LLVM 2.7 Release Notes

This document contains the release notes for the LLVM Compiler Infrastructure, release 2.7. 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.

FIXME: llvm.org moved to new server, mention new logo, Ted and Doug new code owners, web page in llvm-www repos.

The LLVM 2.7 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.

The Clang project is ...

In the LLVM 2.7 time-frame, the Clang team has made many improvements:

FIXME: C++! Include a link to cxx_compatibility.html
FIXME: Static Analyzer improvements?
CIndex API and Python bindings: Clang now includes a C API as part of the CIndex library. Although we make make some changes to the API in the future, it is intended to be stable and has been designed for use by external projects. See the Clang doxygen CIndex documentation for more details. The CIndex API also includings an preliminary set of Python bindings.
ARM Support: Clang now has ABI support for both the Darwin and Linux ARM ABIs. Coupled with many improvements to the LLVM ARM backend, Clang is now suitable for use as a a beta quality ARM compiler.

Previously announced in the 2.4, 2.5, and 2.6 LLVM releases, the Clang project also includes an early stage static source code analysis tool for automatically finding bugs in C and Objective-C programs. The tool performs checks to find bugs that occur on a specific path within a program.

In the LLVM 2.7 time-frame, the analyzer core has sprouted legs and...

The VMKit project is an implementation of a JVM and a CLI Virtual Machine (Microsoft .NET is an implementation of the CLI) using LLVM for static and just-in-time compilation.

With the release of LLVM 2.7, VMKit has shifted to a great framework for writing virtual machines. VMKit now offers precise and efficient garbage collection with multi-threading support, thanks to the MMTk memory management toolkit, as well as just in time and ahead of time compilation with LLVM. The major changes in VMKit 0.27 are:

Garbage collection: VMKit now uses the MMTk toolkit for garbage collectors. The first collector to be ported is the MarkSweep collector, which is precise, and drastically improves the performance of VMKit.
Line number information in the JVM: by using the debug metadata of LLVM, the JVM now supports precise line number information, useful when printing a stack trace.
Interface calls in the JVM: we implemented a variant of the Interface Method Table technique for interface calls in the JVM.

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.

DragonEgg is a port of llvm-gcc to gcc-4.5. Unlike llvm-gcc, which makes many intrusive changes to the underlying gcc-4.2 code, dragonegg in theory does not require any gcc-4.5 modifications whatsoever (currently one small patch is needed). This is thanks to the new gcc plugin architecture, which makes it possible to modify the behaviour of gcc at runtime by loading a plugin, which is nothing more than a dynamic library which conforms to the gcc plugin interface. DragonEgg is a gcc plugin that causes the LLVM optimizers to be run instead of the gcc optimizers, and the LLVM code generators instead of the gcc code generators, just like llvm-gcc. To use it, you add "-fplugin=path/dragonegg.so" to the gcc-4.5 command line, and gcc-4.5 magically becomes llvm-gcc-4.5!

DragonEgg is still a work in progress. Currently C works very well, while C++, Ada and Fortran work fairly well. 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.

DragonEgg has not yet been released. Once gcc-4.5 has been released, dragonegg will probably be released as part of the following LLVM release.

The LLVM Machine Code (MC) Toolkit project is ...

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 2.7.

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 0.43 and later have been tested and are known to work with LLVM 2.7 (and continue to work with older LLVM releases >= 2.5).

Roadsend PHP (rphp) is an open source implementation of the PHP programming language that uses LLVM for its optimizer, JIT and static compiler. This is a reimplementation of an earlier project that is now based on LLVM.

Unladen Swallow is a branch of Python intended to be fully compatible and significantly faster. It uses LLVM's optimization passes and JIT compiler.

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.

SAFECode is a memory safe C compiler built using LLVM. It takes standard, unannotated C code, analyzes the code to ensure that memory accesses and array indexing operations are safe, and instruments the code with run-time checks when safety cannot be proven statically.

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 2.7 includes several major new capabilities:

Extensible metadata solid. Debug info improvements: using metadata instead of llvm.dbg global variables. This brings several enhancements including improved compile times. New instruction selector. GHC Haskell ABI/ calling conv support. Pre-Alpha support for unions in IR. New InlineHint and StackAlignment function attributes Code generator MC'ized except for debug info and EH. New SCEV AA pass: -scev-aa Inliner reuses arrays allocas when inlining multiple callers to reduce stack usage. MC encoding and disassembler apis. Optimal Edge Profiling? Instcombine is now a library, has its own IRBuilder to simplify itself. New llvm/Support/Regex.h API. FileCheck now does regex's Many subtle pointer invalidation bugs in Callgraph have been fixed and it now uses asserting value handles. MC Disassembler (with blog post), MCInstPrinter. Many X86 backend and AsmPrinter simplifications Various tools like llc and opt now read either .ll or .bc files as input. Malloc and free instructions got removed, along with LowerAllocations pass. compiler-rt support for ARM. completely llvm-gcc NEON support. Can transcode from GAS to intel syntax with "llvm-mc foo.s -output-asm-variant=1" JIT debug information with GDB 7.0 New CodeGen Level CSE CMake can now run tests, what other improvements? ARM/Thumb using reg scavenging for stack object address materialization (PEI). New SSAUpdater and MachineSSAUpdater classes for unstructured ssa updating, changed jump threading, GVN, etc to use it which simplified them and speed them up. Combiner-AA improvements, why not on by default? Pre-regalloc tail duplication x86 sibcall / tailcall optimization in CCC mode. New LSR with "full strength reduction" mode. Description? Codegen level OptimizeExtsPass pass, takes advantage of x86 subregs. Better code size analysis in loop unswitch, inliner code split out to a new CodeMetrics class for reuse. The ARM backend now has good support for ARMv4 backend (tested on StrongARM hardware), previously only supported ARMv4T and newer. Half-float support in APFloat Indirect branch + address of label (blog post), particularly useful for interpreters. Many changes to the pass ordering for improved optimization effectiveness. BasicAA improved to be less dependent on "type safe" pointers, it can now look through bitcasts more aggressively. GVN PHI Translation improvements. blog post: http://blog.llvm.org/2009/12/advanced-topics-in-redundant-load.html llvm.objectsize. MachineSSAUpdater.h PostRA scheduler for X86? llvm.dbg.value, not being used by default though, more in 2.8. Many improvements to debug info Support for the GCC option -fno-schedule-insns non-temporal load/store libllvm2.7.so?? configure with --enable-shared dbgs() and -debug-buffer-size=N New MicroBlaze backend. http://en.wikipedia.org/wiki/MicroBlaze XMM subreg modeling for extraction of the low element. Opt now works conservatively if no target data is set (is this fully working?) Target data now has notion of 'native' integer data types which optimizations can use. ARM backend generates instructions in unified assembly syntax. New Analysis/InstructionSimplify.h interface for simplifying instructions that don't exist. Jump threading is now much more aggressive at simplifying correlated conditionals and threading blocks with otherwise complex logic. CondProp pass removed (functionality merged into jump threading). X86 and XCore supports returning arbitrary return values, returning too many values is supported by returning through a hidden pointer. verbose-asm now produces information about spill slots and loop nests Defaults to RTTI off (smaller code size!), packagers should build with make REQUIRE_RTTI=1. AndersAA got removed PredSimplify, LoopVR, GVNPRE, RSProfiling (random sampling profiling) got removed. LLVM command line tools now overwrite their output, before they would only do this with -f. DOUT removed, use DEBUG(errs() instead. Much stuff converted to use raw_ostream instead of std::ostream. TargetAsmInfo renamed to MCAsmInfo llvm/ADT/iterator.h gone.

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

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:

Also, -anders-aa was removed

The JIT now defaults to compiling eagerly to avoid a race condition in the lazy JIT. Clients that still want the lazy JIT can switch it on by calling ExecutionEngine::DisableLazyCompilation(false).
It is now possible to create more than one JIT instance in the same process. These JITs can generate machine code in parallel, although you still have to obey the other threading restrictions.

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:

New features of the X86 target include:

New features of the PIC16 target include:

Things not yet supported:

Variable arguments.
Interrupts/programs.

New features of the ARM target include:

New features of other targets include:

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 2.6, this section lists some "gotchas" that you may run into upgrading from the previous release.

The LLVM interpreter now defaults to not using libffi even if you have it installed. This makes it more likely that an LLVM built on one system will work when copied to a similar system. To use libffi, configure with --enable-libffi.

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

ModuleProvider has been removed and its methods moved to Module and GlobalValue. Most clients can remove uses of ExistingModuleProvider, replace getBitcodeModuleProvider with getLazyBitcodeModule, and pass their Module to functions that used to accept ModuleProvider. Clients who wrote their own ModuleProviders will need to derive from GVMaterializer instead and use Module::setMaterializer to attach it to a Module.
GhostLinkage has given up the ghost. GlobalValues that have not yet been read from their backing storage have the same linkage they will have after being read in. Clients must replace calls to GlobalValue::hasNotBeenReadFromBitcode with GlobalValue::isMaterializable.
FIXME: Debug info has been totally redone. Add pointers to new APIs. Substantial caveats about compatibility of .ll and .bc files.
The llvm/Support/DataTypes.h header has moved to llvm/System/DataTypes.h.
The isInteger, isIntOrIntVector, isFloatingPoint, isFPOrFPVector and isFPOrFPVector methods have been renamed isIntegerTy, isIntOrIntVectorTy, isFloatingPointTy, isFPOrFPVectorTy and isFPOrFPVectorTy respectively.

LLVM is known to work on the following platforms:

Intel and AMD machines (IA32, X86-64, AMD64, EMT-64) running Red Hat Linux, Fedora Core, FreeBSD and AuroraUX (and probably other unix-like systems).
PowerPC and X86-based Mac OS X systems, running 10.3 and above in 32-bit and 64-bit modes.
Intel and AMD machines running on Win32 using MinGW libraries (native).
Intel and AMD machines running on Win32 with the Cygwin libraries (limited support is available for native builds with Visual C++).
Sun x86 and AMD64 machines running Solaris 10, OpenSolaris 0906.
Alpha-based machines running Debian GNU/Linux.

The core LLVM infrastructure uses GNU autoconf to adapt itself to the machine and operating system on which it is built. However, minor porting may be required to get LLVM to work on new platforms. We welcome your portability patches and reports of successful builds or error messages.

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.

LLVM will not correctly compile on Solaris and/or OpenSolaris using the stock GCC 3.x.x series 'out the box', See: Broken versions of GCC and other tools. However, A Modern GCC Build for x86/x86-64 has been made available from the third party AuroraUX Project that has been meticulously tested for bootstrapping LLVM & Clang.

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 MSIL, Alpha, SPU, MIPS, PIC16, Blackfin, MSP430, SystemZ and MicroBlaze backends are experimental.
The llc "-filetype=asm" (the default) is the only supported value for this option. The MachO writer is experimental, and works much better in mainline SVN.

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'.
The X86 backend generates inefficient floating point code when configured to generate code for systems that don't have SSE2.
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 several bugs and 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, the llvm-gcc and 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.

Support for the Advanced SIMD (Neon) instruction set is still incomplete and not well tested. Some features may not work at all, and the code quality may be poor in some cases.
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 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.

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).

If you run into GCC extensions which are not supported, please let us know.

Fortran support generally works, but there are still several unresolved bugs in Bugzilla. Please see the tools/gfortran component for details.

The llvm-gcc 4.2 Ada compiler works fairly well; however, this is not a mature technology, and problems should be expected.

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.