This document contains the release notes for the LLVM Compiler Infrastructure, release 2.6. 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 2.6 distribution currently consists of code from the core LLVM repository —which roughly includes the LLVM optimizers, code generators and supporting tools — and the llvm-gcc repository. In addition to this code, the LLVM Project includes other sub-projects that are in development. The two which are the most actively developed are the Clang Project and the VMKit Project.

The Clang project is an effort to build a set of new 'LLVM native' front-end technologies for the C family of languages. LLVM 2.6 is the first release to officially include Clang, and it provides a production quality C and Objective-C compiler. If you are interested in fast compiles and good diagnostics, we encourage you to try it out.

In addition to supporting these languages, C++ support is also well under way, and mainline Clang is able to parse the libstdc++ 4.2 headers and even codegen simple apps. If you are interested in Clang C++ support or any other Clang feature, we strongly encourage you to get involved on the Clang front-end mailing list.

In the LLVM 2.6 time-frame, the Clang team has made many improvements:

• C and Objective-C support are now considered production quality.
• AuroraUX / FreeBSD & OpenBSD Toolchain support.
• Most of Objective-C 2.0 is now supported with the GNU runtime.
• Many many bugs are fixed and many features have been added.

UPDATE! Previously announced in the 2.4 and 2.5 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 a growing set of checks to find bugs that occur on a specific path within a program.

In the LLVM 2.6 time-frame there have been many significant improvements to XYZ.

The set of checks performed by the static analyzer continues to expand, and future plans for the tool include full source-level inter-procedural analysis and deeper checks such as buffer overrun detection. There are many opportunities to extend and enhance the static analyzer, and anyone interested in working on this project is encouraged to get involved!

UPDATE! The VMKit project is an implementation of a JVM and a CLI Virtual Machines (Microsoft .NET is an implementation of the CLI) using the Just-In-Time compiler of LLVM.

Following LLVM 2.6, VMKit has its XYZ release that you can find on its webpage. The release includes bug fixes, cleanup and new features. The major changes are:

• Something wonderful!

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 compiling into a runtime call to the "__fixunsdfdi" function. The compiler-rt library provides optimized implementations of this and other low-level routines.

All of the code in the compiler-rt project is available under the standard LLVM License, a "BSD-style" license.

The new LLVM klee project is a symbolic execution framework for programs in LLVM bitcode form. Klee tries to symbolically evaluate "all" paths through the application and records state transitions that lead to fault states. This allows it to construct testcases that lead to faults and can even be used to verify algorithms. For more details, please see the OSDI 2008 paper about Klee.

Duncan needs to write me.

UPDATE! blah MC: MCSection, MCAsmInfo MCInstPrinter did it make it in? MCInst (X86 using it so far) Rewrite of X86 GV selection logic: TargetOperand flags on ExternalSymbol, GV, etc operands. Can parse and re-print out an darwin-x86 .s file. TargetLoweringObjectFile, MCSectionKind Verrrry early start of a macho writer.

Rubinius is an environment for running Ruby code which strives to write as much of the core class implementation in Ruby as possible. Combined with a bytecode interpreting VM, it uses LLVM to optimize and compile ruby code down to machine code. Techniques such as type feedback, method inlining, and uncommon traps are all used to remove dynamism from ruby execution and increase performance.

Since LLVM 2.5, Rubinius has made several major leaps forward, implementing a counter based JIT, type feedback, and speculative method inlining.

MacRuby is an implementation of Ruby on top of core Mac OS X technologies, such as the Objective-C common runtime and garbage collector, and the CoreFoundation framework. It is principally developed by Apple and aims at enabling the creation of full-fledged Mac OS X applications.

MacRuby uses LLVM for optimization passes, JIT and AOT compilation of Ruby expressions. It also uses zero-cost DWARF exceptions to implement Ruby exception handling.

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 0.31 and later have been tested and are known to work with LLVM 2.6 (and continue to work with older LLVM releases >= 2.3 as well).

LDC is an implementation of the D Programming Language using the LLVM optimizer and code generator. The LDC project works great with the LLVM 2.6 release. General improvements in this cycle have included new inline asm constraint handling, better debug info support, general bugfixes, and better x86-64 support. This has allowed some major improvements in LDC, getting us much closer to being as fully featured as the original DMD compiler from DigitalMars.

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.

LLVM-Lua uses LLVM to add JIT & static compiling support to the Lua VM. Lua bytecode is analyzed to remove type checks, then LLVM is used to compile those bytecodes down to machine code.

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 2.6 includes several major new capabilities:

• New compiler-rt, klee, and machine code toolkit subprojects.
• LLVM debug info now generates line number information, even when optimizations are enabled. This allows statistical sampling tools like oprofile and Shark to map samples back to source lines.
• LLVM now includes new experiemental backends to support the MSP430, SystemZ, and BlackFin architectures.
• LLVM supports a new Gold Linker Plugin which enables support for transparent link-time optimization on ELF targets when used with the Gold binutils linker.
• LLVMContext, llvm_start_multithreaded: ProgrammersManual.html#threading
• FileCheck
• LLVM 2.6 includes a brand new experimental LLVM bindings to the Ada2005 programming language.

LLVM IR has several new features that are used by our existing front-ends and can be useful if you are writing a front-end for LLVM:

• Getelementpr instruction now allows any integer type for array/pointer indexes.
• Inbounds for GEP
• NSW/NUW/exact div
• LSR promotes int induction variables to 64-bit on 64-bit targets, major perf boost for numerical code.
• LSR now analyzes pointer expressions (e.g. getelementptrs), not just integers.
• new linkage types linkonce_odr, weak_odr, linker_private, and available_externally.
• New fadd, fsub, fmul instructions and classes.
• Target intrinsics can now return multiple results.

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

• SRoA improvements for vector unions, memset, arbitrary weird bitfield accesses etc. It now produces "strange" sized integers.
• Inliner reuse stack space when inlining arrays?
• Enabled GVN Load PRE.
• New Static Single Information (SSI) construction pass (not used by anything yet, experimental).

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:

• -asm-verbose now prints location info (with -g) and loop nest info.
• Tblgen now supports multiclass inheritance and a number of new string and list operations like !(subst), !(foreach), !car, !cdr, !null, !if, !cast. These make the .td files more expressive and allow more aggressive factoring of duplication across instruction patterns.
• New MachineVerifier pass.
• Machine LICM, hoists things like constant pool loads, loads from readonly stubs, vector constant synthesization code, etc.
• Machine Sinking
• target-specific intrinsics (r63765)
• Regalloc improvements for commuting, various spiller peephole optimizations, cross-class coalescing.
• llc -enable-value-prop, propagation of value info (sign/zero ext info) from one MBB to another
• Regalloc hints for allocation stuff: Evan r73381/r73671. Finished/enabled?
• Stack slot coloring for register spills (denser stack frames)
• SelectionDAGS: New BuildVectorSDNode (r65296), and ISD::VECTOR_SHUFFLE (r69952 / PR2957)
• Experimental support for shrink wrapping support in PEI.
• Experimental support for writing ELF .o files directly from the compiler, it works well for many simple C testcases, but doesn't support exception handling, debug info, inline assembly, etc.
• Targets can now specify register allocation hints through MachineRegisterInfo:: setRegAllocationHint. A regalloc hint consists 1) hint type, 2) physical register number. A hint type of zero specifies a register allocation preference. Other hint type values are target specific which are resolved by TargetRegisterInfo::ResolveRegAllocHint. An example of which is the ARM target can uses register hint to request that the register allocator provide an even / odd register pair to two virtual registers. It is important to note the register allocation hints are just hints. There is no guarantee the register allocators will be able to satisfy the hints.

New features of the X86 target include:

• Preliminary support for addrspace 256 -> GS, 257 -> FS, known problems: CodeGenerator.html#x86_memory
• Support for softfloat modes, typically used by OS kernels.
• X86-64: better modeling of implicit zero extensions, eliminates a lot of redundant zexts
• X86-64 TLS support for local exec and initial exec.
• Better modeling of H registerts as subregs.
• Vector icmp/fcmp now work with SSE codegen.
• SSE 4.2 support.
• all global variable reference logic is now in ClassifyGlobalReference.

New features of the PIC16 target include:

• Support for floating-point, indirect function calls, and passing/returning aggregate types to functions.
• The code generator is able to generate debug info into output COFF files.
• Support for placing an object into a specific section or at a specific address in memory.

Things not yet supported:

• Variable arguments.
• Interrupts/programs.

New features of the ARM target include:

• Preliminary support for processors, such as the Cortex-A8 and Cortex-A9, that implement version v7-A of the ARM architecture. The ARM backend now supports both the Thumb2 and Advanced SIMD (Neon) instruction sets. The AAPCS-VFP "hard float" calling conventions are also supported with the -float-abi=hard flag. These features are still somewhat experimental and subject to change. The Neon intrinsics, in particular, may change in future releases of LLVM.
ARM AAPCS-VFP hard float ABI is supported. ARM calling convention code is now tblgen generated instead of manual. ARM: NEON support. neonfp for doing single precision fp with neon instead of VFP.

New features of other targets include:

• Add support for the PowerPC 64-bit SVR4 ABI.
• Mips now supports O32 Calling Convention.

• New EngineBuilder class for creating JITs: r76276
New PrettyStackTrace, crashes of llvm tools should give some indication of what the compiler was doing at the time of the crash (e.g. running a pass), and print out command line arguments. StringRef class, Twine class. New WeakVH and AssertingVH and CallbackVH classes. New llvm/ADT/Triple class. llvm_report_error() error handling API (llvm/Support/ErrorHandling.h) New llvm/System/Atomic.h, llvm/System/RWMutex.h for portable atomic ops, rw locks. New SourceMgr, SMLoc classes for simple parsers with caret diagnostics and #include support, (used by tablegen, llvm-mc, the .ll parser, FileCheck, etc)

Other miscellaneous features include:

• interpreter + libffi
• JIT now supports generating more than 16M of code.
• Users can now register a JITEventListener to receive callbacks when the JIT emits or frees machine code. The OProfile support uses this mechanism.
JIT support for oprofile (r75279), configure with --with-oprofile. Now we get line # and function info for JIT'd functions.
• LLVM profile information support has been significantly improved to produce correct use counts, and has support for edge profiling with reduced runtime overhead. Combined, the generated profile information is both more correct and imposes about half as much overhead (2.6. from 12% to 6% overhead on SPEC CPU2000).
• Many extensions to the C APIs.
• LLVMC: * Dynamic plugins now work on Windows. * New option property: init. Makes possible to provide default values for options defined in plugins (interface to cl::init). * New example: Skeleton, shows how to create a standalone LLVMC-based driver. * New example: mcc16, a driver for the PIC16 toolchain.

If you're already an LLVM user or developer with out-of-tree changes based on LLVM 2.5, this section lists some "gotchas" that you may run into upgrading from the previous release.

• The Itanium (IA64) backend has been removed. It was not supported and bitrotted.
• The BigBlock register allocator has been removed, it also bitrotted.
• The C Backend (-march=c) is no longer considered part of the LLVM release criteria. We still want it to work, but no one is maintaining it and it lacks support for arbitrary precision integers and other important IR features.

LLVM build now builds all libraries as .a files instead of some libraries as relinked .o files. This requires some APIs like InitializeAllTargets.h. TargetRegistry!

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

API Cleanup: no use of hash_set/hash_map, no more llvm::OStream Use raw_ostream for everything, killed off llvm/Streams.h and DOUT

• LLVM's global uniquing tables for Types and Constants have been privatized into members of an LLVMContext. A number of APIs now take an LLVMContext as a parameter. To smooth the transition for clients that will only ever use a single context, the new getGlobalContext() API can be used to access a default global context which can be passed in any and all cases where a context is required.
• The getABITypeSize methods are now called getAllocSize.
• The Add, Sub, and Mul operators are no longer overloaded for floating-point types. Floating-point addition, subtraction, and multiplication are now represented with new operators FAdd, FSub, and FMul. In the IRBuilder API, CreateAdd, CreateSub, CreateMul, and CreateNeg should only be used for integer arithmetic now; CreateFAdd, CreateFSub, CreateFMul, and CreateFNeg should now be used for floating-point arithmetic.
• The DynamicLibrary class can no longer be constructed, its functionality has moved to static member functions.
• raw_fd_ostream's constructor for opening a given filename now takes an extra Force argument. If Force is set to false, an error will be reported if a file with the given name already exists. If Force is set to true, the file will be silently truncated (which is the behavior before this flag was added).
• SCEVHandle no longer exists, because reference counting is no longer done for SCEV* objects, instead const SCEV* should be used.
• Many APIs, notably llvm::Value, now use the StringRef and Twine classes instead of passing const char* or std::string, as described in the Programmer's Manual. Most clients should be unaffected by this transition, unless they are used to Value::getName() returning a string. Here are some tips on updating to 2.6:
  • getNameStr() is still available, and matches the old behavior. Replacing getName() calls with this is an safe option, although more efficient alternatives are now possible.
  • If you were just relying on getName() being able to be sent to a std::ostream, consider migrating to llvm::raw_ostream.
  • If you were using getName().c_str() to get a const char* pointer to the name, you can use getName().data(). Note that this string (as before), may not be the entire name if the name containts embedded null characters.
  • If you were using operator plus on the result of getName() and treating the result as an std::string, you can either uses Twine::str to get the result as an std::string, or could move to a Twine based design.
  • isName() should be replaced with comparison against getName() (this is now efficient).
• The registration interfaces for backend Targets has changed (what was previously TargetMachineRegistry). For backend authors, see the Writing An LLVM Backend guide. For clients, the notable API changes are:
  • TargetMachineRegistry has been renamed to TargetRegistry.
  • Clients should move to using the TargetRegistry::lookupTarget() function to find targets.
• llvm-dis now fails if output file exists, instead of dumping to stdout. FIXME: describe any other tool changes due to the raw_fd_ostream change. FIXME: This is not an API change, maybe there should be a tool changes section?
• temporarely due to Context API change passes should call doInitialization() method of the pass they inherit from, otherwise Context is NULL. FIXME: remove this entry when this is no longer needed.

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 UltraSPARC workstations running Solaris 10.
• 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/x64 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, and PIC16 backends are experimental.
• The llc "-filetype=asm" (the default) is the only supported value for this option.

• 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).

• The O32 ABI is not fully supported.
• 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.

llvm-gcc does not currently support Link-Time Optimization on most platforms "out-of-the-box". Please inquire on the LLVMdev mailing list if you are interested.

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.

The C++ front-end is considered to be fully tested and works for a number of non-trivial programs, including LLVM itself, Qt, Mozilla, etc.

• Exception handling works well on the X86 and PowerPC targets. Currently only Linux and Darwin targets are supported (both 32 and 64 bit).

• 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.

The Llvm.Linkage module is broken, and has incorrect values. Only Llvm.Linkage.External, Llvm.Linkage.Available_externally, and Llvm.Linkage.Link_once will be correct. If you need any of the other linkage modes, you'll have to write an external C library in order to expose the functionality. This has been fixed in the trunk.

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.