This document contains the release notes for the LLVM Compiler Infrastructure, release 2.8. Here we describe the status of LLVM, including major improvements from the previous release and significant known problems. All LLVM releases may be downloaded from the LLVM releases web site.

For more information about LLVM, including information about the latest release, please check out the main LLVM web site. If you have questions or comments, the LLVM Developer's Mailing List is a good place to send them.

Note that if you are reading this file from a Subversion checkout or the main LLVM web page, this document applies to the next release, not the current one. To see the release notes for a specific release, please see the releases page.

The LLVM 2.8 distribution currently consists of code from the core LLVM repository (which roughly includes the LLVM optimizers, code generators and supporting tools), the Clang repository and the llvm-gcc repository. In addition to this code, the LLVM Project includes other sub-projects that are in development. Here we include updates on these subprojects.

Clang is an LLVM front end for the C, C++, and Objective-C languages. Clang aims to provide a better user experience through expressive diagnostics, a high level of conformance to language standards, fast compilation, and low memory use. Like LLVM, Clang provides a modular, library-based architecture that makes it suitable for creating or integrating with other development tools. Clang is considered a production-quality compiler for C, Objective-C, C++ and Objective-C++ on x86 (32- and 64-bit), and for darwin-arm targets.

In the LLVM 2.8 time-frame, the Clang team has made many improvements:

• Surely these guys have done something
• X86-64 abi improvements? Did they make it in?

The Clang Static Analyzer project is an effort to use static source code analysis techniques to automatically find bugs in C and Objective-C programs (and hopefully C++ in the future!). The tool is very good at finding bugs that occur on specific paths through code, such as on error conditions.

The LLVM 2.8 release fixes a number of bugs and slightly improves precision over 2.7, but there are no major new features in the release.

DragonEgg is a port of llvm-gcc to gcc-4.5. Unlike llvm-gcc, dragonegg in theory does not require any gcc-4.5 modifications whatsoever (currently one small patch is needed) thanks to the new gcc plugin architecture. DragonEgg is a gcc plugin that makes gcc-4.5 use the LLVM optimizers and code generators instead of gcc's, just like with llvm-gcc.

DragonEgg is still a work in progress, but it is able to compile a lot of code, for example all of gcc, LLVM and clang. Currently Ada, C, C++ and Fortran work well, while all other languages either don't work at all or only work poorly. For the moment only the x86-32 and x86-64 targets are supported, and only on linux and darwin (darwin may need additional gcc patches).

The 2.8 release has the following notable changes:

• The plugin loads faster due to exporting fewer symbols.
• Additional vector operations such as addps256 are now supported.
• Ada global variables with no initial value are no longer zero initialized, resulting in better optimization.
• The '-fplugin-arg-dragonegg-enable-gcc-optzns' flag now runs all gcc optimizers, rather than just a handful.
• Fortran programs using common variables now link correctly.
• GNU OMP constructs no longer crash the compiler.

The VMKit project is an implementation of a Java Virtual Machine (Java VM or JVM) that uses LLVM for static and just-in-time compilation. As of LLVM 2.8, VMKit now supports copying garbage collectors, and can be configured to use MMTk's copy mark-sweep garbage collector. In LLVM 2.8, the VMKit .NET VM is no longer being maintained.

The new LLVM compiler-rt project is a simple library that provides an implementation of the low-level target-specific hooks required by code generation and other runtime components. For example, when compiling for a 32-bit target, converting a double to a 64-bit unsigned integer is compiled into a runtime call to the "__fixunsdfdi" function. The compiler-rt library provides highly optimized implementations of this and other low-level routines (some are 3x faster than the equivalent libgcc routines).

All of the code in the compiler-rt project is available under the standard LLVM License, a "BSD-style" license. New in LLVM 2.8, compiler_rt now supports soft floating point (for targets that don't have a real floating point unit), and includes an extensive testsuite for the "blocks" language feature and the blocks runtime included in compiler_rt.

LLDB is a brand new member of the LLVM umbrella of projects. LLDB is a next generation, high-performance debugger. It is built as a set of reusable components which highly leverage existing libraries in the larger LLVM Project, such as the Clang expression parser, the LLVM disassembler and the LLVM JIT.

LLDB is in early development and not included as part of the LLVM 2.8 release, but is mature enough to support basic debugging scenarios on Mac OS X in C, Objective-C and C++. We'd really like help extending and expanding LLDB to support new platforms, new languages, new architectures, and new features.

libc++ is another new member of the LLVM family. It is an implementation of the C++ standard library, written from the ground up to specifically target the forthcoming C++'0X standard and focus on delivering great performance.

As of the LLVM 2.8 release, libc++ is virtually feature complete, but would benefit from more testing and better integration with Clang++. It is also looking forward to the C++ committee finalizing the C++'0x standard.

An exciting aspect of LLVM is that it is used as an enabling technology for a lot of other language and tools projects. This section lists some of the projects that have already been updated to work with LLVM 2.8.

TCE is a toolset for designing application-specific processors (ASP) based on the Transport triggered architecture (TTA). The toolset provides a complete co-design flow from C/C++ programs down to synthesizable VHDL and parallel program binaries. Processor customization points include the register files, function units, supported operations, and the interconnection network.

TCE uses llvm-gcc/Clang and LLVM for C/C++ language support, target independent optimizations and also for parts of code generation. It generates new LLVM-based code generators "on the fly" for the designed TTA processors and loads them in to the compiler backend as runtime libraries to avoid per-target recompilation of larger parts of the compiler chain.

Horizon is a bytecode language and compiler written on top of LLVM, intended for producing single-address-space managed code operating systems that run faster than the equivalent multiple-address-space C systems. More in-depth blurb is available on the wiki.

Clam AntiVirus is an open source (GPL) anti-virus toolkit for UNIX, designed especially for e-mail scanning on mail gateways. Since version 0.96 it has bytecode signatures that allow writing detections for complex malware. It uses LLVM's JIT to speed up the execution of bytecode on X86,X86-64,PPC32/64, falling back to its own interpreter otherwise. The git version was updated to work with LLVM 2.8

The ClamAV bytecode compiler uses Clang and LLVM to compile a C-like language, insert runtime checks, and generate ClamAV bytecode.

Pure is an algebraic/functional programming language based on term rewriting. Programs are collections of equations which are used to evaluate expressions in a symbolic fashion. Pure offers dynamic typing, eager and lazy evaluation, lexical closures, a hygienic macro system (also based on term rewriting), built-in list and matrix support (including list and matrix comprehensions) and an easy-to-use C interface. The interpreter uses LLVM as a backend to JIT-compile Pure programs to fast native code.

Pure versions 0.44 and later have been tested and are known to work with LLVM 2.8 (and continue to work with older LLVM releases >= 2.5).

GHC is an open source, state-of-the-art programming suite for Haskell, a standard lazy functional programming language. It includes an optimizing static compiler generating good code for a variety of platforms, together with an interactive system for convenient, quick development.

In addition to the existing C and native code generators, GHC 7.0 now supports an LLVM code generator. GHC supports LLVM 2.7 and later.

Clay is a new systems programming language that is specifically designed for generic programming. It makes generic programming very concise thanks to whole program type propagation. It uses LLVM as its backend.

llvm-py has been updated to work with LLVM 2.8. llvm-py provides Python bindings for LLVM, allowing you to write a compiler backend or a VM in Python.

FAUST is a compiled language for real-time audio signal processing. The name FAUST stands for Functional AUdio STream. Its programming model combines two approaches: functional programming and block diagram composition. In addition with the C, C++, JAVA output formats, the Faust compiler can now generate LLVM bitcode, and works with LLVM 2.7 and 2.8.

Jade (Just-in-time Adaptive Decoder Engine) is a generic video decoder engine using LLVM for just-in-time compilation of video decoder configurations. Those configurations are designed by MPEG Reconfigurable Video Coding (RVC) committee. MPEG RVC standard is built on a stream-based dataflow representation of decoders. It is composed of a standard library of coding tools written in RVC-CAL language and a dataflow configuration &emdash; block diagram &emdash; of a decoder.

Jade project is hosted as part of the Open RVC-CAL Compiler and requires it to translate the RVC-CAL standard library of video coding tools into an LLVM assembly code.

Neko LLVM JIT replaces the standard Neko JIT with an LLVM-based implementation. While not fully complete, it is already providing a 1.5x speedup on 64-bit systems. Neko LLVM JIT requires LLVM 2.8 or later.

Crack aims to provide the ease of development of a scripting language with the performance of a compiled language. The language derives concepts from C++, Java and Python, incorporating object-oriented programming, operator overloading and strong typing. Crack 0.2 works with LLVM 2.7, and the forthcoming Crack 0.2.1 release builds on LLVM 2.8.

DTMC provides support for Transactional Memory, which is an easy-to-use and efficient way to synchronize accesses to shared memory. Transactions can contain normal C/C++ code (e.g., __transaction { list.remove(x); x.refCount--; }) and will be executed virtually atomically and isolated from other transactions.

Kai (Japanese 会 for meeting/gathering) is an experimental interpreter that provides a highly extensible runtime environment and explicit control over the compilation process. Programs are defined using nested symbolic expressions, which are all parsed into first-class values with minimal intrinsic semantics. Kai can generate optimised code at run-time (using LLVM) in order to exploit the nature of the underlying hardware and to integrate with external software libraries. It is a unique exploration into world of dynamic code compilation, and the interaction between high level and low level semantics.

OSL is a shading language designed for use in physically based renderers and in particular production rendering. By using LLVM instead of the interpreter, it was able to meet its performance goals (>= C-code) while retaining the benefits of runtime specialization and a portable high-level language.

This release includes a huge number of bug fixes, performance tweaks and minor improvements. Some of the major improvements and new features are listed in this section.

LLVM 2.8 includes several major new capabilities:

• As mentioned above, libc++ and LLDB are major new additions to the LLVM collective.
• LLVM 2.8 now has pretty decent support for debugging optimized code. You should be able to reliably get debug info for function arguments, assuming that the value is actually available where you have stopped.

LLVM IR has several new features for better support of new targets and that expose new optimization opportunities:

• The memcpy, memmove, and memset intrinsics now take address space qualified pointers and a bit to indicate whether the transfer is "volatile" or not.
• Per-instruction debug info metadata is much faster and uses less memory by using the new DebugLoc class.
• LLVM IR now has a more formalized concept of "trap values", which allow the optimizer to optimize more aggressively in the presence of undefined behavior, while still producing predictable results.
• LLVM IR now supports two new linkage types (linker_private_weak and linker_private_weak_def_auto) which map onto some obscure MachO concepts.

In addition to a large array of minor performance tweaks and bug fixes, this release includes a few major enhancements and additions to the optimizers:

• As mentioned above, the optimizer now has support for updating debug information as it goes. A key aspect of this is the new llvm.dbg.value intrinsic. This intrinsic represents debug info for variables that are promoted to SSA values (typically by mem2reg or the -scalarrepl passes).
• The JumpThreading pass is now much more aggressive about implied value relations, allowing it to thread conditions like "a == 4" when a is known to be 13 in one of the predecessors of a block. It does this in conjunction with the new LazyValueInfo analysis pass.
• The new RegionInfo analysis pass identifies single-entry single-exit regions in the CFG. You can play with it with the "opt -regions analyze" or "opt -view-regions" commands.
• The loop optimizer has significantly improve strength reduction and analysis capabilities. Notably it is able to build on the trap value and signed integer overflow information to optimize <= and >= loops.
• The CallGraphSCCPassManager now has some basic support for iterating within an SCC when a optimizer devirtualizes a function call. This allows inlining through indirect call sites that are devirtualized by store-load forwarding and other optimizations.
• The new -loweratomic pass is available to lower atomic instructions into their non-atomic form. This can be useful to optimize generic code that expects to run in a single-threaded environment.

The LLVM Machine Code (aka MC) subsystem was created to solve a number of problems in the realm of assembly, disassembly, object file format handling, and a number of other related areas that CPU instruction-set level tools work in.

The MC subproject has made great leaps in LLVM 2.8. For example, support for directly writing .o files from LLC (and clang) now works reliably for darwin/x86[-64] (including inline assembly support) and the integrated assembler is turned on by default in Clang for these targets. This provides improved compile times among other things.

• The entire compiler has converted over to using the MCStreamer assembler API instead of writing out a .s file textually.
• The "assembler parser" is far more mature than in 2.7, supporting a full complement of directives, now supports assembler macros, etc.
• The "assembler backend" has been completed, including support for relaxation relocation processing and all the other things that an assembler does.
• The MachO file format support is now fully functional and works.
• The MC disassembler now fully supports ARM and Thumb. ARM assembler support is still in early development though.
• The X86 MC assembler now supports the X86 AES and AVX instruction set.
• Work on ELF and COFF support is well underway, but isn't useful yet in LLVM 2.8. Please contact the llvmdev mailing list if you're interested in this.

For more information, please see the Intro to the LLVM MC Project Blog Post.

We have put a significant amount of work into the code generator infrastructure, which allows us to implement more aggressive algorithms and make it run faster:

MachineCSE tuned and on by default. Rewrote tblgen's type inference for backends to be more consistent and diagnose more target bugs. This also allows limited support for writing patterns for instructions that return multiple results, e.g. a virtual register and a flag result. Stuff that used 'parallel' before should use this. New -regalloc=fast, =local got removed New -regalloc=default option that chooses a register allocator based on the -O optimization level. New SubRegIndex tblgen class for targets -> jakob Bottom up fast isel. Simple Load reuse. No more machinedce. IR ABI: <3 x float> is passed as <4 x float> instead of 3 floats. New COPY instruction. copyRegToReg -> copyPhysReg, isMoveInstr is gone. RenderMachineFunction: -rendermf SplitKit? Evan: Teach bottom up pre-ra scheduler to track register pressure. Work in progress. Evan: Add an ILP scheduler. On x86_64, this is a win for all tests in CFP2000. It also sped up 256.bzip2 by 16%. New OptimizeExts+OptimizeCmps -> PeepholeOptimizer pass New LocalStackSlotAllocation.cpp pass (jimg) Atomics now get legalized when not natively supported (jim g) -ffunction-sections and -fdata-sections are supported on ELF targets. -momit-leaf-frame-pointer now supported.

New features of the X86 target include:

• The X86 backend now supports holding X87 floating point stack values in registers across basic blocks, dramatically improving performance of code that uses long double, and when targetting CPUs that don't support SSE.
New SSEDomainFix pass: On Nehalem and newer CPUs there is a 2 cycle latency penalty on using a register in a different domain than where it was defined. Some instructions have equvivalents for different domains, like por/orps/orpd. The SSEDomainFix pass tries to minimize the number of domain crossings by changing between equvivalent opcodes where possible. X86 backend attempts to promote 16-bit integer operations to 32-bits to avoid 0x66 prefixes, which are slow on some microarchitectures and bloat the code on others. New support for X86 "thiscall" calling convention (x86_thiscallcc in IR) for windows. New llvm.x86.int intrinsic (for int $42 and int3) Verbose assembly decodes X86 shuffle instructions, e.g.: insertps $113, %xmm3, %xmm0 ## xmm0 = zero,xmm0[1,2],xmm3[1] unpcklps %xmm1, %xmm0 ## xmm0 = xmm0[0],xmm1[0],xmm0[1],xmm1[1] pshufd $1, %xmm1, %xmm1 ## xmm1 = xmm1[1,0,0,0] X86 ABI: <2 x float> in IR no longer maps onto MMX, it turns into <4 x float> new GHC calling convention

New features of the ARM target include:

NEON: Better performance for QQQQ (4-consecutive Q register) instructions. New reg sequence abstraction? ARM: Better scheduling (list-hybrid, hybrid?) ARM: Tail call support. ARM: General performance work and tuning. ARM: Half float support through intrinsics LangRef.html#int_fp16
• ARMGlobalMerge:
• The ARM NEON intrinsics have been substantially reworked to reduce redundancy and improve code generation. Some of the major changes are:
  1. All of the NEON load and store intrinsics (llvm.arm.neon.vld* and llvm.arm.neon.vst*) take an extra parameter to specify the alignment in bytes of the memory being accessed.
  2. The llvm.arm.neon.vaba intrinsic (vector absolute difference and accumulate) has been removed. This operation is now represented using the llvm.arm.neon.vabd intrinsic (vector absolute difference) followed by a vector add.
  3. The llvm.arm.neon.vabdl and llvm.arm.neon.vabal intrinsics (lengthening vector absolute difference with and without accumlation) have been removed. They are represented using the llvm.arm.neon.vabd intrinsic (vector absolute difference) followed by a vector zero-extend operation, and for vabal, a vector add.
  4. The llvm.arm.neon.vmovn intrinsic has been removed. Calls of this intrinsic are now replaced by vector truncate operations.
  5. The llvm.arm.neon.vmovls and llvm.arm.neon.vmovlu intrinsics have been removed. They are now represented as vector sign-extend (vmovls) and zero-extend (vmovlu) operations.
  6. The llvm.arm.neon.vaddl*, llvm.arm.neon.vaddw*, llvm.arm.neon.vsubl*, and llvm.arm.neon.vsubw* intrinsics (lengthening vector add and subtract) have been removed. They are replaced by vector add and vector subtract operations where one (vaddw, vsubw) or both (vaddl, vsubl) of the operands are either sign-extended or zero-extended.
  7. The llvm.arm.neon.vmulls, llvm.arm.neon.vmullu, llvm.arm.neon.vmlal*, and llvm.arm.neon.vmlsl* intrinsics (lengthening vector multiply with and without accumulation and subtraction) have been removed. These operations are now represented as vector multiplications where the operands are either sign-extended or zero-extended, followed by a vector add for vmlal or a vector subtract for vmlsl. Note that the polynomial vector multiply intrinsic, llvm.arm.neon.vmullp, remains unchanged.

This release includes a number of new APIs that are used internally, which may also be useful for external clients.

Other miscellaneous features include:

If you're already an LLVM user or developer with out-of-tree changes based on LLVM 2.7, this section lists some "gotchas" that you may run into upgrading from the previous release.

• The build configuration machinery changed the output directory names. It wasn't clear to many people that "Release-Asserts" build was a release build without asserts. To make this more clear, "Release" does not include assertions and "Release+Asserts" does (likewise, "Debug" and "Debug+Asserts").
• The MSIL Backend was removed, it was unsupported and broken.
• The ABCD, SSI, and SCCVN passes were removed. These were not fully functional and their behavior has been or will be subsumed by the LazyValueInfo pass.
• The LLVM IR 'Union' feature was removed. While this is a desirable feature for LLVM IR to support, the existing implementation was half baked and barely useful. We'd really like anyone interested to resurrect the work and finish it for a future release.
• If you're used to reading .ll files, you'll probably notice that .ll file dumps don't produce #uses comments anymore. To get them, run a .bc file through "llvm-dis --show-annotations".
• Target triples are now stored in a normalized form, and all inputs from humans are expected to be normalized by Triple::normalize before being stored in a module triple or passed to another library.

In addition, many APIs have changed in this release. Some of the major LLVM API changes are:

• LLVM 2.8 changes the internal order of operands in InvokeInst and CallInst. To be portable across releases, please use the CallSite class and the high-level accessors, such as getCalledValue and setUnwindDest.
• You can no longer pass use_iterators directly to cast<> (and similar), because these routines tend to perform costly dereference operations more than once. You have to dereference the iterators yourself and pass them in.
• llvm.memcpy.*, llvm.memset.*, llvm.memmove.* intrinsics take an extra parameter now ("i1 isVolatile"), totaling 5 parameters, and the pointer operands are now address-space qualified. If you were creating these intrinsic calls and prototypes yourself (as opposed to using Intrinsic::getDeclaration), you can use UpgradeIntrinsicFunction/UpgradeIntrinsicCall to be portable accross releases.
• SetCurrentDebugLocation takes a DebugLoc now instead of a MDNode. Change your code to use SetCurrentDebugLocation(DebugLoc::getFromDILocation(...)).
• The RegisterPass and RegisterAnalysisGroup templates are considered deprecated, but continue to function in LLVM 2.8. Clients are strongly advised to use the upcoming INITIALIZE_PASS() and INITIALIZE_AG_PASS() macros instead.
• The constructor for the Triple class no longer tries to understand odd triple specifications. Frontends should ensure that they only pass valid triples to LLVM. The Triple::normalize utility method has been added to help front-ends deal with funky triples.
• Some APIs got renamed:
  • llvm_report_error -> report_fatal_error
  • llvm_install_error_handler -> install_fatal_error_handler
  • llvm::DwarfExceptionHandling -> llvm::JITExceptionHandling
  • VISIBILITY_HIDDEN -> LLVM_LIBRARY_VISIBILITY

This section contains significant known problems with the LLVM system, listed by component. If you run into a problem, please check the LLVM bug database and submit a bug if there isn't already one.

The following components of this LLVM release are either untested, known to be broken or unreliable, or are in early development. These components should not be relied on, and bugs should not be filed against them, but they may be useful to some people. In particular, if you would like to work on one of these components, please contact us on the LLVMdev list.

• The Alpha, Blackfin, CellSPU, MicroBlaze, MSP430, MIPS, PIC16, SystemZ and XCore backends are experimental.
• llc "-filetype=obj" is experimental on all targets other than darwin-i386 and darwin-x86_64.

• The X86 backend does not yet support all inline assembly that uses the X86 floating point stack. It supports the 'f' and 't' constraints, but not 'u'.
• Win64 code generation wasn't widely tested. Everything should work, but we expect small issues to happen. Also, llvm-gcc cannot build the mingw64 runtime currently due to lack of support for the 'u' inline assembly constraint and for X87 floating point inline assembly.
• The X86-64 backend does not yet support the LLVM IR instruction va_arg. Currently, front-ends support variadic argument constructs on X86-64 by lowering them manually.

• The Linux PPC32/ABI support needs testing for the interpreter and static compilation, and lacks support for debug information.

• Thumb mode works only on ARMv6 or higher processors. On sub-ARMv6 processors, thumb programs can crash or produce wrong results (PR1388).
• Compilation for ARM Linux OABI (old ABI) is supported but not fully tested.

• The SPARC backend only supports the 32-bit SPARC ABI (-m32); it does not support the 64-bit SPARC ABI (-m64).

• 64-bit MIPS targets are not supported yet.

• On 21164s, some rare FP arithmetic sequences which may trap do not have the appropriate nops inserted to ensure restartability.

The C backend has numerous problems and is not being actively maintained. Depending on it for anything serious is not advised.

• The C backend has only basic support for inline assembly code.
• The C backend violates the ABI of common C++ programs, preventing intermixing between C++ compiled by the CBE and C++ code compiled with llc or native compilers.
• The C backend does not support all exception handling constructs.
• The C backend does not support arbitrary precision integers.

llvm-gcc is generally very stable for the C family of languages. The only major language feature of GCC not supported by llvm-gcc is the __builtin_apply family of builtins. However, some extensions are only supported on some targets. For example, trampolines are only supported on some targets (these are used when you take the address of a nested function).

Fortran support generally works, but there are still several unresolved bugs in Bugzilla. Please see the tools/gfortran component for details. Note that llvm-gcc is missing major Fortran performance work in the frontend and library that went into GCC after 4.2. If you are interested in Fortran, we recommend that you consider using dragonegg instead.

The llvm-gcc 4.2 Ada compiler has basic functionality. However, this is not a mature technology, and problems should be expected. For example:

• The Ada front-end currently only builds on X86-32. This is mainly due to lack of trampoline support (pointers to nested functions) on other platforms. However, it also fails to build on X86-64 which does support trampolines.
• The Ada front-end fails to bootstrap. This is due to lack of LLVM support for setjmp/longjmp style exception handling, which is used internally by the compiler. Workaround: configure with --disable-bootstrap.
• The c380004, c393010 and cxg2021 ACATS tests fail (c380004 also fails with gcc-4.2 mainline). If the compiler is built with checks disabled then c393010 causes the compiler to go into an infinite loop, using up all system memory.
• Some GCC specific Ada tests continue to crash the compiler.
• The -E binder option (exception backtraces) does not work and will result in programs crashing if an exception is raised. Workaround: do not use -E.
• Only discrete types are allowed to start or finish at a non-byte offset in a record. Workaround: do not pack records or use representation clauses that result in a field of a non-discrete type starting or finishing in the middle of a byte.
• The lli interpreter considers 'main' as generated by the Ada binder to be invalid. Workaround: hand edit the file to use pointers for argv and envp rather than integers.
• The -fstack-check option is ignored.

A wide variety of additional information is available on the LLVM web page, in particular in the documentation section. The web page also contains versions of the API documentation which is up-to-date with the Subversion version of the source code. You can access versions of these documents specific to this release by going into the "llvm/doc/" directory in the LLVM tree.

If you have any questions or comments about LLVM, please feel free to contact us via the mailing lists.