mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2024-12-25 21:16:19 +00:00
178 lines
7.4 KiB
Plaintext
178 lines
7.4 KiB
Plaintext
//===- README.txt - Information about the X86 backend and related files ---===//
|
|
//
|
|
// This file contains random notes and points of interest about the X86 backend.
|
|
//
|
|
//===----------------------------------------------------------------------===//
|
|
|
|
===========
|
|
I. Overview
|
|
===========
|
|
|
|
This directory contains a machine description for the X86 processor family.
|
|
Currently this machine description is used for a high performance code generator
|
|
used by the LLVM JIT and static code generators. One of the main objectives
|
|
that we would like to support with this project is to build a nice clean code
|
|
generator that may be extended in the future in a variety of ways: new targets,
|
|
new optimizations, new transformations, etc.
|
|
|
|
This document describes the current state of the X86 code generator, along with
|
|
implementation notes, design decisions, and other stuff.
|
|
|
|
|
|
===================================
|
|
II. Architecture / Design Decisions
|
|
===================================
|
|
|
|
We designed the infrastructure into the generic LLVM machine specific
|
|
representation, which allows us to support as many targets as possible with our
|
|
framework. This framework should allow us to share many common machine specific
|
|
transformations (register allocation, instruction scheduling, etc...) among all
|
|
of the backends that may eventually be supported by LLVM, and ensures that the
|
|
JIT and static compiler backends are largely shared.
|
|
|
|
At the high-level, LLVM code is translated to a machine specific representation
|
|
formed out of MachineFunction, MachineBasicBlock, and MachineInstr instances
|
|
(defined in include/llvm/CodeGen). This representation is completely target
|
|
agnostic, representing instructions in their most abstract form: an opcode and a
|
|
series of operands. This representation is designed to support both SSA
|
|
representation for machine code, as well as a register allocated, non-SSA form.
|
|
|
|
Because the Machine* representation must work regardless of the target machine,
|
|
it contains very little semantic information about the program. To get semantic
|
|
information about the program, a layer of Target description datastructures are
|
|
used, defined in include/llvm/Target.
|
|
|
|
Note that there is some amount of complexity that the X86 backend contains due
|
|
to the Sparc backend's legacy requirements. These should eventually fade away
|
|
as the project progresses.
|
|
|
|
|
|
SSA Instruction Representation
|
|
------------------------------
|
|
Target machine instructions are represented as instances of MachineInstr, and
|
|
all specific machine instruction types should have an entry in the
|
|
X86InstrInfo.td file. In the X86 backend, there are two particularly
|
|
interesting forms of machine instruction: those that produce a value (such as
|
|
add), and those that do not (such as a store).
|
|
|
|
Instructions that produce a value use Operand #0 as the "destination" register.
|
|
When printing the assembly code with the built-in machine instruction printer,
|
|
these destination registers will be printed to the left side of an '=' sign, as
|
|
in: %reg1027 = add %reg1026, %reg1025
|
|
|
|
This `add' MachineInstruction contains three "operands": the first is the
|
|
destination register (#1027), the second is the first source register (#1026)
|
|
and the third is the second source register (#1025). Never forget the
|
|
destination register will show up in the MachineInstr operands vector. The code
|
|
to generate this instruction looks like this:
|
|
|
|
BuildMI(BB, X86::ADDrr32, 2, 1027).addReg(1026).addReg(1025);
|
|
|
|
The first argument to BuildMI is the basic block to append the machine
|
|
instruction to, the second is the opcode, the third is the number of operands,
|
|
the fourth is the destination register. The two addReg calls specify operands
|
|
in order.
|
|
|
|
MachineInstrs that do not produce a value do not have this implicit first
|
|
operand, they simply have #operands = #uses. To create them, simply do not
|
|
specify a destination register to the BuildMI call.
|
|
|
|
|
|
======================
|
|
IV. Source Code Layout
|
|
======================
|
|
|
|
The LLVM code generator is composed of source files primarily in the following
|
|
locations:
|
|
|
|
include/llvm/CodeGen
|
|
--------------------
|
|
This directory contains header files that are used to represent the program in a
|
|
machine specific representation. It currently also contains a bunch of stuff
|
|
used by the Sparc backend that we don't want to get mixed up in, such as
|
|
register allocation internals.
|
|
|
|
include/llvm/Target
|
|
-------------------
|
|
This directory contains header files that are used to interpret the machine
|
|
specific representation of the program. This allows us to write generic
|
|
transformations that will work on any target that implements the interfaces
|
|
defined in this directory. The only classes used by the X86 backend so far are
|
|
the TargetMachine, TargetData, MachineInstrInfo, and MRegisterInfo classes.
|
|
|
|
lib/CodeGen
|
|
-----------
|
|
This directory will contain all of the target independent transformations (for
|
|
example, register allocation) that we write. These transformations should only
|
|
use information exposed through the Target interface, they should not include
|
|
any target specific header files.
|
|
|
|
lib/Target/X86
|
|
--------------
|
|
This directory contains the machine description for X86 that is required to the
|
|
rest of the compiler working. It contains any code that is truly specific to
|
|
the X86 backend, for example the instruction selector and machine code emitter.
|
|
|
|
lib/ExecutionEngine/JIT
|
|
-----------------------
|
|
This directory contains the top-level code for the JIT compiler. This code
|
|
basically boils down to a call to TargetMachine::addPassesToJITCompile, and
|
|
handles the compile-dispatch-recompile cycle.
|
|
|
|
test/Regression/CodeGen/X86
|
|
---------------------------
|
|
This directory contains regression tests for the X86 code generator.
|
|
|
|
|
|
==================================================
|
|
V. Strange Things, or, Things That Should Be Known
|
|
==================================================
|
|
|
|
Representing memory in MachineInstrs
|
|
------------------------------------
|
|
|
|
The x86 has a very, uhm, flexible, way of accessing memory. It is capable of
|
|
addressing memory addresses of the following form directly in integer
|
|
instructions (which use ModR/M addressing):
|
|
|
|
Base+[1,2,4,8]*IndexReg+Disp32
|
|
|
|
Wow, that's crazy. In order to represent this, LLVM tracks no less that 4
|
|
operands for each memory operand of this form. This means that the "load" form
|
|
of 'mov' has the following "Operands" in this order:
|
|
|
|
Index: 0 | 1 2 3 4
|
|
Meaning: DestReg, | BaseReg, Scale, IndexReg, Displacement
|
|
OperandTy: VirtReg, | VirtReg, UnsImm, VirtReg, SignExtImm
|
|
|
|
Stores and all other instructions treat the four memory operands in the same
|
|
way, in the same order.
|
|
|
|
|
|
==========================
|
|
VI. TODO / Future Projects
|
|
==========================
|
|
|
|
Ideas for Improvements:
|
|
-----------------------
|
|
1. Implement an *optimal* linear time instruction selector
|
|
2. Implement lots of nifty runtime optimizations
|
|
3. Implement new targets: IA64? X86-64? M68k? MMIX? Who knows...
|
|
|
|
Infrastructure Improvements:
|
|
----------------------------
|
|
|
|
1. X86/Printer.cpp and Sparc/EmitAssembly.cpp both have copies of what is
|
|
roughly the same code, used to output constants in a form the assembler
|
|
can understand. These functions should be shared at some point. They
|
|
should be rewritten to pass around iostreams instead of strings. The
|
|
list of functions is as follows:
|
|
|
|
isStringCompatible
|
|
toOctal
|
|
ConstantExprToString
|
|
valToExprString
|
|
getAsCString
|
|
printSingleConstantValue (with TypeToDataDirective inlined)
|
|
printConstantValueOnly
|