0C0; text-align: center; }
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.unknown:before { content: "?" }
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.no { background-color: #C11B17 }
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.no:before { content: "N" }
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.partial { background-color: #F88017 }
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.yes { background-color: #0F0; }
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.yes:before { content: "Y" }
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.na { background-color: #6666FF; }
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.na:before { content: "N/A" }
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</style>
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.. contents::
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:local:
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.. warning::
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This is a work in progress.
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Introduction
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============
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The LLVM target-independent code generator is a framework that provides a suite
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of reusable components for translating the LLVM internal representation to the
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machine code for a specified target---either in assembly form (suitable for a
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static compiler) or in binary machine code format (usable for a JIT
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compiler). The LLVM target-independent code generator consists of six main
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components:
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1. `Abstract target description`_ interfaces which capture important properties
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about various aspects of the machine, independently of how they will be used.
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These interfaces are defined in ``include/llvm/Target/``.
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2. Classes used to represent the `code being generated`_ for a target. These
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classes are intended to be abstract enough to represent the machine code for
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*any* target machine. These classes are defined in
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``include/llvm/CodeGen/``. At this level, concepts like "constant pool
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entries" and "jump tables" are explicitly exposed.
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3. Classes and algorithms used to represent code as the object file level, the
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`MC Layer`_. These classes represent assembly level constructs like labels,
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sections, and instructions. At this level, concepts like "constant pool
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entries" and "jump tables" don't exist.
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4. `Target-independent algorithms`_ used to implement various phases of native
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code generation (register allocation, scheduling, stack frame representation,
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etc). This code lives in ``lib/CodeGen/``.
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5. `Implementations of the abstract target description interfaces`_ for
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particular targets. These machine descriptions make use of the components
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provided by LLVM, and can optionally provide custom target-specific passes,
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to build complete code generators for a specific target. Target descriptions
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live in ``lib/Target/``.
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6. The target-independent JIT components. The LLVM JIT is completely target
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independent (it uses the ``TargetJITInfo`` structure to interface for
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target-specific issues. The code for the target-independent JIT lives in
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``lib/ExecutionEngine/JIT``.
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Depending on which part of the code generator you are interested in working on,
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different pieces of this will be useful to you. In any case, you should be
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familiar with the `target description`_ and `machine code representation`_
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classes. If you want to add a backend for a new target, you will need to
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`implement the target description`_ classes for your new target and understand
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the :doc:`LLVM code representation <LangRef>`. If you are interested in
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implementing a new `code generation algorithm`_, it should only depend on the
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target-description and machine code representation classes, ensuring that it is
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portable.
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Required components in the code generator
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-----------------------------------------
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The two pieces of the LLVM code generator are the high-level interface to the
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code generator and the set of reusable components that can be used to build
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target-specific backends. The two most important interfaces (:raw-html:`<tt>`
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`TargetMachine`_ :raw-html:`</tt>` and :raw-html:`<tt>` `DataLayout`_
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:raw-html:`</tt>`) are the only ones that are required to be defined for a
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backend to fit into the LLVM system, but the others must be defined if the
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reusable code generator components are going to be used.
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This design has two important implications. The first is that LLVM can support
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completely non-traditional code generation targets. For example, the C backend
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does not require register allocation, instruction selection, or any of the other
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standard components provided by the system. As such, it only implements these
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two interfaces, and does its own thing. Note that C backend was removed from the
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trunk since LLVM 3.1 release. Another example of a code generator like this is a
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(purely hypothetical) backend that converts LLVM to the GCC RTL form and uses
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GCC to emit machine code for a target.
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This design also implies that it is possible to design and implement radically
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different code generators in the LLVM system that do not make use of any of the
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built-in components. Doing so is not recommended at all, but could be required
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for radically different targets that do not fit into the LLVM machine
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description model: FPGAs for example.
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.. _high-level design of the code generator:
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The high-level design of the code generator
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-------------------------------------------
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The LLVM target-independent code generator is designed to support efficient and
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quality code generation for standard register-based microprocessors. Code
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generation in this model is divided into the following stages:
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1. `Instruction Selection`_ --- This phase determines an efficient way to
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express the input LLVM code in the target instruction set. This stage
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produces the initial code for the program in the target instruction set, then
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makes use of virtual registers in SSA form and physical registers that
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represent any required register assignments due to target constraints or
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calling conventions. This step turns the LLVM code into a DAG of target
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instructions.
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2. `Scheduling and Formation`_ --- This phase takes the DAG of target
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instructions produced by the instruction selection phase, determines an
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ordering of the instructions, then emits the instructions as :raw-html:`<tt>`
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`MachineInstr`_\s :raw-html:`</tt>` with that ordering. Note that we
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describe this in the `instruction selection section`_ because it operates on
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a `SelectionDAG`_.
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3. `SSA-based Machine Code Optimizations`_ --- This optional stage consists of a
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series of machine-code optimizations that operate on the SSA-form produced by
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the instruction selector. Optimizations like modulo-scheduling or peephole
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optimization work here.
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4. `Register Allocation`_ --- The target code is transformed from an infinite
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virtual register file in SSA form to the concrete register file used by the
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target. This phase introduces spill code and eliminates all virtual register
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references from the program.
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5. `Prolog/Epilog Code Insertion`_ --- Once the machine code has been generated
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for the function and the amount of stack space required is known (used for
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LLVM alloca's and spill slots), the prolog and epilog code for the function
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can be inserted and "abstract stack location references" can be eliminated.
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This stage is responsible for implementing optimizations like frame-pointer
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elimination and stack packing.
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6. `Late Machine Code Optimizations`_ --- Optimizations that operate on "final"
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machine code can go here, such as spill code scheduling and peephole
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optimizations.
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7. `Code Emission`_ --- The final stage actually puts out the code for the
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current function, either in the target assembler format or in machine
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code.
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The code generator is based on the assumption that the instruction selector will
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use an optimal pattern matching selector to create high-quality sequences of
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native instructions. Alternative code generator designs based on pattern
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expansion and aggressive iterative peephole optimization are much slower. This
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design permits efficient compilation (important for JIT environments) and
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aggressive optimization (used when generating code offline) by allowing
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components of varying levels of sophistication to be used for any step of
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compilation.
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In addition to these stages, target implementations can insert arbitrary
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target-specific passes into the flow. For example, the X86 target uses a
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special pass to handle the 80x87 floating point stack architecture. Other
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targets with unusual requirements can be supported with custom passes as needed.
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Using TableGen for target description
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-------------------------------------
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The target description classes require a detailed description of the target
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architecture. These target descriptions often have a large amount of common
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information (e.g., an ``add`` instruction is almost identical to a ``sub``
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instruction). In order to allow the maximum amount of commonality to be
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factored out, the LLVM code generator uses the
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:doc:`TableGen/index` tool to describe big chunks of the
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target machine, which allows the use of domain-specific and target-specific
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abstractions to reduce the amount of repetition.
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As LLVM continues to be developed and refined, we plan to move more and more of
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the target description to the ``.td`` form. Doing so gives us a number of
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advantages. The most important is that it makes it easier to port LLVM because
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it reduces the amount of C++ code that has to be written, and the surface area
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of the code generator that needs to be understood before someone can get
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something working. Second, it makes it easier to change things. In particular,
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if tables and other things are all emitted by ``tblgen``, we only need a change
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in one place (``tblgen``) to update all of the targets to a new interface.
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.. _Abstract target description:
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.. _target description:
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Target description classes
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==========================
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The LLVM target description classes (located in the ``include/llvm/Target``
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directory) provide an abstract description of the target machine independent of
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any particular client. These classes are designed to capture the *abstract*
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properties of the target (such as the instructions and registers it has), and do
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not incorporate any particular pieces of code generation algorithms.
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All of the target description classes (except the :raw-html:`<tt>` `DataLayout`_
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:raw-html:`</tt>` class) are designed to be subclassed by the concrete target
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implementation, and have virtual methods implemented. To get to these
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implementations, the :raw-html:`<tt>` `TargetMachine`_ :raw-html:`</tt>` class
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provides accessors that should be implemented by the target.
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.. _TargetMachine:
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The ``TargetMachine`` class
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---------------------------
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The ``TargetMachine`` class provides virtual methods that are used to access the
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target-specific implementations of the various target description classes via
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the ``get*Info`` methods (``getInstrInfo``, ``getRegisterInfo``,
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``getFrameInfo``, etc.). This class is designed to be specialized by a concrete
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target implementation (e.g., ``X86TargetMachine``) which implements the various
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virtual methods. The only required target description class is the
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:raw-html:`<tt>` `DataLayout`_ :raw-html:`</tt>` class, but if the code
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generator components are to be used, the other interfaces should be implemented
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as well.
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.. _DataLayout:
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The ``DataLayout`` class
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------------------------
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The ``DataLayout`` class is the only required target description class, and it
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is the only class that is not extensible (you cannot derive a new class from
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it). ``DataLayout`` specifies information about how the target lays out memory
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for structures, the alignment requirements for various data types, the size of
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pointers in the target, and whether the target is little-endian or
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big-endian.
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.. _TargetLowering:
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The ``TargetLowering`` class
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----------------------------
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The ``TargetLowering`` class is used by SelectionDAG based instruction selectors
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primarily to describe how LLVM code should be lowered to SelectionDAG
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operations. Among other things, this class indicates:
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* an initial register class to use for various ``ValueType``\s,
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* which operations are natively supported by the target machine,
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* the return type of ``setcc`` operations,
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* the type to use for shift amounts, and
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* various high-level characteristics, like whether it is profitable to turn
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division by a constant into a multiplication sequence.
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.. _TargetRegisterInfo:
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The ``TargetRegisterInfo`` class
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--------------------------------
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The ``TargetRegisterInfo`` class is used to describe the register file of the
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target and any interactions between the registers.
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Registers are represented in the code generator by unsigned integers. Physical
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registers (those that actually exist in the target description) are unique
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small numbers, and virtual registers are generally large. Note that
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register ``#0`` is reserved as a flag value.
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Each register in the processor description has an associated
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``TargetRegisterDesc`` entry, which provides a textual name for the register
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(used for assembly output and debugging dumps) and a set of aliases (used to
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indicate whether one register overlaps with another).
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In addition to the per-register description, the ``TargetRegisterInfo`` class
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exposes a set of processor specific register classes (instances of the
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``TargetRegisterClass`` class). Each register class contains sets of registers
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that have the same properties (for example, they are all 32-bit integer
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registers). Each SSA virtual register created by the instruction selector has
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an associated register class. When the register allocator runs, it replaces
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virtual registers with a physical register in the set.
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The target-specific implementations of these classes is auto-generated from a
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:doc:`TableGen/index` description of the register file.
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.. _TargetInstrInfo:
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The ``TargetInstrInfo`` class
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-----------------------------
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The ``TargetInstrInfo`` class is used to describe the machine instructions
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supported by the target. Descriptions define things like the mnemonic for
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the opcode, the number of operands, the list of implicit register uses and defs,
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whether the instruction has certain target-independent properties (accesses
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memory, is commutable, etc), and holds any target-specific flags.
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The ``TargetFrameLowering`` class
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---------------------------------
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The ``TargetFrameLowering`` class is used to provide information about the stack
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frame layout of the target. It holds the direction of stack growth, the known
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stack alignment on entry to each function, and the offset to the local area.
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The offset to the local area is the offset from the stack pointer on function
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entry to the first location where function data (local variables, spill
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locations) can be stored.
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The ``TargetSubtarget`` class
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-----------------------------
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The ``TargetSubtarget`` class is used to provide information about the specific
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chip set being targeted. A sub-target informs code generation of which
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instructions are supported, instruction latencies and instruction execution
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itinerary; i.e., which processing units are used, in what order, and for how
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long.
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The ``TargetJITInfo`` class
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---------------------------
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The ``TargetJITInfo`` class exposes an abstract interface used by the
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Just-In-Time code generator to perform target-specific activities, such as
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emitting stubs. If a ``TargetMachine`` supports JIT code generation, it should
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provide one of these objects through the ``getJITInfo`` method.
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.. _code being generated:
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.. _machine code representation:
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Machine code description classes
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================================
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At the high-level, LLVM code is translated to a machine specific representation
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formed out of :raw-html:`<tt>` `MachineFunction`_ :raw-html:`</tt>`,
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:raw-html:`<tt>` `MachineBasicBlock`_ :raw-html:`</tt>`, and :raw-html:`<tt>`
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`MachineInstr`_ :raw-html:`</tt>` instances (defined in
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``include/llvm/CodeGen``). This representation is completely target agnostic,
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representing instructions in their most abstract form: an opcode and a series of
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operands. This representation is designed to support both an SSA representation
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for machine code, as well as a register allocated, non-SSA form.
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.. _MachineInstr:
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The ``MachineInstr`` class
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--------------------------
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Target machine instructions are represented as instances of the ``MachineInstr``
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class. This class is an extremely abstract way of representing machine
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instructions. In particular, it only keeps track of an opcode number and a set
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of operands.
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The opcode number is a simple unsigned integer that only has meaning to a
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specific backend. All of the instructions for a target should be defined in the
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``*InstrInfo.td`` file for the target. The opcode enum values are auto-generated
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from this description. The ``MachineInstr`` class does not have any information
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about how to interpret the instruction (i.e., what the semantics of the
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instruction are); for that you must refer to the :raw-html:`<tt>`
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`TargetInstrInfo`_ :raw-html:`</tt>` class.
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The operands of a machine instruction can be of several different types: a
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register reference, a constant integer, a basic block reference, etc. In
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addition, a machine operand should be marked as a def or a use of the value
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(though only registers are allowed to be defs).
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By convention, the LLVM code generator orders instruction operands so that all
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register definitions come before the register uses, even on architectures that
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are normally printed in other orders. For example, the SPARC add instruction:
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"``add %i1, %i2, %i3``" adds the "%i1", and "%i2" registers and stores the
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result into the "%i3" register. In the LLVM code generator, the operands should
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be stored as "``%i3, %i1, %i2``": with the destination first.
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Keeping destination (definition) operands at the beginning of the operand list
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has several advantages. In particular, the debugging printer will print the
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instruction like this:
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.. code-block:: llvm
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%r3 = add %i1, %i2
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Also if the first operand is a def, it is easier to `create instructions`_ whose
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only def is the first operand.
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.. _create instructions:
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Using the ``MachineInstrBuilder.h`` functions
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Machine instructions are created by using the ``BuildMI`` functions, located in
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the ``include/llvm/CodeGen/MachineInstrBuilder.h`` file. The ``BuildMI``
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functions make it easy to build arbitrary machine instructions. Usage of the
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``BuildMI`` functions look like this:
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.. code-block:: c++
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// Create a 'DestReg = mov 42' (rendered in X86 assembly as 'mov DestReg, 42')
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// instruction. The '1' specifies how many operands will be added.
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MachineInstr *MI = BuildMI(X86::MOV32ri, 1, DestReg).addImm(42);
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// Create the same instr, but insert it at the end of a basic block.
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MachineBasicBlock &MBB = ...
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BuildMI(MBB, X86::MOV32ri, 1, DestReg).addImm(42);
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// Create the same instr, but insert it before a specified iterator point.
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MachineBasicBlock::iterator MBBI = ...
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BuildMI(MBB, MBBI, X86::MOV32ri, 1, DestReg).addImm(42);
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// Create a 'cmp Reg, 0' instruction, no destination reg.
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MI = BuildMI(X86::CMP32ri, 2).addReg(Reg).addImm(0);
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// Create an 'sahf' instruction which takes no operands and stores nothing.
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MI = BuildMI(X86::SAHF, 0);
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// Create a self looping branch instruction.
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BuildMI(MBB, X86::JNE, 1).addMBB(&MBB);
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The key thing to remember with the ``BuildMI`` functions is that you have to
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specify the number of operands that the machine instruction will take. This
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allows for efficient memory allocation. You also need to specify if operands
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default to be uses of values, not definitions. If you need to add a definition
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operand (other than the optional destination register), you must explicitly mark
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it as such:
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.. code-block:: c++
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MI.addReg(Reg, RegState::Define);
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Fixed (preassigned) registers
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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One important issue that the code generator needs to be aware of is the presence
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of fixed registers. In particular, there are often places in the instruction
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stream where the register allocator *must* arrange for a particular value to be
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in a particular register. This can occur due to limitations of the instruction
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set (e.g., the X86 can only do a 32-bit divide with the ``EAX``/``EDX``
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registers), or external factors like calling conventions. In any case, the
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instruction selector should emit code that copies a virtual register into or out
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of a physical register when needed.
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For example, consider this simple LLVM example:
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.. code-block:: llvm
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define i32 @test(i32 %X, i32 %Y) {
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%Z = sdiv i32 %X, %Y
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ret i32 %Z
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}
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The X86 instruction selector might produce this machine code for the ``div`` and
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``ret``:
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.. code-block:: llvm
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;; Start of div
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%EAX = mov %reg1024 ;; Copy X (in reg1024) into EAX
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%reg1027 = sar %reg1024, 31
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%EDX = mov %reg1027 ;; Sign extend X into EDX
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idiv %reg1025 ;; Divide by Y (in reg1025)
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%reg1026 = mov %EAX ;; Read the result (Z) out of EAX
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;; Start of ret
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%EAX = mov %reg1026 ;; 32-bit return value goes in EAX
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ret
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By the end of code generation, the register allocator would coalesce the
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registers and delete the resultant identity moves producing the following
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code:
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.. code-block:: llvm
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;; X is in EAX, Y is in ECX
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mov %EAX, %EDX
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sar %EDX, 31
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idiv %ECX
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ret
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This approach is extremely general (if it can handle the X86 architecture, it
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can handle anything!) and allows all of the target specific knowledge about the
|
|
instruction stream to be isolated in the instruction selector. Note that
|
|
physical registers should have a short lifetime for good code generation, and
|
|
all physical registers are assumed dead on entry to and exit from basic blocks
|
|
(before register allocation). Thus, if you need a value to be live across basic
|
|
block boundaries, it *must* live in a virtual register.
|
|
|
|
Call-clobbered registers
|
|
^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Some machine instructions, like calls, clobber a large number of physical
|
|
registers. Rather than adding ``<def,dead>`` operands for all of them, it is
|
|
possible to use an ``MO_RegisterMask`` operand instead. The register mask
|
|
operand holds a bit mask of preserved registers, and everything else is
|
|
considered to be clobbered by the instruction.
|
|
|
|
Machine code in SSA form
|
|
^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
``MachineInstr``'s are initially selected in SSA-form, and are maintained in
|
|
SSA-form until register allocation happens. For the most part, this is
|
|
trivially simple since LLVM is already in SSA form; LLVM PHI nodes become
|
|
machine code PHI nodes, and virtual registers are only allowed to have a single
|
|
definition.
|
|
|
|
After register allocation, machine code is no longer in SSA-form because there
|
|
are no virtual registers left in the code.
|
|
|
|
.. _MachineBasicBlock:
|
|
|
|
The ``MachineBasicBlock`` class
|
|
-------------------------------
|
|
|
|
The ``MachineBasicBlock`` class contains a list of machine instructions
|
|
(:raw-html:`<tt>` `MachineInstr`_ :raw-html:`</tt>` instances). It roughly
|
|
corresponds to the LLVM code input to the instruction selector, but there can be
|
|
a one-to-many mapping (i.e. one LLVM basic block can map to multiple machine
|
|
basic blocks). The ``MachineBasicBlock`` class has a "``getBasicBlock``" method,
|
|
which returns the LLVM basic block that it comes from.
|
|
|
|
.. _MachineFunction:
|
|
|
|
The ``MachineFunction`` class
|
|
-----------------------------
|
|
|
|
The ``MachineFunction`` class contains a list of machine basic blocks
|
|
(:raw-html:`<tt>` `MachineBasicBlock`_ :raw-html:`</tt>` instances). It
|
|
corresponds one-to-one with the LLVM function input to the instruction selector.
|
|
In addition to a list of basic blocks, the ``MachineFunction`` contains a a
|
|
``MachineConstantPool``, a ``MachineFrameInfo``, a ``MachineFunctionInfo``, and
|
|
a ``MachineRegisterInfo``. See ``include/llvm/CodeGen/MachineFunction.h`` for
|
|
more information.
|
|
|
|
``MachineInstr Bundles``
|
|
------------------------
|
|
|
|
LLVM code generator can model sequences of instructions as MachineInstr
|
|
bundles. A MI bundle can model a VLIW group / pack which contains an arbitrary
|
|
number of parallel instructions. It can also be used to model a sequential list
|
|
of instructions (potentially with data dependencies) that cannot be legally
|
|
separated (e.g. ARM Thumb2 IT blocks).
|
|
|
|
Conceptually a MI bundle is a MI with a number of other MIs nested within:
|
|
|
|
::
|
|
|
|
--------------
|
|
| Bundle | ---------
|
|
-------------- \
|
|
| ----------------
|
|
| | MI |
|
|
| ----------------
|
|
| |
|
|
| ----------------
|
|
| | MI |
|
|
| ----------------
|
|
| |
|
|
| ----------------
|
|
| | MI |
|
|
| ----------------
|
|
|
|
|
--------------
|
|
| Bundle | --------
|
|
-------------- \
|
|
| ----------------
|
|
| | MI |
|
|
| ----------------
|
|
| |
|
|
| ----------------
|
|
| | MI |
|
|
| ----------------
|
|
| |
|
|
| ...
|
|
|
|
|
--------------
|
|
| Bundle | --------
|
|
-------------- \
|
|
|
|
|
...
|
|
|
|
MI bundle support does not change the physical representations of
|
|
MachineBasicBlock and MachineInstr. All the MIs (including top level and nested
|
|
ones) are stored as sequential list of MIs. The "bundled" MIs are marked with
|
|
the 'InsideBundle' flag. A top level MI with the special BUNDLE opcode is used
|
|
to represent the start of a bundle. It's legal to mix BUNDLE MIs with indiviual
|
|
MIs that are not inside bundles nor represent bundles.
|
|
|
|
MachineInstr passes should operate on a MI bundle as a single unit. Member
|
|
methods have been taught to correctly handle bundles and MIs inside bundles.
|
|
The MachineBasicBlock iterator has been modified to skip over bundled MIs to
|
|
enforce the bundle-as-a-single-unit concept. An alternative iterator
|
|
instr_iterator has been added to MachineBasicBlock to allow passes to iterate
|
|
over all of the MIs in a MachineBasicBlock, including those which are nested
|
|
inside bundles. The top level BUNDLE instruction must have the correct set of
|
|
register MachineOperand's that represent the cumulative inputs and outputs of
|
|
the bundled MIs.
|
|
|
|
Packing / bundling of MachineInstr's should be done as part of the register
|
|
allocation super-pass. More specifically, the pass which determines what MIs
|
|
should be bundled together must be done after code generator exits SSA form
|
|
(i.e. after two-address pass, PHI elimination, and copy coalescing). Bundles
|
|
should only be finalized (i.e. adding BUNDLE MIs and input and output register
|
|
MachineOperands) after virtual registers have been rewritten into physical
|
|
registers. This requirement eliminates the need to add virtual register operands
|
|
to BUNDLE instructions which would effectively double the virtual register def
|
|
and use lists.
|
|
|
|
.. _MC Layer:
|
|
|
|
The "MC" Layer
|
|
==============
|
|
|
|
The MC Layer is used to represent and process code at the raw machine code
|
|
level, devoid of "high level" information like "constant pools", "jump tables",
|
|
"global variables" or anything like that. At this level, LLVM handles things
|
|
like label names, machine instructions, and sections in the object file. The
|
|
code in this layer is used for a number of important purposes: the tail end of
|
|
the code generator uses it to write a .s or .o file, and it is also used by the
|
|
llvm-mc tool to implement standalone machine code assemblers and disassemblers.
|
|
|
|
This section describes some of the important classes. There are also a number
|
|
of important subsystems that interact at this layer, they are described later in
|
|
this manual.
|
|
|
|
.. _MCStreamer:
|
|
|
|
The ``MCStreamer`` API
|
|
----------------------
|
|
|
|
MCStreamer is best thought of as an assembler API. It is an abstract API which
|
|
is *implemented* in different ways (e.g. to output a .s file, output an ELF .o
|
|
file, etc) but whose API correspond directly to what you see in a .s file.
|
|
MCStreamer has one method per directive, such as EmitLabel, EmitSymbolAttribute,
|
|
SwitchSection, EmitValue (for .byte, .word), etc, which directly correspond to
|
|
assembly level directives. It also has an EmitInstruction method, which is used
|
|
to output an MCInst to the streamer.
|
|
|
|
This API is most important for two clients: the llvm-mc stand-alone assembler is
|
|
effectively a parser that parses a line, then invokes a method on MCStreamer. In
|
|
the code generator, the `Code Emission`_ phase of the code generator lowers
|
|
higher level LLVM IR and Machine* constructs down to the MC layer, emitting
|
|
directives through MCStreamer.
|
|
|
|
On the implementation side of MCStreamer, there are two major implementations:
|
|
one for writing out a .s file (MCAsmStreamer), and one for writing out a .o
|
|
file (MCObjectStreamer). MCAsmStreamer is a straight-forward implementation
|
|
that prints out a directive for each method (e.g. ``EmitValue -> .byte``), but
|
|
MCObjectStreamer implements a full assembler.
|
|
|
|
For target specific directives, the MCStreamer has a MCTargetStreamer instance.
|
|
Each target that needs it defines a class that inherits from it and is a lot
|
|
like MCStreamer itself: It has one method per directive and two classes that
|
|
inherit from it, a target object streamer and a target asm streamer. The target
|
|
asm streamer just prints it (``emitFnStart -> .fnstrart``), and the object
|
|
streamer implement the assembler logic for it.
|
|
|
|
To make llvm use these classes, the target initialization must call
|
|
TargetRegistry::RegisterAsmStreamer and TargetRegistry::RegisterMCObjectStreamer
|
|
passing callbacks that allocate the corresponding target streamer and pass it
|
|
to createAsmStreamer or to the appropriate object streamer constructor.
|
|
|
|
The ``MCContext`` class
|
|
-----------------------
|
|
|
|
The MCContext class is the owner of a variety of uniqued data structures at the
|
|
MC layer, including symbols, sections, etc. As such, this is the class that you
|
|
interact with to create symbols and sections. This class can not be subclassed.
|
|
|
|
The ``MCSymbol`` class
|
|
----------------------
|
|
|
|
The MCSymbol class represents a symbol (aka label) in the assembly file. There
|
|
are two interesting kinds of symbols: assembler temporary symbols, and normal
|
|
symbols. Assembler temporary symbols are used and processed by the assembler
|
|
but are discarded when the object file is produced. The distinction is usually
|
|
represented by adding a prefix to the label, for example "L" labels are
|
|
assembler temporary labels in MachO.
|
|
|
|
MCSymbols are created by MCContext and uniqued there. This means that MCSymbols
|
|
can be compared for pointer equivalence to find out if they are the same symbol.
|
|
Note that pointer inequality does not guarantee the labels will end up at
|
|
different addresses though. It's perfectly legal to output something like this
|
|
to the .s file:
|
|
|
|
::
|
|
|
|
foo:
|
|
bar:
|
|
.byte 4
|
|
|
|
In this case, both the foo and bar symbols will have the same address.
|
|
|
|
The ``MCSection`` class
|
|
-----------------------
|
|
|
|
The ``MCSection`` class represents an object-file specific section. It is
|
|
subclassed by object file specific implementations (e.g. ``MCSectionMachO``,
|
|
``MCSectionCOFF``, ``MCSectionELF``) and these are created and uniqued by
|
|
MCContext. The MCStreamer has a notion of the current section, which can be
|
|
changed with the SwitchToSection method (which corresponds to a ".section"
|
|
directive in a .s file).
|
|
|
|
.. _MCInst:
|
|
|
|
The ``MCInst`` class
|
|
--------------------
|
|
|
|
The ``MCInst`` class is a target-independent representation of an instruction.
|
|
It is a simple class (much more so than `MachineInstr`_) that holds a
|
|
target-specific opcode and a vector of MCOperands. MCOperand, in turn, is a
|
|
simple discriminated union of three cases: 1) a simple immediate, 2) a target
|
|
register ID, 3) a symbolic expression (e.g. "``Lfoo-Lbar+42``") as an MCExpr.
|
|
|
|
MCInst is the common currency used to represent machine instructions at the MC
|
|
layer. It is the type used by the instruction encoder, the instruction printer,
|
|
and the type generated by the assembly parser and disassembler.
|
|
|
|
.. _Target-independent algorithms:
|
|
.. _code generation algorithm:
|
|
|
|
Target-independent code generation algorithms
|
|
=============================================
|
|
|
|
This section documents the phases described in the `high-level design of the
|
|
code generator`_. It explains how they work and some of the rationale behind
|
|
their design.
|
|
|
|
.. _Instruction Selection:
|
|
.. _instruction selection section:
|
|
|
|
Instruction Selection
|
|
---------------------
|
|
|
|
Instruction Selection is the process of translating LLVM code presented to the
|
|
code generator into target-specific machine instructions. There are several
|
|
well-known ways to do this in the literature. LLVM uses a SelectionDAG based
|
|
instruction selector.
|
|
|
|
Portions of the DAG instruction selector are generated from the target
|
|
description (``*.td``) files. Our goal is for the entire instruction selector
|
|
to be generated from these ``.td`` files, though currently there are still
|
|
things that require custom C++ code.
|
|
|
|
.. _SelectionDAG:
|
|
|
|
Introduction to SelectionDAGs
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
The SelectionDAG provides an abstraction for code representation in a way that
|
|
is amenable to instruction selection using automatic techniques
|
|
(e.g. dynamic-programming based optimal pattern matching selectors). It is also
|
|
well-suited to other phases of code generation; in particular, instruction
|
|
scheduling (SelectionDAG's are very close to scheduling DAGs post-selection).
|
|
Additionally, the SelectionDAG provides a host representation where a large
|
|
variety of very-low-level (but target-independent) `optimizations`_ may be
|
|
performed; ones which require extensive information about the instructions
|
|
efficiently supported by the target.
|
|
|
|
The SelectionDAG is a Directed-Acyclic-Graph whose nodes are instances of the
|
|
``SDNode`` class. The primary payload of the ``SDNode`` is its operation code
|
|
(Opcode) that indicates what operation the node performs and the operands to the
|
|
operation. The various operation node types are described at the top of the
|
|
``include/llvm/CodeGen/ISDOpcodes.h`` file.
|
|
|
|
Although most operations define a single value, each node in the graph may
|
|
define multiple values. For example, a combined div/rem operation will define
|
|
both the dividend and the remainder. Many other situations require multiple
|
|
values as well. Each node also has some number of operands, which are edges to
|
|
the node defining the used value. Because nodes may define multiple values,
|
|
edges are represented by instances of the ``SDValue`` class, which is a
|
|
``<SDNode, unsigned>`` pair, indicating the node and result value being used,
|
|
respectively. Each value produced by an ``SDNode`` has an associated ``MVT``
|
|
(Machine Value Type) indicating what the type of the value is.
|
|
|
|
SelectionDAGs contain two different kinds of values: those that represent data
|
|
flow and those that represent control flow dependencies. Data values are simple
|
|
edges with an integer or floating point value type. Control edges are
|
|
represented as "chain" edges which are of type ``MVT::Other``. These edges
|
|
provide an ordering between nodes that have side effects (such as loads, stores,
|
|
calls, returns, etc). All nodes that have side effects should take a token
|
|
chain as input and produce a new one as output. By convention, token chain
|
|
inputs are always operand #0, and chain results are always the last value
|
|
produced by an operation. However, after instruction selection, the
|
|
machine nodes have their chain after the instruction's operands, and
|
|
may be followed by glue nodes.
|
|
|
|
A SelectionDAG has designated "Entry" and "Root" nodes. The Entry node is
|
|
always a marker node with an Opcode of ``ISD::EntryToken``. The Root node is
|
|
the final side-effecting node in the token chain. For example, in a single basic
|
|
block function it would be the return node.
|
|
|
|
One important concept for SelectionDAGs is the notion of a "legal" vs.
|
|
"illegal" DAG. A legal DAG for a target is one that only uses supported
|
|
operations and supported types. On a 32-bit PowerPC, for example, a DAG with a
|
|
value of type i1, i8, i16, or i64 would be illegal, as would a DAG that uses a
|
|
SREM or UREM operation. The `legalize types`_ and `legalize operations`_ phases
|
|
are responsible for turning an illegal DAG into a legal DAG.
|
|
|
|
.. _SelectionDAG-Process:
|
|
|
|
SelectionDAG Instruction Selection Process
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
SelectionDAG-based instruction selection consists of the following steps:
|
|
|
|
#. `Build initial DAG`_ --- This stage performs a simple translation from the
|
|
input LLVM code to an illegal SelectionDAG.
|
|
|
|
#. `Optimize SelectionDAG`_ --- This stage performs simple optimizations on the
|
|
SelectionDAG to simplify it, and recognize meta instructions (like rotates
|
|
and ``div``/``rem`` pairs) for targets that support these meta operations.
|
|
This makes the resultant code more efficient and the `select instructions
|
|
from DAG`_ phase (below) simpler.
|
|
|
|
#. `Legalize SelectionDAG Types`_ --- This stage transforms SelectionDAG nodes
|
|
to eliminate any types that are unsupported on the target.
|
|
|
|
#. `Optimize SelectionDAG`_ --- The SelectionDAG optimizer is run to clean up
|
|
redundancies exposed by type legalization.
|
|
|
|
#. `Legalize SelectionDAG Ops`_ --- This stage transforms SelectionDAG nodes to
|
|
eliminate any operations that are unsupported on the target.
|
|
|
|
#. `Optimize SelectionDAG`_ --- The SelectionDAG optimizer is run to eliminate
|
|
inefficiencies introduced by operation legalization.
|
|
|
|
#. `Select instructions from DAG`_ --- Finally, the target instruction selector
|
|
matches the DAG operations to target instructions. This process translates
|
|
the target-independent input DAG into another DAG of target instructions.
|
|
|
|
#. `SelectionDAG Scheduling and Formation`_ --- The last phase assigns a linear
|
|
order to the instructions in the target-instruction DAG and emits them into
|
|
the MachineFunction being compiled. This step uses traditional prepass
|
|
scheduling techniques.
|
|
|
|
After all of these steps are complete, the SelectionDAG is destroyed and the
|
|
rest of the code generation passes are run.
|
|
|
|
One great way to visualize what is going on here is to take advantage of a few
|
|
LLC command line options. The following options pop up a window displaying the
|
|
SelectionDAG at specific times (if you only get errors printed to the console
|
|
while using this, you probably `need to configure your
|
|
system <ProgrammersManual.html#viewing-graphs-while-debugging-code>`_ to add support for it).
|
|
|
|
* ``-view-dag-combine1-dags`` displays the DAG after being built, before the
|
|
first optimization pass.
|
|
|
|
* ``-view-legalize-dags`` displays the DAG before Legalization.
|
|
|
|
* ``-view-dag-combine2-dags`` displays the DAG before the second optimization
|
|
pass.
|
|
|
|
* ``-view-isel-dags`` displays the DAG before the Select phase.
|
|
|
|
* ``-view-sched-dags`` displays the DAG before Scheduling.
|
|
|
|
The ``-view-sunit-dags`` displays the Scheduler's dependency graph. This graph
|
|
is based on the final SelectionDAG, with nodes that must be scheduled together
|
|
bundled into a single scheduling-unit node, and with immediate operands and
|
|
other nodes that aren't relevant for scheduling omitted.
|
|
|
|
The option ``-filter-view-dags`` allows to select the name of the basic block
|
|
that you are interested to visualize and filters all the previous
|
|
``view-*-dags`` options.
|
|
|
|
.. _Build initial DAG:
|
|
|
|
Initial SelectionDAG Construction
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
The initial SelectionDAG is na\ :raw-html:`ï`\ vely peephole expanded from
|
|
the LLVM input by the ``SelectionDAGBuilder`` class. The intent of this pass
|
|
is to expose as much low-level, target-specific details to the SelectionDAG as
|
|
possible. This pass is mostly hard-coded (e.g. an LLVM ``add`` turns into an
|
|
``SDNode add`` while a ``getelementptr`` is expanded into the obvious
|
|
arithmetic). This pass requires target-specific hooks to lower calls, returns,
|
|
varargs, etc. For these features, the :raw-html:`<tt>` `TargetLowering`_
|
|
:raw-html:`</tt>` interface is used.
|
|
|
|
.. _legalize types:
|
|
.. _Legalize SelectionDAG Types:
|
|
.. _Legalize SelectionDAG Ops:
|
|
|
|
SelectionDAG LegalizeTypes Phase
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
The Legalize phase is in charge of converting a DAG to only use the types that
|
|
are natively supported by the target.
|
|
|
|
There are two main ways of converting values of unsupported scalar types to
|
|
values of supported types: converting small types to larger types ("promoting"),
|
|
and breaking up large integer types into smaller ones ("expanding"). For
|
|
example, a target might require that all f32 values are promoted to f64 and that
|
|
all i1/i8/i16 values are promoted to i32. The same target might require that
|
|
all i64 values be expanded into pairs of i32 values. These changes can insert
|
|
sign and zero extensions as needed to make sure that the final code has the same
|
|
behavior as the input.
|
|
|
|
There are two main ways of converting values of unsupported vector types to
|
|
value of supported types: splitting vector types, multiple times if necessary,
|
|
until a legal type is found, and extending vector types by adding elements to
|
|
the end to round them out to legal types ("widening"). If a vector gets split
|
|
all the way down to single-element parts with no supported vector type being
|
|
found, the elements are converted to scalars ("scalarizing").
|
|
|
|
A target implementation tells the legalizer which types are supported (and which
|
|
register class to use for them) by calling the ``addRegisterClass`` method in
|
|
its ``TargetLowering`` constructor.
|
|
|
|
.. _legalize operations:
|
|
.. _Legalizer:
|
|
|
|
SelectionDAG Legalize Phase
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
The Legalize phase is in charge of converting a DAG to only use the operations
|
|
that are natively supported by the target.
|
|
|
|
Targets often have weird constraints, such as not supporting every operation on
|
|
every supported datatype (e.g. X86 does not support byte conditional moves and
|
|
PowerPC does not support sign-extending loads from a 16-bit memory location).
|
|
Legalize takes care of this by open-coding another sequence of operations to
|
|
emulate the operation ("expansion"), by promoting one type to a larger type that
|
|
supports the operation ("promotion"), or by using a target-specific hook to
|
|
implement the legalization ("custom").
|
|
|
|
A target implementation tells the legalizer which operations are not supported
|
|
(and which of the above three actions to take) by calling the
|
|
``setOperationAction`` method in its ``TargetLowering`` constructor.
|
|
|
|
Prior to the existence of the Legalize passes, we required that every target
|
|
`selector`_ supported and handled every operator and type even if they are not
|
|
natively supported. The introduction of the Legalize phases allows all of the
|
|
canonicalization patterns to be shared across targets, and makes it very easy to
|
|
optimize the canonicalized code because it is still in the form of a DAG.
|
|
|
|
.. _optimizations:
|
|
.. _Optimize SelectionDAG:
|
|
.. _selector:
|
|
|
|
SelectionDAG Optimization Phase: the DAG Combiner
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
The SelectionDAG optimization phase is run multiple times for code generation,
|
|
immediately after the DAG is built and once after each legalization. The first
|
|
run of the pass allows the initial code to be cleaned up (e.g. performing
|
|
optimizations that depend on knowing that the operators have restricted type
|
|
inputs). Subsequent runs of the pass clean up the messy code generated by the
|
|
Legalize passes, which allows Legalize to be very simple (it can focus on making
|
|
code legal instead of focusing on generating *good* and legal code).
|
|
|
|
One important class of optimizations performed is optimizing inserted sign and
|
|
zero extension instructions. We currently use ad-hoc techniques, but could move
|
|
to more rigorous techniques in the future. Here are some good papers on the
|
|
subject:
|
|
|
|
"`Widening integer arithmetic <http://www.eecs.harvard.edu/~nr/pubs/widen-abstract.html>`_" :raw-html:`<br>`
|
|
Kevin Redwine and Norman Ramsey :raw-html:`<br>`
|
|
International Conference on Compiler Construction (CC) 2004
|
|
|
|
"`Effective sign extension elimination <http://portal.acm.org/citation.cfm?doid=512529.512552>`_" :raw-html:`<br>`
|
|
Motohiro Kawahito, Hideaki Komatsu, and Toshio Nakatani :raw-html:`<br>`
|
|
Proceedings of the ACM SIGPLAN 2002 Conference on Programming Language Design
|
|
and Implementation.
|
|
|
|
.. _Select instructions from DAG:
|
|
|
|
SelectionDAG Select Phase
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
The Select phase is the bulk of the target-specific code for instruction
|
|
selection. This phase takes a legal SelectionDAG as input, pattern matches the
|
|
instructions supported by the target to this DAG, and produces a new DAG of
|
|
target code. For example, consider the following LLVM fragment:
|
|
|
|
.. code-block:: llvm
|
|
|
|
%t1 = fadd float %W, %X
|
|
%t2 = fmul float %t1, %Y
|
|
%t3 = fadd float %t2, %Z
|
|
|
|
This LLVM code corresponds to a SelectionDAG that looks basically like this:
|
|
|
|
.. code-block:: llvm
|
|
|
|
(fadd:f32 (fmul:f32 (fadd:f32 W, X), Y), Z)
|
|
|
|
If a target supports floating point multiply-and-add (FMA) operations, one of
|
|
the adds can be merged with the multiply. On the PowerPC, for example, the
|
|
output of the instruction selector might look like this DAG:
|
|
|
|
::
|
|
|
|
(FMADDS (FADDS W, X), Y, Z)
|
|
|
|
The ``FMADDS`` instruction is a ternary instruction that multiplies its first
|
|
two operands and adds the third (as single-precision floating-point numbers).
|
|
The ``FADDS`` instruction is a simple binary single-precision add instruction.
|
|
To perform this pattern match, the PowerPC backend includes the following
|
|
instruction definitions:
|
|
|
|
.. code-block:: text
|
|
:emphasize-lines: 4-5,9
|
|
|
|
def FMADDS : AForm_1<59, 29,
|
|
(ops F4RC:$FRT, F4RC:$FRA, F4RC:$FRC, F4RC:$FRB),
|
|
"fmadds $FRT, $FRA, $FRC, $FRB",
|
|
[(set F4RC:$FRT, (fadd (fmul F4RC:$FRA, F4RC:$FRC),
|
|
F4RC:$FRB))]>;
|
|
def FADDS : AForm_2<59, 21,
|
|
(ops F4RC:$FRT, F4RC:$FRA, F4RC:$FRB),
|
|
"fadds $FRT, $FRA, $FRB",
|
|
[(set F4RC:$FRT, (fadd F4RC:$FRA, F4RC:$FRB))]>;
|
|
|
|
The highlighted portion of the instruction definitions indicates the pattern
|
|
used to match the instructions. The DAG operators (like ``fmul``/``fadd``)
|
|
are defined in the ``include/llvm/Target/TargetSelectionDAG.td`` file.
|
|
"``F4RC``" is the register class of the input and result values.
|
|
|
|
The TableGen DAG instruction selector generator reads the instruction patterns
|
|
in the ``.td`` file and automatically builds parts of the pattern matching code
|
|
for your target. It has the following strengths:
|
|
|
|
* At compiler-compiler time, it analyzes your instruction patterns and tells you
|
|
if your patterns make sense or not.
|
|
|
|
* It can handle arbitrary constraints on operands for the pattern match. In
|
|
particular, it is straight-forward to say things like "match any immediate
|
|
that is a 13-bit sign-extended value". For examples, see the ``immSExt16``
|
|
and related ``tblgen`` classes in the PowerPC backend.
|
|
|
|
* It knows several important identities for the patterns defined. For example,
|
|
it knows that addition is commutative, so it allows the ``FMADDS`` pattern
|
|
above to match "``(fadd X, (fmul Y, Z))``" as well as "``(fadd (fmul X, Y),
|
|
Z)``", without the target author having to specially handle this case.
|
|
|
|
* It has a full-featured type-inferencing system. In particular, you should
|
|
rarely have to explicitly tell the system what type parts of your patterns
|
|
are. In the ``FMADDS`` case above, we didn't have to tell ``tblgen`` that all
|
|
of the nodes in the pattern are of type 'f32'. It was able to infer and
|
|
propagate this knowledge from the fact that ``F4RC`` has type 'f32'.
|
|
|
|
* Targets can define their own (and rely on built-in) "pattern fragments".
|
|
Pattern fragments are chunks of reusable patterns that get inlined into your
|
|
patterns during compiler-compiler time. For example, the integer "``(not
|
|
x)``" operation is actually defined as a pattern fragment that expands as
|
|
"``(xor x, -1)``", since the SelectionDAG does not have a native '``not``'
|
|
operation. Targets can define their own short-hand fragments as they see fit.
|
|
See the definition of '``not``' and '``ineg``' for examples.
|
|
|
|
* In addition to instructions, targets can specify arbitrary patterns that map
|
|
to one or more instructions using the 'Pat' class. For example, the PowerPC
|
|
has no way to load an arbitrary integer immediate into a register in one
|
|
instruction. To tell tblgen how to do this, it defines:
|
|
|
|
::
|
|
|
|
// Arbitrary immediate support. Implement in terms of LIS/ORI.
|
|
def : Pat<(i32 imm:$imm),
|
|
(ORI (LIS (HI16 imm:$imm)), (LO16 imm:$imm))>;
|
|
|
|
If none of the single-instruction patterns for loading an immediate into a
|
|
register match, this will be used. This rule says "match an arbitrary i32
|
|
immediate, turning it into an ``ORI`` ('or a 16-bit immediate') and an ``LIS``
|
|
('load 16-bit immediate, where the immediate is shifted to the left 16 bits')
|
|
instruction". To make this work, the ``LO16``/``HI16`` node transformations
|
|
are used to manipulate the input immediate (in this case, take the high or low
|
|
16-bits of the immediate).
|
|
|
|
* When using the 'Pat' class to map a pattern to an instruction that has one
|
|
or more complex operands (like e.g. `X86 addressing mode`_), the pattern may
|
|
either specify the operand as a whole using a ``ComplexPattern``, or else it
|
|
may specify the components of the complex operand separately. The latter is
|
|
done e.g. for pre-increment instructions by the PowerPC back end:
|
|
|
|
::
|
|
|
|
def STWU : DForm_1<37, (outs ptr_rc:$ea_res), (ins GPRC:$rS, memri:$dst),
|
|
"stwu $rS, $dst", LdStStoreUpd, []>,
|
|
RegConstraint<"$dst.reg = $ea_res">, NoEncode<"$ea_res">;
|
|
|
|
def : Pat<(pre_store GPRC:$rS, ptr_rc:$ptrreg, iaddroff:$ptroff),
|
|
(STWU GPRC:$rS, iaddroff:$ptroff, ptr_rc:$ptrreg)>;
|
|
|
|
Here, the pair of ``ptroff`` and ``ptrreg`` operands is matched onto the
|
|
complex operand ``dst`` of class ``memri`` in the ``STWU`` instruction.
|
|
|
|
* While the system does automate a lot, it still allows you to write custom C++
|
|
code to match special cases if there is something that is hard to
|
|
express.
|
|
|
|
While it has many strengths, the system currently has some limitations,
|
|
primarily because it is a work in progress and is not yet finished:
|
|
|
|
* Overall, there is no way to define or match SelectionDAG nodes that define
|
|
multiple values (e.g. ``SMUL_LOHI``, ``LOAD``, ``CALL``, etc). This is the
|
|
biggest reason that you currently still *have to* write custom C++ code
|
|
for your instruction selector.
|
|
|
|
* There is no great way to support matching complex addressing modes yet. In
|
|
the future, we will extend pattern fragments to allow them to define multiple
|
|
values (e.g. the four operands of the `X86 addressing mode`_, which are
|
|
currently matched with custom C++ code). In addition, we'll extend fragments
|
|
so that a fragment can match multiple different patterns.
|
|
|
|
* We don't automatically infer flags like ``isStore``/``isLoad`` yet.
|
|
|
|
* We don't automatically generate the set of supported registers and operations
|
|
for the `Legalizer`_ yet.
|
|
|
|
* We don't have a way of tying in custom legalized nodes yet.
|
|
|
|
Despite these limitations, the instruction selector generator is still quite
|
|
useful for most of the binary and logical operations in typical instruction
|
|
sets. If you run into any problems or can't figure out how to do something,
|
|
please let Chris know!
|
|
|
|
.. _Scheduling and Formation:
|
|
.. _SelectionDAG Scheduling and Formation:
|
|
|
|
SelectionDAG Scheduling and Formation Phase
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
The scheduling phase takes the DAG of target instructions from the selection
|
|
phase and assigns an order. The scheduler can pick an order depending on
|
|
various constraints of the machines (i.e. order for minimal register pressure or
|
|
try to cover instruction latencies). Once an order is established, the DAG is
|
|
converted to a list of :raw-html:`<tt>` `MachineInstr`_\s :raw-html:`</tt>` and
|
|
the SelectionDAG is destroyed.
|
|
|
|
Note that this phase is logically separate from the instruction selection phase,
|
|
but is tied to it closely in the code because it operates on SelectionDAGs.
|
|
|
|
Future directions for the SelectionDAG
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
#. Optional function-at-a-time selection.
|
|
|
|
#. Auto-generate entire selector from ``.td`` file.
|
|
|
|
.. _SSA-based Machine Code Optimizations:
|
|
|
|
SSA-based Machine Code Optimizations
|
|
------------------------------------
|
|
|
|
To Be Written
|
|
|
|
Live Intervals
|
|
--------------
|
|
|
|
Live Intervals are the ranges (intervals) where a variable is *live*. They are
|
|
used by some `register allocator`_ passes to determine if two or more virtual
|
|
registers which require the same physical register are live at the same point in
|
|
the program (i.e., they conflict). When this situation occurs, one virtual
|
|
register must be *spilled*.
|
|
|
|
Live Variable Analysis
|
|
^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
The first step in determining the live intervals of variables is to calculate
|
|
the set of registers that are immediately dead after the instruction (i.e., the
|
|
instruction calculates the value, but it is never used) and the set of registers
|
|
that are used by the instruction, but are never used after the instruction
|
|
(i.e., they are killed). Live variable information is computed for
|
|
each *virtual* register and *register allocatable* physical register
|
|
in the function. This is done in a very efficient manner because it uses SSA to
|
|
sparsely compute lifetime information for virtual registers (which are in SSA
|
|
form) and only has to track physical registers within a block. Before register
|
|
allocation, LLVM can assume that physical registers are only live within a
|
|
single basic block. This allows it to do a single, local analysis to resolve
|
|
physical register lifetimes within each basic block. If a physical register is
|
|
not register allocatable (e.g., a stack pointer or condition codes), it is not
|
|
tracked.
|
|
|
|
Physical registers may be live in to or out of a function. Live in values are
|
|
typically arguments in registers. Live out values are typically return values in
|
|
registers. Live in values are marked as such, and are given a dummy "defining"
|
|
instruction during live intervals analysis. If the last basic block of a
|
|
function is a ``return``, then it's marked as using all live out values in the
|
|
function.
|
|
|
|
``PHI`` nodes need to be handled specially, because the calculation of the live
|
|
variable information from a depth first traversal of the CFG of the function
|
|
won't guarantee that a virtual register used by the ``PHI`` node is defined
|
|
before it's used. When a ``PHI`` node is encountered, only the definition is
|
|
handled, because the uses will be handled in other basic blocks.
|
|
|
|
For each ``PHI`` node of the current basic block, we simulate an assignment at
|
|
the end of the current basic block and traverse the successor basic blocks. If a
|
|
successor basic block has a ``PHI`` node and one of the ``PHI`` node's operands
|
|
is coming from the current basic block, then the variable is marked as *alive*
|
|
within the current basic block and all of its predecessor basic blocks, until
|
|
the basic block with the defining instruction is encountered.
|
|
|
|
Live Intervals Analysis
|
|
^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
We now have the information available to perform the live intervals analysis and
|
|
build the live intervals themselves. We start off by numbering the basic blocks
|
|
and machine instructions. We then handle the "live-in" values. These are in
|
|
physical registers, so the physical register is assumed to be killed by the end
|
|
of the basic block. Live intervals for virtual registers are computed for some
|
|
ordering of the machine instructions ``[1, N]``. A live interval is an interval
|
|
``[i, j)``, where ``1 >= i >= j > N``, for which a variable is live.
|
|
|
|
.. note::
|
|
More to come...
|
|
|
|
.. _Register Allocation:
|
|
.. _register allocator:
|
|
|
|
Register Allocation
|
|
-------------------
|
|
|
|
The *Register Allocation problem* consists in mapping a program
|
|
:raw-html:`<b><tt>` P\ :sub:`v`\ :raw-html:`</tt></b>`, that can use an unbounded
|
|
number of virtual registers, to a program :raw-html:`<b><tt>` P\ :sub:`p`\
|
|
:raw-html:`</tt></b>` that contains a finite (possibly small) number of physical
|
|
registers. Each target architecture has a different number of physical
|
|
registers. If the number of physical registers is not enough to accommodate all
|
|
the virtual registers, some of them will have to be mapped into memory. These
|
|
virtuals are called *spilled virtuals*.
|
|
|
|
How registers are represented in LLVM
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
In LLVM, physical registers are denoted by integer numbers that normally range
|
|
from 1 to 1023. To see how this numbering is defined for a particular
|
|
architecture, you can read the ``GenRegisterNames.inc`` file for that
|
|
architecture. For instance, by inspecting
|
|
``lib/Target/X86/X86GenRegisterInfo.inc`` we see that the 32-bit register
|
|
``EAX`` is denoted by 43, and the MMX register ``MM0`` is mapped to 65.
|
|
|
|
Some architectures contain registers that share the same physical location. A
|
|
notable example is the X86 platform. For instance, in the X86 architecture, the
|
|
registers ``EAX``, ``AX`` and ``AL`` share the first eight bits. These physical
|
|
registers are marked as *aliased* in LLVM. Given a particular architecture, you
|
|
can check which registers are aliased by inspecting its ``RegisterInfo.td``
|
|
file. Moreover, the class ``MCRegAliasIterator`` enumerates all the physical
|
|
registers aliased to a register.
|
|
|
|
Physical registers, in LLVM, are grouped in *Register Classes*. Elements in the
|
|
same register class are functionally equivalent, and can be interchangeably
|
|
used. Each virtual register can only be mapped to physical registers of a
|
|
particular class. For instance, in the X86 architecture, some virtuals can only
|
|
be allocated to 8 bit registers. A register class is described by
|
|
``TargetRegisterClass`` objects. To discover if a virtual register is
|
|
compatible with a given physical, this code can be used:
|
|
|
|
.. code-block:: c++
|
|
|
|
bool RegMapping_Fer::compatible_class(MachineFunction &mf,
|
|
unsigned v_reg,
|
|
unsigned p_reg) {
|
|
assert(TargetRegisterInfo::isPhysicalRegister(p_reg) &&
|
|
"Target register must be physical");
|
|
const TargetRegisterClass *trc = mf.getRegInfo().getRegClass(v_reg);
|
|
return trc->contains(p_reg);
|
|
}
|
|
|
|
Sometimes, mostly for debugging purposes, it is useful to change the number of
|
|
physical registers available in the target architecture. This must be done
|
|
statically, inside the ``TargetRegsterInfo.td`` file. Just ``grep`` for
|
|
``RegisterClass``, the last parameter of which is a list of registers. Just
|
|
commenting some out is one simple way to avoid them being used. A more polite
|
|
way is to explicitly exclude some registers from the *allocation order*. See the
|
|
definition of the ``GR8`` register class in
|
|
``lib/Target/X86/X86RegisterInfo.td`` for an example of this.
|
|
|
|
Virtual registers are also denoted by integer numbers. Contrary to physical
|
|
registers, different virtual registers never share the same number. Whereas
|
|
physical registers are statically defined in a ``TargetRegisterInfo.td`` file
|
|
and cannot be created by the application developer, that is not the case with
|
|
virtual registers. In order to create new virtual registers, use the method
|
|
``MachineRegisterInfo::createVirtualRegister()``. This method will return a new
|
|
virtual register. Use an ``IndexedMap<Foo, VirtReg2IndexFunctor>`` to hold
|
|
information per virtual register. If you need to enumerate all virtual
|
|
registers, use the function ``TargetRegisterInfo::index2VirtReg()`` to find the
|
|
virtual register numbers:
|
|
|
|
.. code-block:: c++
|
|
|
|
for (unsigned i = 0, e = MRI->getNumVirtRegs(); i != e; ++i) {
|
|
unsigned VirtReg = TargetRegisterInfo::index2VirtReg(i);
|
|
stuff(VirtReg);
|
|
}
|
|
|
|
Before register allocation, the operands of an instruction are mostly virtual
|
|
registers, although physical registers may also be used. In order to check if a
|
|
given machine operand is a register, use the boolean function
|
|
``MachineOperand::isRegister()``. To obtain the integer code of a register, use
|
|
``MachineOperand::getReg()``. An instruction may define or use a register. For
|
|
instance, ``ADD reg:1026 := reg:1025 reg:1024`` defines the registers 1024, and
|
|
uses registers 1025 and 1026. Given a register operand, the method
|
|
``MachineOperand::isUse()`` informs if that register is being used by the
|
|
instruction. The method ``MachineOperand::isDef()`` informs if that registers is
|
|
being defined.
|
|
|
|
We will call physical registers present in the LLVM bitcode before register
|
|
allocation *pre-colored registers*. Pre-colored registers are used in many
|
|
different situations, for instance, to pass parameters of functions calls, and
|
|
to store results of particular instructions. There are two types of pre-colored
|
|
registers: the ones *implicitly* defined, and those *explicitly*
|
|
defined. Explicitly defined registers are normal operands, and can be accessed
|
|
with ``MachineInstr::getOperand(int)::getReg()``. In order to check which
|
|
registers are implicitly defined by an instruction, use the
|
|
``TargetInstrInfo::get(opcode)::ImplicitDefs``, where ``opcode`` is the opcode
|
|
of the target instruction. One important difference between explicit and
|
|
implicit physical registers is that the latter are defined statically for each
|
|
instruction, whereas the former may vary depending on the program being
|
|
compiled. For example, an instruction that represents a function call will
|
|
always implicitly define or use the same set of physical registers. To read the
|
|
registers implicitly used by an instruction, use
|
|
``TargetInstrInfo::get(opcode)::ImplicitUses``. Pre-colored registers impose
|
|
constraints on any register allocation algorithm. The register allocator must
|
|
make sure that none of them are overwritten by the values of virtual registers
|
|
while still alive.
|
|
|
|
Mapping virtual registers to physical registers
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
There are two ways to map virtual registers to physical registers (or to memory
|
|
slots). The first way, that we will call *direct mapping*, is based on the use
|
|
of methods of the classes ``TargetRegisterInfo``, and ``MachineOperand``. The
|
|
second way, that we will call *indirect mapping*, relies on the ``VirtRegMap``
|
|
class in order to insert loads and stores sending and getting values to and from
|
|
memory.
|
|
|
|
The direct mapping provides more flexibility to the developer of the register
|
|
allocator; however, it is more error prone, and demands more implementation
|
|
work. Basically, the programmer will have to specify where load and store
|
|
instructions should be inserted in the target function being compiled in order
|
|
to get and store values in memory. To assign a physical register to a virtual
|
|
register present in a given operand, use ``MachineOperand::setReg(p_reg)``. To
|
|
insert a store instruction, use ``TargetInstrInfo::storeRegToStackSlot(...)``,
|
|
and to insert a load instruction, use ``TargetInstrInfo::loadRegFromStackSlot``.
|
|
|
|
The indirect mapping shields the application developer from the complexities of
|
|
inserting load and store instructions. In order to map a virtual register to a
|
|
physical one, use ``VirtRegMap::assignVirt2Phys(vreg, preg)``. In order to map
|
|
a certain virtual register to memory, use
|
|
``VirtRegMap::assignVirt2StackSlot(vreg)``. This method will return the stack
|
|
slot where ``vreg``'s value will be located. If it is necessary to map another
|
|
virtual register to the same stack slot, use
|
|
``VirtRegMap::assignVirt2StackSlot(vreg, stack_location)``. One important point
|
|
to consider when using the indirect mapping, is that even if a virtual register
|
|
is mapped to memory, it still needs to be mapped to a physical register. This
|
|
physical register is the location where the virtual register is supposed to be
|
|
found before being stored or after being reloaded.
|
|
|
|
If the indirect strategy is used, after all the virtual registers have been
|
|
mapped to physical registers or stack slots, it is necessary to use a spiller
|
|
object to place load and store instructions in the code. Every virtual that has
|
|
been mapped to a stack slot will be stored to memory after being defined and will
|
|
be loaded before being used. The implementation of the spiller tries to recycle
|
|
load/store instructions, avoiding unnecessary instructions. For an example of
|
|
how to invoke the spiller, see ``RegAllocLinearScan::runOnMachineFunction`` in
|
|
``lib/CodeGen/RegAllocLinearScan.cpp``.
|
|
|
|
Handling two address instructions
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
With very rare exceptions (e.g., function calls), the LLVM machine code
|
|
instructions are three address instructions. That is, each instruction is
|
|
expected to define at most one register, and to use at most two registers.
|
|
However, some architectures use two address instructions. In this case, the
|
|
defined register is also one of the used registers. For instance, an instruction
|
|
such as ``ADD %EAX, %EBX``, in X86 is actually equivalent to ``%EAX = %EAX +
|
|
%EBX``.
|
|
|
|
In order to produce correct code, LLVM must convert three address instructions
|
|
that represent two address instructions into true two address instructions. LLVM
|
|
provides the pass ``TwoAddressInstructionPass`` for this specific purpose. It
|
|
must be run before register allocation takes place. After its execution, the
|
|
resulting code may no longer be in SSA form. This happens, for instance, in
|
|
situations where an instruction such as ``%a = ADD %b %c`` is converted to two
|
|
instructions such as:
|
|
|
|
::
|
|
|
|
%a = MOVE %b
|
|
%a = ADD %a %c
|
|
|
|
Notice that, internally, the second instruction is represented as ``ADD
|
|
%a[def/use] %c``. I.e., the register operand ``%a`` is both used and defined by
|
|
the instruction.
|
|
|
|
The SSA deconstruction phase
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
An important transformation that happens during register allocation is called
|
|
the *SSA Deconstruction Phase*. The SSA form simplifies many analyses that are
|
|
performed on the control flow graph of programs. However, traditional
|
|
instruction sets do not implement PHI instructions. Thus, in order to generate
|
|
executable code, compilers must replace PHI instructions with other instructions
|
|
that preserve their semantics.
|
|
|
|
There are many ways in which PHI instructions can safely be removed from the
|
|
target code. The most traditional PHI deconstruction algorithm replaces PHI
|
|
instructions with copy instructions. That is the strategy adopted by LLVM. The
|
|
SSA deconstruction algorithm is implemented in
|
|
``lib/CodeGen/PHIElimination.cpp``. In order to invoke this pass, the identifier
|
|
``PHIEliminationID`` must be marked as required in the code of the register
|
|
allocator.
|
|
|
|
Instruction folding
|
|
^^^^^^^^^^^^^^^^^^^
|
|
|
|
*Instruction folding* is an optimization performed during register allocation
|
|
that removes unnecessary copy instructions. For instance, a sequence of
|
|
instructions such as:
|
|
|
|
::
|
|
|
|
%EBX = LOAD %mem_address
|
|
%EAX = COPY %EBX
|
|
|
|
can be safely substituted by the single instruction:
|
|
|
|
::
|
|
|
|
%EAX = LOAD %mem_address
|
|
|
|
Instructions can be folded with the
|
|
``TargetRegisterInfo::foldMemoryOperand(...)`` method. Care must be taken when
|
|
folding instructions; a folded instruction can be quite different from the
|
|
original instruction. See ``LiveIntervals::addIntervalsForSpills`` in
|
|
``lib/CodeGen/LiveIntervalAnalysis.cpp`` for an example of its use.
|
|
|
|
Built in register allocators
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
The LLVM infrastructure provides the application developer with three different
|
|
register allocators:
|
|
|
|
* *Fast* --- This register allocator is the default for debug builds. It
|
|
allocates registers on a basic block level, attempting to keep values in
|
|
registers and reusing registers as appropriate.
|
|
|
|
* *Basic* --- This is an incremental approach to register allocation. Live
|
|
ranges are assigned to registers one at a time in an order that is driven by
|
|
heuristics. Since code can be rewritten on-the-fly during allocation, this
|
|
framework allows interesting allocators to be developed as extensions. It is
|
|
not itself a production register allocator but is a potentially useful
|
|
stand-alone mode for triaging bugs and as a performance baseline.
|
|
|
|
* *Greedy* --- *The default allocator*. This is a highly tuned implementation of
|
|
the *Basic* allocator that incorporates global live range splitting. This
|
|
allocator works hard to minimize the cost of spill code.
|
|
|
|
* *PBQP* --- A Partitioned Boolean Quadratic Programming (PBQP) based register
|
|
allocator. This allocator works by constructing a PBQP problem representing
|
|
the register allocation problem under consideration, solving this using a PBQP
|
|
solver, and mapping the solution back to a register assignment.
|
|
|
|
The type of register allocator used in ``llc`` can be chosen with the command
|
|
line option ``-regalloc=...``:
|
|
|
|
.. code-block:: bash
|
|
|
|
$ llc -regalloc=linearscan file.bc -o ln.s
|
|
$ llc -regalloc=fast file.bc -o fa.s
|
|
$ llc -regalloc=pbqp file.bc -o pbqp.s
|
|
|
|
.. _Prolog/Epilog Code Insertion:
|
|
|
|
Prolog/Epilog Code Insertion
|
|
----------------------------
|
|
|
|
Compact Unwind
|
|
|
|
Throwing an exception requires *unwinding* out of a function. The information on
|
|
how to unwind a given function is traditionally expressed in DWARF unwind
|
|
(a.k.a. frame) info. But that format was originally developed for debuggers to
|
|
backtrace, and each Frame Description Entry (FDE) requires ~20-30 bytes per
|
|
function. There is also the cost of mapping from an address in a function to the
|
|
corresponding FDE at runtime. An alternative unwind encoding is called *compact
|
|
unwind* and requires just 4-bytes per function.
|
|
|
|
The compact unwind encoding is a 32-bit value, which is encoded in an
|
|
architecture-specific way. It specifies which registers to restore and from
|
|
where, and how to unwind out of the function. When the linker creates a final
|
|
linked image, it will create a ``__TEXT,__unwind_info`` section. This section is
|
|
a small and fast way for the runtime to access unwind info for any given
|
|
function. If we emit compact unwind info for the function, that compact unwind
|
|
info will be encoded in the ``__TEXT,__unwind_info`` section. If we emit DWARF
|
|
unwind info, the ``__TEXT,__unwind_info`` section will contain the offset of the
|
|
FDE in the ``__TEXT,__eh_frame`` section in the final linked image.
|
|
|
|
For X86, there are three modes for the compact unwind encoding:
|
|
|
|
*Function with a Frame Pointer (``EBP`` or ``RBP``)*
|
|
``EBP/RBP``-based frame, where ``EBP/RBP`` is pushed onto the stack
|
|
immediately after the return address, then ``ESP/RSP`` is moved to
|
|
``EBP/RBP``. Thus to unwind, ``ESP/RSP`` is restored with the current
|
|
``EBP/RBP`` value, then ``EBP/RBP`` is restored by popping the stack, and the
|
|
return is done by popping the stack once more into the PC. All non-volatile
|
|
registers that need to be restored must have been saved in a small range on
|
|
the stack that starts ``EBP-4`` to ``EBP-1020`` (``RBP-8`` to
|
|
``RBP-1020``). The offset (divided by 4 in 32-bit mode and 8 in 64-bit mode)
|
|
is encoded in bits 16-23 (mask: ``0x00FF0000``). The registers saved are
|
|
encoded in bits 0-14 (mask: ``0x00007FFF``) as five 3-bit entries from the
|
|
following table:
|
|
|
|
============== ============= ===============
|
|
Compact Number i386 Register x86-64 Register
|
|
============== ============= ===============
|
|
1 ``EBX`` ``RBX``
|
|
2 ``ECX`` ``R12``
|
|
3 ``EDX`` ``R13``
|
|
4 ``EDI`` ``R14``
|
|
5 ``ESI`` ``R15``
|
|
6 ``EBP`` ``RBP``
|
|
============== ============= ===============
|
|
|
|
*Frameless with a Small Constant Stack Size (``EBP`` or ``RBP`` is not used as a frame pointer)*
|
|
To return, a constant (encoded in the compact unwind encoding) is added to the
|
|
``ESP/RSP``. Then the return is done by popping the stack into the PC. All
|
|
non-volatile registers that need to be restored must have been saved on the
|
|
stack immediately after the return address. The stack size (divided by 4 in
|
|
32-bit mode and 8 in 64-bit mode) is encoded in bits 16-23 (mask:
|
|
``0x00FF0000``). There is a maximum stack size of 1024 bytes in 32-bit mode
|
|
and 2048 in 64-bit mode. The number of registers saved is encoded in bits 9-12
|
|
(mask: ``0x00001C00``). Bits 0-9 (mask: ``0x000003FF``) contain which
|
|
registers were saved and their order. (See the
|
|
``encodeCompactUnwindRegistersWithoutFrame()`` function in
|
|
``lib/Target/X86FrameLowering.cpp`` for the encoding algorithm.)
|
|
|
|
*Frameless with a Large Constant Stack Size (``EBP`` or ``RBP`` is not used as a frame pointer)*
|
|
This case is like the "Frameless with a Small Constant Stack Size" case, but
|
|
the stack size is too large to encode in the compact unwind encoding. Instead
|
|
it requires that the function contains "``subl $nnnnnn, %esp``" in its
|
|
prolog. The compact encoding contains the offset to the ``$nnnnnn`` value in
|
|
the function in bits 9-12 (mask: ``0x00001C00``).
|
|
|
|
.. _Late Machine Code Optimizations:
|
|
|
|
Late Machine Code Optimizations
|
|
-------------------------------
|
|
|
|
.. note::
|
|
|
|
To Be Written
|
|
|
|
.. _Code Emission:
|
|
|
|
Code Emission
|
|
-------------
|
|
|
|
The code emission step of code generation is responsible for lowering from the
|
|
code generator abstractions (like `MachineFunction`_, `MachineInstr`_, etc) down
|
|
to the abstractions used by the MC layer (`MCInst`_, `MCStreamer`_, etc). This
|
|
is done with a combination of several different classes: the (misnamed)
|
|
target-independent AsmPrinter class, target-specific subclasses of AsmPrinter
|
|
(such as SparcAsmPrinter), and the TargetLoweringObjectFile class.
|
|
|
|
Since the MC layer works at the level of abstraction of object files, it doesn't
|
|
have a notion of functions, global variables etc. Instead, it thinks about
|
|
labels, directives, and instructions. A key class used at this time is the
|
|
MCStreamer class. This is an abstract API that is implemented in different ways
|
|
(e.g. to output a .s file, output an ELF .o file, etc) that is effectively an
|
|
"assembler API". MCStreamer has one method per directive, such as EmitLabel,
|
|
EmitSymbolAttribute, SwitchSection, etc, which directly correspond to assembly
|
|
level directives.
|
|
|
|
If you are interested in implementing a code generator for a target, there are
|
|
three important things that you have to implement for your target:
|
|
|
|
#. First, you need a subclass of AsmPrinter for your target. This class
|
|
implements the general lowering process converting MachineFunction's into MC
|
|
label constructs. The AsmPrinter base class provides a number of useful
|
|
methods and routines, and also allows you to override the lowering process in
|
|
some important ways. You should get much of the lowering for free if you are
|
|
implementing an ELF, COFF, or MachO target, because the
|
|
TargetLoweringObjectFile class implements much of the common logic.
|
|
|
|
#. Second, you need to implement an instruction printer for your target. The
|
|
instruction printer takes an `MCInst`_ and renders it to a raw_ostream as
|
|
text. Most of this is automatically generated from the .td file (when you
|
|
specify something like "``add $dst, $src1, $src2``" in the instructions), but
|
|
you need to implement routines to print operands.
|
|
|
|
#. Third, you need to implement code that lowers a `MachineInstr`_ to an MCInst,
|
|
usually implemented in "<target>MCInstLower.cpp". This lowering process is
|
|
often target specific, and is responsible for turning jump table entries,
|
|
constant pool indices, global variable addresses, etc into MCLabels as
|
|
appropriate. This translation layer is also responsible for expanding pseudo
|
|
ops used by the code generator into the actual machine instructions they
|
|
correspond to. The MCInsts that are generated by this are fed into the
|
|
instruction printer or the encoder.
|
|
|
|
Finally, at your choosing, you can also implement a subclass of MCCodeEmitter
|
|
which lowers MCInst's into machine code bytes and relocations. This is
|
|
important if you want to support direct .o file emission, or would like to
|
|
implement an assembler for your target.
|
|
|
|
VLIW Packetizer
|
|
---------------
|
|
|
|
In a Very Long Instruction Word (VLIW) architecture, the compiler is responsible
|
|
for mapping instructions to functional-units available on the architecture. To
|
|
that end, the compiler creates groups of instructions called *packets* or
|
|
*bundles*. The VLIW packetizer in LLVM is a target-independent mechanism to
|
|
enable the packetization of machine instructions.
|
|
|
|
Mapping from instructions to functional units
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Instructions in a VLIW target can typically be mapped to multiple functional
|
|
units. During the process of packetizing, the compiler must be able to reason
|
|
about whether an instruction can be added to a packet. This decision can be
|
|
complex since the compiler has to examine all possible mappings of instructions
|
|
to functional units. Therefore to alleviate compilation-time complexity, the
|
|
VLIW packetizer parses the instruction classes of a target and generates tables
|
|
at compiler build time. These tables can then be queried by the provided
|
|
machine-independent API to determine if an instruction can be accommodated in a
|
|
packet.
|
|
|
|
How the packetization tables are generated and used
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
The packetizer reads instruction classes from a target's itineraries and creates
|
|
a deterministic finite automaton (DFA) to represent the state of a packet. A DFA
|
|
consists of three major elements: inputs, states, and transitions. The set of
|
|
inputs for the generated DFA represents the instruction being added to a
|
|
packet. The states represent the possible consumption of functional units by
|
|
instructions in a packet. In the DFA, transitions from one state to another
|
|
occur on the addition of an instruction to an existing packet. If there is a
|
|
legal mapping of functional units to instructions, then the DFA contains a
|
|
corresponding transition. The absence of a transition indicates that a legal
|
|
mapping does not exist and that the instruction cannot be added to the packet.
|
|
|
|
To generate tables for a VLIW target, add *Target*\ GenDFAPacketizer.inc as a
|
|
target to the Makefile in the target directory. The exported API provides three
|
|
functions: ``DFAPacketizer::clearResources()``,
|
|
``DFAPacketizer::reserveResources(MachineInstr *MI)``, and
|
|
``DFAPacketizer::canReserveResources(MachineInstr *MI)``. These functions allow
|
|
a target packetizer to add an instruction to an existing packet and to check
|
|
whether an instruction can be added to a packet. See
|
|
``llvm/CodeGen/DFAPacketizer.h`` for more information.
|
|
|
|
Implementing a Native Assembler
|
|
===============================
|
|
|
|
Though you're probably reading this because you want to write or maintain a
|
|
compiler backend, LLVM also fully supports building a native assembler.
|
|
We've tried hard to automate the generation of the assembler from the .td files
|
|
(in particular the instruction syntax and encodings), which means that a large
|
|
part of the manual and repetitive data entry can be factored and shared with the
|
|
compiler.
|
|
|
|
Instruction Parsing
|
|
-------------------
|
|
|
|
.. note::
|
|
|
|
To Be Written
|
|
|
|
|
|
Instruction Alias Processing
|
|
----------------------------
|
|
|
|
Once the instruction is parsed, it enters the MatchInstructionImpl function.
|
|
The MatchInstructionImpl function performs alias processing and then does actual
|
|
matching.
|
|
|
|
Alias processing is the phase that canonicalizes different lexical forms of the
|
|
same instructions down to one representation. There are several different kinds
|
|
of alias that are possible to implement and they are listed below in the order
|
|
that they are processed (which is in order from simplest/weakest to most
|
|
complex/powerful). Generally you want to use the first alias mechanism that
|
|
meets the needs of your instruction, because it will allow a more concise
|
|
description.
|
|
|
|
Mnemonic Aliases
|
|
^^^^^^^^^^^^^^^^
|
|
|
|
The first phase of alias processing is simple instruction mnemonic remapping for
|
|
classes of instructions which are allowed with two different mnemonics. This
|
|
phase is a simple and unconditionally remapping from one input mnemonic to one
|
|
output mnemonic. It isn't possible for this form of alias to look at the
|
|
operands at all, so the remapping must apply for all forms of a given mnemonic.
|
|
Mnemonic aliases are defined simply, for example X86 has:
|
|
|
|
::
|
|
|
|
def : MnemonicAlias<"cbw", "cbtw">;
|
|
def : MnemonicAlias<"smovq", "movsq">;
|
|
def : MnemonicAlias<"fldcww", "fldcw">;
|
|
def : MnemonicAlias<"fucompi", "fucomip">;
|
|
def : MnemonicAlias<"ud2a", "ud2">;
|
|
|
|
... and many others. With a MnemonicAlias definition, the mnemonic is remapped
|
|
simply and directly. Though MnemonicAlias's can't look at any aspect of the
|
|
instruction (such as the operands) they can depend on global modes (the same
|
|
ones supported by the matcher), through a Requires clause:
|
|
|
|
::
|
|
|
|
def : MnemonicAlias<"pushf", "pushfq">, Requires<[In64BitMode]>;
|
|
def : MnemonicAlias<"pushf", "pushfl">, Requires<[In32BitMode]>;
|
|
|
|
In this example, the mnemonic gets mapped into a different one depending on
|
|
the current instruction set.
|
|
|
|
Instruction Aliases
|
|
^^^^^^^^^^^^^^^^^^^
|
|
|
|
The most general phase of alias processing occurs while matching is happening:
|
|
it provides new forms for the matcher to match along with a specific instruction
|
|
to generate. An instruction alias has two parts: the string to match and the
|
|
instruction to generate. For example:
|
|
|
|
::
|
|
|
|
def : InstAlias<"movsx $src, $dst", (MOVSX16rr8W GR16:$dst, GR8 :$src)>;
|
|
def : InstAlias<"movsx $src, $dst", (MOVSX16rm8W GR16:$dst, i8mem:$src)>;
|
|
def : InstAlias<"movsx $src, $dst", (MOVSX32rr8 GR32:$dst, GR8 :$src)>;
|
|
def : InstAlias<"movsx $src, $dst", (MOVSX32rr16 GR32:$dst, GR16 :$src)>;
|
|
def : InstAlias<"movsx $src, $dst", (MOVSX64rr8 GR64:$dst, GR8 :$src)>;
|
|
def : InstAlias<"movsx $src, $dst", (MOVSX64rr16 GR64:$dst, GR16 :$src)>;
|
|
def : InstAlias<"movsx $src, $dst", (MOVSX64rr32 GR64:$dst, GR32 :$src)>;
|
|
|
|
This shows a powerful example of the instruction aliases, matching the same
|
|
mnemonic in multiple different ways depending on what operands are present in
|
|
the assembly. The result of instruction aliases can include operands in a
|
|
different order than the destination instruction, and can use an input multiple
|
|
times, for example:
|
|
|
|
::
|
|
|
|
def : InstAlias<"clrb $reg", (XOR8rr GR8 :$reg, GR8 :$reg)>;
|
|
def : InstAlias<"clrw $reg", (XOR16rr GR16:$reg, GR16:$reg)>;
|
|
def : InstAlias<"clrl $reg", (XOR32rr GR32:$reg, GR32:$reg)>;
|
|
def : InstAlias<"clrq $reg", (XOR64rr GR64:$reg, GR64:$reg)>;
|
|
|
|
This example also shows that tied operands are only listed once. In the X86
|
|
backend, XOR8rr has two input GR8's and one output GR8 (where an input is tied
|
|
to the output). InstAliases take a flattened operand list without duplicates
|
|
for tied operands. The result of an instruction alias can also use immediates
|
|
and fixed physical registers which are added as simple immediate operands in the
|
|
result, for example:
|
|
|
|
::
|
|
|
|
// Fixed Immediate operand.
|
|
def : InstAlias<"aad", (AAD8i8 10)>;
|
|
|
|
// Fixed register operand.
|
|
def : InstAlias<"fcomi", (COM_FIr ST1)>;
|
|
|
|
// Simple alias.
|
|
def : InstAlias<"fcomi $reg", (COM_FIr RST:$reg)>;
|
|
|
|
Instruction aliases can also have a Requires clause to make them subtarget
|
|
specific.
|
|
|
|
If the back-end supports it, the instruction printer can automatically emit the
|
|
alias rather than what's being aliased. It typically leads to better, more
|
|
readable code. If it's better to print out what's being aliased, then pass a '0'
|
|
as the third parameter to the InstAlias definition.
|
|
|
|
Instruction Matching
|
|
--------------------
|
|
|
|
.. note::
|
|
|
|
To Be Written
|
|
|
|
.. _Implementations of the abstract target description interfaces:
|
|
.. _implement the target description:
|
|
|
|
Target-specific Implementation Notes
|
|
====================================
|
|
|
|
This section of the document explains features or design decisions that are
|
|
specific to the code generator for a particular target. First we start with a
|
|
table that summarizes what features are supported by each target.
|
|
|
|
.. _target-feature-matrix:
|
|
|
|
Target Feature Matrix
|
|
---------------------
|
|
|
|
Note that this table does not include the C backend or Cpp backends, since they
|
|
do not use the target independent code generator infrastructure. It also
|
|
doesn't list features that are not supported fully by any target yet. It
|
|
considers a feature to be supported if at least one subtarget supports it. A
|
|
feature being supported means that it is useful and works for most cases, it
|
|
does not indicate that there are zero known bugs in the implementation. Here is
|
|
the key:
|
|
|
|
:raw-html:`<table border="1" cellspacing="0">`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<th>Unknown</th>`
|
|
:raw-html:`<th>Not Applicable</th>`
|
|
:raw-html:`<th>No support</th>`
|
|
:raw-html:`<th>Partial Support</th>`
|
|
:raw-html:`<th>Complete Support</th>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td class="unknown"></td>`
|
|
:raw-html:`<td class="na"></td>`
|
|
:raw-html:`<td class="no"></td>`
|
|
:raw-html:`<td class="partial"></td>`
|
|
:raw-html:`<td class="yes"></td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`</table>`
|
|
|
|
Here is the table:
|
|
|
|
:raw-html:`<table width="689" border="1" cellspacing="0">`
|
|
:raw-html:`<tr><td></td>`
|
|
:raw-html:`<td colspan="13" align="center" style="background-color:#ffc">Target</td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<th>Feature</th>`
|
|
:raw-html:`<th>ARM</th>`
|
|
:raw-html:`<th>Hexagon</th>`
|
|
:raw-html:`<th>MSP430</th>`
|
|
:raw-html:`<th>Mips</th>`
|
|
:raw-html:`<th>NVPTX</th>`
|
|
:raw-html:`<th>PowerPC</th>`
|
|
:raw-html:`<th>Sparc</th>`
|
|
:raw-html:`<th>SystemZ</th>`
|
|
:raw-html:`<th>X86</th>`
|
|
:raw-html:`<th>XCore</th>`
|
|
:raw-html:`</tr>`
|
|
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td><a href="#feat_reliable">is generally reliable</a></td>`
|
|
:raw-html:`<td class="yes"></td> <!-- ARM -->`
|
|
:raw-html:`<td class="yes"></td> <!-- Hexagon -->`
|
|
:raw-html:`<td class="unknown"></td> <!-- MSP430 -->`
|
|
:raw-html:`<td class="yes"></td> <!-- Mips -->`
|
|
:raw-html:`<td class="yes"></td> <!-- NVPTX -->`
|
|
:raw-html:`<td class="yes"></td> <!-- PowerPC -->`
|
|
:raw-html:`<td class="yes"></td> <!-- Sparc -->`
|
|
:raw-html:`<td class="yes"></td> <!-- SystemZ -->`
|
|
:raw-html:`<td class="yes"></td> <!-- X86 -->`
|
|
:raw-html:`<td class="yes"></td> <!-- XCore -->`
|
|
:raw-html:`</tr>`
|
|
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td><a href="#feat_asmparser">assembly parser</a></td>`
|
|
:raw-html:`<td class="no"></td> <!-- ARM -->`
|
|
:raw-html:`<td class="no"></td> <!-- Hexagon -->`
|
|
:raw-html:`<td class="no"></td> <!-- MSP430 -->`
|
|
:raw-html:`<td class="no"></td> <!-- Mips -->`
|
|
:raw-html:`<td class="no"></td> <!-- NVPTX -->`
|
|
:raw-html:`<td class="no"></td> <!-- PowerPC -->`
|
|
:raw-html:`<td class="no"></td> <!-- Sparc -->`
|
|
:raw-html:`<td class="yes"></td> <!-- SystemZ -->`
|
|
:raw-html:`<td class="yes"></td> <!-- X86 -->`
|
|
:raw-html:`<td class="no"></td> <!-- XCore -->`
|
|
:raw-html:`</tr>`
|
|
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td><a href="#feat_disassembler">disassembler</a></td>`
|
|
:raw-html:`<td class="yes"></td> <!-- ARM -->`
|
|
:raw-html:`<td class="no"></td> <!-- Hexagon -->`
|
|
:raw-html:`<td class="no"></td> <!-- MSP430 -->`
|
|
:raw-html:`<td class="no"></td> <!-- Mips -->`
|
|
:raw-html:`<td class="na"></td> <!-- NVPTX -->`
|
|
:raw-html:`<td class="no"></td> <!-- PowerPC -->`
|
|
:raw-html:`<td class="yes"></td> <!-- SystemZ -->`
|
|
:raw-html:`<td class="no"></td> <!-- Sparc -->`
|
|
:raw-html:`<td class="yes"></td> <!-- X86 -->`
|
|
:raw-html:`<td class="yes"></td> <!-- XCore -->`
|
|
:raw-html:`</tr>`
|
|
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td><a href="#feat_inlineasm">inline asm</a></td>`
|
|
:raw-html:`<td class="yes"></td> <!-- ARM -->`
|
|
:raw-html:`<td class="yes"></td> <!-- Hexagon -->`
|
|
:raw-html:`<td class="unknown"></td> <!-- MSP430 -->`
|
|
:raw-html:`<td class="no"></td> <!-- Mips -->`
|
|
:raw-html:`<td class="yes"></td> <!-- NVPTX -->`
|
|
:raw-html:`<td class="yes"></td> <!-- PowerPC -->`
|
|
:raw-html:`<td class="unknown"></td> <!-- Sparc -->`
|
|
:raw-html:`<td class="yes"></td> <!-- SystemZ -->`
|
|
:raw-html:`<td class="yes"></td> <!-- X86 -->`
|
|
:raw-html:`<td class="yes"></td> <!-- XCore -->`
|
|
:raw-html:`</tr>`
|
|
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td><a href="#feat_jit">jit</a></td>`
|
|
:raw-html:`<td class="partial"><a href="#feat_jit_arm">*</a></td> <!-- ARM -->`
|
|
:raw-html:`<td class="no"></td> <!-- Hexagon -->`
|
|
:raw-html:`<td class="unknown"></td> <!-- MSP430 -->`
|
|
:raw-html:`<td class="yes"></td> <!-- Mips -->`
|
|
:raw-html:`<td class="na"></td> <!-- NVPTX -->`
|
|
:raw-html:`<td class="yes"></td> <!-- PowerPC -->`
|
|
:raw-html:`<td class="unknown"></td> <!-- Sparc -->`
|
|
:raw-html:`<td class="yes"></td> <!-- SystemZ -->`
|
|
:raw-html:`<td class="yes"></td> <!-- X86 -->`
|
|
:raw-html:`<td class="no"></td> <!-- XCore -->`
|
|
:raw-html:`</tr>`
|
|
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td><a href="#feat_objectwrite">.o file writing</a></td>`
|
|
:raw-html:`<td class="no"></td> <!-- ARM -->`
|
|
:raw-html:`<td class="no"></td> <!-- Hexagon -->`
|
|
:raw-html:`<td class="no"></td> <!-- MSP430 -->`
|
|
:raw-html:`<td class="no"></td> <!-- Mips -->`
|
|
:raw-html:`<td class="na"></td> <!-- NVPTX -->`
|
|
:raw-html:`<td class="no"></td> <!-- PowerPC -->`
|
|
:raw-html:`<td class="no"></td> <!-- Sparc -->`
|
|
:raw-html:`<td class="yes"></td> <!-- SystemZ -->`
|
|
:raw-html:`<td class="yes"></td> <!-- X86 -->`
|
|
:raw-html:`<td class="no"></td> <!-- XCore -->`
|
|
:raw-html:`</tr>`
|
|
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td><a hr:raw-html:`ef="#feat_tailcall">tail calls</a></td>`
|
|
:raw-html:`<td class="yes"></td> <!-- ARM -->`
|
|
:raw-html:`<td class="yes"></td> <!-- Hexagon -->`
|
|
:raw-html:`<td class="unknown"></td> <!-- MSP430 -->`
|
|
:raw-html:`<td class="no"></td> <!-- Mips -->`
|
|
:raw-html:`<td class="no"></td> <!-- NVPTX -->`
|
|
:raw-html:`<td class="yes"></td> <!-- PowerPC -->`
|
|
:raw-html:`<td class="unknown"></td> <!-- Sparc -->`
|
|
:raw-html:`<td class="no"></td> <!-- SystemZ -->`
|
|
:raw-html:`<td class="yes"></td> <!-- X86 -->`
|
|
:raw-html:`<td class="no"></td> <!-- XCore -->`
|
|
:raw-html:`</tr>`
|
|
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td><a href="#feat_segstacks">segmented stacks</a></td>`
|
|
:raw-html:`<td class="no"></td> <!-- ARM -->`
|
|
:raw-html:`<td class="no"></td> <!-- Hexagon -->`
|
|
:raw-html:`<td class="no"></td> <!-- MSP430 -->`
|
|
:raw-html:`<td class="no"></td> <!-- Mips -->`
|
|
:raw-html:`<td class="no"></td> <!-- NVPTX -->`
|
|
:raw-html:`<td class="no"></td> <!-- PowerPC -->`
|
|
:raw-html:`<td class="no"></td> <!-- Sparc -->`
|
|
:raw-html:`<td class="no"></td> <!-- SystemZ -->`
|
|
:raw-html:`<td class="partial"><a href="#feat_segstacks_x86">*</a></td> <!-- X86 -->`
|
|
:raw-html:`<td class="no"></td> <!-- XCore -->`
|
|
:raw-html:`</tr>`
|
|
|
|
:raw-html:`</table>`
|
|
|
|
.. _feat_reliable:
|
|
|
|
Is Generally Reliable
|
|
^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
This box indicates whether the target is considered to be production quality.
|
|
This indicates that the target has been used as a static compiler to compile
|
|
large amounts of code by a variety of different people and is in continuous use.
|
|
|
|
.. _feat_asmparser:
|
|
|
|
Assembly Parser
|
|
^^^^^^^^^^^^^^^
|
|
|
|
This box indicates whether the target supports parsing target specific .s files
|
|
by implementing the MCAsmParser interface. This is required for llvm-mc to be
|
|
able to act as a native assembler and is required for inline assembly support in
|
|
the native .o file writer.
|
|
|
|
.. _feat_disassembler:
|
|
|
|
Disassembler
|
|
^^^^^^^^^^^^
|
|
|
|
This box indicates whether the target supports the MCDisassembler API for
|
|
disassembling machine opcode bytes into MCInst's.
|
|
|
|
.. _feat_inlineasm:
|
|
|
|
Inline Asm
|
|
^^^^^^^^^^
|
|
|
|
This box indicates whether the target supports most popular inline assembly
|
|
constraints and modifiers.
|
|
|
|
.. _feat_jit:
|
|
|
|
JIT Support
|
|
^^^^^^^^^^^
|
|
|
|
This box indicates whether the target supports the JIT compiler through the
|
|
ExecutionEngine interface.
|
|
|
|
.. _feat_jit_arm:
|
|
|
|
The ARM backend has basic support for integer code in ARM codegen mode, but
|
|
lacks NEON and full Thumb support.
|
|
|
|
.. _feat_objectwrite:
|
|
|
|
.o File Writing
|
|
^^^^^^^^^^^^^^^
|
|
|
|
This box indicates whether the target supports writing .o files (e.g. MachO,
|
|
ELF, and/or COFF) files directly from the target. Note that the target also
|
|
must include an assembly parser and general inline assembly support for full
|
|
inline assembly support in the .o writer.
|
|
|
|
Targets that don't support this feature can obviously still write out .o files,
|
|
they just rely on having an external assembler to translate from a .s file to a
|
|
.o file (as is the case for many C compilers).
|
|
|
|
.. _feat_tailcall:
|
|
|
|
Tail Calls
|
|
^^^^^^^^^^
|
|
|
|
This box indicates whether the target supports guaranteed tail calls. These are
|
|
calls marked "`tail <LangRef.html#i_call>`_" and use the fastcc calling
|
|
convention. Please see the `tail call section`_ for more details.
|
|
|
|
.. _feat_segstacks:
|
|
|
|
Segmented Stacks
|
|
^^^^^^^^^^^^^^^^
|
|
|
|
This box indicates whether the target supports segmented stacks. This replaces
|
|
the traditional large C stack with many linked segments. It is compatible with
|
|
the `gcc implementation <http://gcc.gnu.org/wiki/SplitStacks>`_ used by the Go
|
|
front end.
|
|
|
|
.. _feat_segstacks_x86:
|
|
|
|
Basic support exists on the X86 backend. Currently vararg doesn't work and the
|
|
object files are not marked the way the gold linker expects, but simple Go
|
|
programs can be built by dragonegg.
|
|
|
|
.. _tail call section:
|
|
|
|
Tail call optimization
|
|
----------------------
|
|
|
|
Tail call optimization, callee reusing the stack of the caller, is currently
|
|
supported on x86/x86-64 and PowerPC. It is performed if:
|
|
|
|
* Caller and callee have the calling convention ``fastcc``, ``cc 10`` (GHC
|
|
calling convention) or ``cc 11`` (HiPE calling convention).
|
|
|
|
* The call is a tail call - in tail position (ret immediately follows call and
|
|
ret uses value of call or is void).
|
|
|
|
* Option ``-tailcallopt`` is enabled.
|
|
|
|
* Platform-specific constraints are met.
|
|
|
|
x86/x86-64 constraints:
|
|
|
|
* No variable argument lists are used.
|
|
|
|
* On x86-64 when generating GOT/PIC code only module-local calls (visibility =
|
|
hidden or protected) are supported.
|
|
|
|
PowerPC constraints:
|
|
|
|
* No variable argument lists are used.
|
|
|
|
* No byval parameters are used.
|
|
|
|
* On ppc32/64 GOT/PIC only module-local calls (visibility = hidden or protected)
|
|
are supported.
|
|
|
|
Example:
|
|
|
|
Call as ``llc -tailcallopt test.ll``.
|
|
|
|
.. code-block:: llvm
|
|
|
|
declare fastcc i32 @tailcallee(i32 inreg %a1, i32 inreg %a2, i32 %a3, i32 %a4)
|
|
|
|
define fastcc i32 @tailcaller(i32 %in1, i32 %in2) {
|
|
%l1 = add i32 %in1, %in2
|
|
%tmp = tail call fastcc i32 @tailcallee(i32 %in1 inreg, i32 %in2 inreg, i32 %in1, i32 %l1)
|
|
ret i32 %tmp
|
|
}
|
|
|
|
Implications of ``-tailcallopt``:
|
|
|
|
To support tail call optimization in situations where the callee has more
|
|
arguments than the caller a 'callee pops arguments' convention is used. This
|
|
currently causes each ``fastcc`` call that is not tail call optimized (because
|
|
one or more of above constraints are not met) to be followed by a readjustment
|
|
of the stack. So performance might be worse in such cases.
|
|
|
|
Sibling call optimization
|
|
-------------------------
|
|
|
|
Sibling call optimization is a restricted form of tail call optimization.
|
|
Unlike tail call optimization described in the previous section, it can be
|
|
performed automatically on any tail calls when ``-tailcallopt`` option is not
|
|
specified.
|
|
|
|
Sibling call optimization is currently performed on x86/x86-64 when the
|
|
following constraints are met:
|
|
|
|
* Caller and callee have the same calling convention. It can be either ``c`` or
|
|
``fastcc``.
|
|
|
|
* The call is a tail call - in tail position (ret immediately follows call and
|
|
ret uses value of call or is void).
|
|
|
|
* Caller and callee have matching return type or the callee result is not used.
|
|
|
|
* If any of the callee arguments are being passed in stack, they must be
|
|
available in caller's own incoming argument stack and the frame offsets must
|
|
be the same.
|
|
|
|
Example:
|
|
|
|
.. code-block:: llvm
|
|
|
|
declare i32 @bar(i32, i32)
|
|
|
|
define i32 @foo(i32 %a, i32 %b, i32 %c) {
|
|
entry:
|
|
%0 = tail call i32 @bar(i32 %a, i32 %b)
|
|
ret i32 %0
|
|
}
|
|
|
|
The X86 backend
|
|
---------------
|
|
|
|
The X86 code generator lives in the ``lib/Target/X86`` directory. This code
|
|
generator is capable of targeting a variety of x86-32 and x86-64 processors, and
|
|
includes support for ISA extensions such as MMX and SSE.
|
|
|
|
X86 Target Triples supported
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
The following are the known target triples that are supported by the X86
|
|
backend. This is not an exhaustive list, and it would be useful to add those
|
|
that people test.
|
|
|
|
* **i686-pc-linux-gnu** --- Linux
|
|
|
|
* **i386-unknown-freebsd5.3** --- FreeBSD 5.3
|
|
|
|
* **i686-pc-cygwin** --- Cygwin on Win32
|
|
|
|
* **i686-pc-mingw32** --- MingW on Win32
|
|
|
|
* **i386-pc-mingw32msvc** --- MingW crosscompiler on Linux
|
|
|
|
* **i686-apple-darwin*** --- Apple Darwin on X86
|
|
|
|
* **x86_64-unknown-linux-gnu** --- Linux
|
|
|
|
X86 Calling Conventions supported
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
The following target-specific calling conventions are known to backend:
|
|
|
|
* **x86_StdCall** --- stdcall calling convention seen on Microsoft Windows
|
|
platform (CC ID = 64).
|
|
|
|
* **x86_FastCall** --- fastcall calling convention seen on Microsoft Windows
|
|
platform (CC ID = 65).
|
|
|
|
* **x86_ThisCall** --- Similar to X86_StdCall. Passes first argument in ECX,
|
|
others via stack. Callee is responsible for stack cleaning. This convention is
|
|
used by MSVC by default for methods in its ABI (CC ID = 70).
|
|
|
|
.. _X86 addressing mode:
|
|
|
|
Representing X86 addressing modes in MachineInstrs
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
The x86 has a very flexible way of accessing memory. It is capable of forming
|
|
memory addresses of the following expression directly in integer instructions
|
|
(which use ModR/M addressing):
|
|
|
|
::
|
|
|
|
SegmentReg: Base + [1,2,4,8] * IndexReg + Disp32
|
|
|
|
In order to represent this, LLVM tracks no less than 5 operands for each memory
|
|
operand of this form. This means that the "load" form of '``mov``' has the
|
|
following ``MachineOperand``\s in this order:
|
|
|
|
::
|
|
|
|
Index: 0 | 1 2 3 4 5
|
|
Meaning: DestReg, | BaseReg, Scale, IndexReg, Displacement Segment
|
|
OperandTy: VirtReg, | VirtReg, UnsImm, VirtReg, SignExtImm PhysReg
|
|
|
|
Stores, and all other instructions, treat the four memory operands in the same
|
|
way and in the same order. If the segment register is unspecified (regno = 0),
|
|
then no segment override is generated. "Lea" operations do not have a segment
|
|
register specified, so they only have 4 operands for their memory reference.
|
|
|
|
X86 address spaces supported
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
x86 has a feature which provides the ability to perform loads and stores to
|
|
different address spaces via the x86 segment registers. A segment override
|
|
prefix byte on an instruction causes the instruction's memory access to go to
|
|
the specified segment. LLVM address space 0 is the default address space, which
|
|
includes the stack, and any unqualified memory accesses in a program. Address
|
|
spaces 1-255 are currently reserved for user-defined code. The GS-segment is
|
|
represented by address space 256, while the FS-segment is represented by address
|
|
space 257. Other x86 segments have yet to be allocated address space
|
|
numbers.
|
|
|
|
While these address spaces may seem similar to TLS via the ``thread_local``
|
|
keyword, and often use the same underlying hardware, there are some fundamental
|
|
differences.
|
|
|
|
The ``thread_local`` keyword applies to global variables and specifies that they
|
|
are to be allocated in thread-local memory. There are no type qualifiers
|
|
involved, and these variables can be pointed to with normal pointers and
|
|
accessed with normal loads and stores. The ``thread_local`` keyword is
|
|
target-independent at the LLVM IR level (though LLVM doesn't yet have
|
|
implementations of it for some configurations)
|
|
|
|
Special address spaces, in contrast, apply to static types. Every load and store
|
|
has a particular address space in its address operand type, and this is what
|
|
determines which address space is accessed. LLVM ignores these special address
|
|
space qualifiers on global variables, and does not provide a way to directly
|
|
allocate storage in them. At the LLVM IR level, the behavior of these special
|
|
address spaces depends in part on the underlying OS or runtime environment, and
|
|
they are specific to x86 (and LLVM doesn't yet handle them correctly in some
|
|
cases).
|
|
|
|
Some operating systems and runtime environments use (or may in the future use)
|
|
the FS/GS-segment registers for various low-level purposes, so care should be
|
|
taken when considering them.
|
|
|
|
Instruction naming
|
|
^^^^^^^^^^^^^^^^^^
|
|
|
|
An instruction name consists of the base name, a default operand size, and a a
|
|
character per operand with an optional special size. For example:
|
|
|
|
::
|
|
|
|
ADD8rr -> add, 8-bit register, 8-bit register
|
|
IMUL16rmi -> imul, 16-bit register, 16-bit memory, 16-bit immediate
|
|
IMUL16rmi8 -> imul, 16-bit register, 16-bit memory, 8-bit immediate
|
|
MOVSX32rm16 -> movsx, 32-bit register, 16-bit memory
|
|
|
|
The PowerPC backend
|
|
-------------------
|
|
|
|
The PowerPC code generator lives in the lib/Target/PowerPC directory. The code
|
|
generation is retargetable to several variations or *subtargets* of the PowerPC
|
|
ISA; including ppc32, ppc64 and altivec.
|
|
|
|
LLVM PowerPC ABI
|
|
^^^^^^^^^^^^^^^^
|
|
|
|
LLVM follows the AIX PowerPC ABI, with two deviations. LLVM uses a PC relative
|
|
(PIC) or static addressing for accessing global values, so no TOC (r2) is
|
|
used. Second, r31 is used as a frame pointer to allow dynamic growth of a stack
|
|
frame. LLVM takes advantage of having no TOC to provide space to save the frame
|
|
pointer in the PowerPC linkage area of the caller frame. Other details of
|
|
PowerPC ABI can be found at `PowerPC ABI
|
|
<http://developer.apple.com/documentation/DeveloperTools/Conceptual/LowLevelABI/Articles/32bitPowerPC.html>`_\
|
|
. Note: This link describes the 32 bit ABI. The 64 bit ABI is similar except
|
|
space for GPRs are 8 bytes wide (not 4) and r13 is reserved for system use.
|
|
|
|
Frame Layout
|
|
^^^^^^^^^^^^
|
|
|
|
The size of a PowerPC frame is usually fixed for the duration of a function's
|
|
invocation. Since the frame is fixed size, all references into the frame can be
|
|
accessed via fixed offsets from the stack pointer. The exception to this is
|
|
when dynamic alloca or variable sized arrays are present, then a base pointer
|
|
(r31) is used as a proxy for the stack pointer and stack pointer is free to grow
|
|
or shrink. A base pointer is also used if llvm-gcc is not passed the
|
|
-fomit-frame-pointer flag. The stack pointer is always aligned to 16 bytes, so
|
|
that space allocated for altivec vectors will be properly aligned.
|
|
|
|
An invocation frame is laid out as follows (low memory at top):
|
|
|
|
:raw-html:`<table border="1" cellspacing="0">`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td>Linkage<br><br></td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td>Parameter area<br><br></td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td>Dynamic area<br><br></td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td>Locals area<br><br></td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td>Saved registers area<br><br></td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`<tr style="border-style: none hidden none hidden;">`
|
|
:raw-html:`<td><br></td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td>Previous Frame<br><br></td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`</table>`
|
|
|
|
The *linkage* area is used by a callee to save special registers prior to
|
|
allocating its own frame. Only three entries are relevant to LLVM. The first
|
|
entry is the previous stack pointer (sp), aka link. This allows probing tools
|
|
like gdb or exception handlers to quickly scan the frames in the stack. A
|
|
function epilog can also use the link to pop the frame from the stack. The
|
|
third entry in the linkage area is used to save the return address from the lr
|
|
register. Finally, as mentioned above, the last entry is used to save the
|
|
previous frame pointer (r31.) The entries in the linkage area are the size of a
|
|
GPR, thus the linkage area is 24 bytes long in 32 bit mode and 48 bytes in 64
|
|
bit mode.
|
|
|
|
32 bit linkage area:
|
|
|
|
:raw-html:`<table border="1" cellspacing="0">`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td>0</td>`
|
|
:raw-html:`<td>Saved SP (r1)</td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td>4</td>`
|
|
:raw-html:`<td>Saved CR</td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td>8</td>`
|
|
:raw-html:`<td>Saved LR</td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td>12</td>`
|
|
:raw-html:`<td>Reserved</td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td>16</td>`
|
|
:raw-html:`<td>Reserved</td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td>20</td>`
|
|
:raw-html:`<td>Saved FP (r31)</td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`</table>`
|
|
|
|
64 bit linkage area:
|
|
|
|
:raw-html:`<table border="1" cellspacing="0">`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td>0</td>`
|
|
:raw-html:`<td>Saved SP (r1)</td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td>8</td>`
|
|
:raw-html:`<td>Saved CR</td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td>16</td>`
|
|
:raw-html:`<td>Saved LR</td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td>24</td>`
|
|
:raw-html:`<td>Reserved</td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td>32</td>`
|
|
:raw-html:`<td>Reserved</td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td>40</td>`
|
|
:raw-html:`<td>Saved FP (r31)</td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`</table>`
|
|
|
|
The *parameter area* is used to store arguments being passed to a callee
|
|
function. Following the PowerPC ABI, the first few arguments are actually
|
|
passed in registers, with the space in the parameter area unused. However, if
|
|
there are not enough registers or the callee is a thunk or vararg function,
|
|
these register arguments can be spilled into the parameter area. Thus, the
|
|
parameter area must be large enough to store all the parameters for the largest
|
|
call sequence made by the caller. The size must also be minimally large enough
|
|
to spill registers r3-r10. This allows callees blind to the call signature,
|
|
such as thunks and vararg functions, enough space to cache the argument
|
|
registers. Therefore, the parameter area is minimally 32 bytes (64 bytes in 64
|
|
bit mode.) Also note that since the parameter area is a fixed offset from the
|
|
top of the frame, that a callee can access its spilt arguments using fixed
|
|
offsets from the stack pointer (or base pointer.)
|
|
|
|
Combining the information about the linkage, parameter areas and alignment. A
|
|
stack frame is minimally 64 bytes in 32 bit mode and 128 bytes in 64 bit mode.
|
|
|
|
The *dynamic area* starts out as size zero. If a function uses dynamic alloca
|
|
then space is added to the stack, the linkage and parameter areas are shifted to
|
|
top of stack, and the new space is available immediately below the linkage and
|
|
parameter areas. The cost of shifting the linkage and parameter areas is minor
|
|
since only the link value needs to be copied. The link value can be easily
|
|
fetched by adding the original frame size to the base pointer. Note that
|
|
allocations in the dynamic space need to observe 16 byte alignment.
|
|
|
|
The *locals area* is where the llvm compiler reserves space for local variables.
|
|
|
|
The *saved registers area* is where the llvm compiler spills callee saved
|
|
registers on entry to the callee.
|
|
|
|
Prolog/Epilog
|
|
^^^^^^^^^^^^^
|
|
|
|
The llvm prolog and epilog are the same as described in the PowerPC ABI, with
|
|
the following exceptions. Callee saved registers are spilled after the frame is
|
|
created. This allows the llvm epilog/prolog support to be common with other
|
|
targets. The base pointer callee saved register r31 is saved in the TOC slot of
|
|
linkage area. This simplifies allocation of space for the base pointer and
|
|
makes it convenient to locate programatically and during debugging.
|
|
|
|
Dynamic Allocation
|
|
^^^^^^^^^^^^^^^^^^
|
|
|
|
.. note::
|
|
|
|
TODO - More to come.
|
|
|
|
The NVPTX backend
|
|
-----------------
|
|
|
|
The NVPTX code generator under lib/Target/NVPTX is an open-source version of
|
|
the NVIDIA NVPTX code generator for LLVM. It is contributed by NVIDIA and is
|
|
a port of the code generator used in the CUDA compiler (nvcc). It targets the
|
|
PTX 3.0/3.1 ISA and can target any compute capability greater than or equal to
|
|
2.0 (Fermi).
|
|
|
|
This target is of production quality and should be completely compatible with
|
|
the official NVIDIA toolchain.
|
|
|
|
Code Generator Options:
|
|
|
|
:raw-html:`<table border="1" cellspacing="0">`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<th>Option</th>`
|
|
:raw-html:`<th>Description</th>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td>sm_20</td>`
|
|
:raw-html:`<td align="left">Set shader model/compute capability to 2.0</td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td>sm_21</td>`
|
|
:raw-html:`<td align="left">Set shader model/compute capability to 2.1</td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td>sm_30</td>`
|
|
:raw-html:`<td align="left">Set shader model/compute capability to 3.0</td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td>sm_35</td>`
|
|
:raw-html:`<td align="left">Set shader model/compute capability to 3.5</td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td>ptx30</td>`
|
|
:raw-html:`<td align="left">Target PTX 3.0</td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`<tr>`
|
|
:raw-html:`<td>ptx31</td>`
|
|
:raw-html:`<td align="left">Target PTX 3.1</td>`
|
|
:raw-html:`</tr>`
|
|
:raw-html:`</table>`
|
|
|