This document contains the release notes for the LLVM Compiler Infrastructure, release 2.4. 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 releases, please see the releases page.

The LLVM 2.4 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 LLVM optimizer and code generator. Clang is continuing to make major strides forward in all areas. Its C and Objective-C parsing support is very solid, and the code generation support is far enough along to build many C applications. While not yet production quality, it is progressing very nicely. In addition, C++ front-end work has started to make significant progress.

Codegen progress/state

The static analysis tool .

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.

...

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 2.4 includes several major new capabilities:

• MRVs got generalized to FCAs. getresult is gone, ret with multiple values is gone.

• -O0 compile times overall much faster

• Attrs changes?

• Initial PIC16 port

• Support the rest of the atomic __sync builtins

• ...

LLVM 2.4 fully supports the llvm-gcc 4.2 front-end, and includes support for the C, C++, Objective-C, Ada, and Fortran front-ends.

• block-pointers
• alpha?
• -flimited-precision
• -flto

New features include:

• vector shifts in the IR: no codegen support yet
• use diet patch landed: saved 15% IR memory footprint
• LLVM IR now directly represents "common" linkage, instead of representing it as a form of weak linkage.
• DebugInfoBuilder
• .ll printing format change: %3 = add i32 4, 2
• opt-size, noinline, alwaysinline function attributes
• ...

In addition to a huge array of bug fixes and minor performance tweaks, the LLVM 2.4 optimizers support a few major enhancements:

• GVN now does local PRE?
• Matthijs' Dead argument elimination rewrite
• Old-ADCE used control dependence and deleted output-free infinite loops. Added a new Loop deletion pass (for deleting output free provably-finite loops) and rewrote ADCE to be simpler faster, and not need control dependence.
• SparsePropagation framework for lattice-based dataflow solvers.
• Tail duplication was removed from the standard optimizer sequence.
• Various helper functions (ComputeMaskedBits, ComputeNumSignBits, etc) were pulled out of instcombine and put into a new ValueTracking.h file, where they can be reused by other passes.
• MarkModRef etc

We 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 writers split out to their own library to avoid JITs having to link them in.
• Big asm writer refactoring + TargetAsmInfo
• 2-addr pass and coalescer can now remat trivial insts to avoid a copy.
• spiller to commute instructions in order to fold a reload
• Stack slot coloring?
• Live intervals renumbering? Is this useful to external people?
• 'is as cheap as a move' instruction flag
• Improvements to selection dag viewing
• fast isel
• Selection dag speedups
• asmwriter + raw_ostream -> fastah
• Partitioned Boolean Quadratic Programming (PBQP) based register allocator.
• ...

New target-specific features include:

• Exception handling is supported by default on Linux/x86-64.
• Position Independent Code (PIC) is now supported on Linux/x86-64.
• ...

New target-specific features include:

• MIPS floating point support?
• PowerPC now supports trampolines.
• ....

New features include:

• llvmc2 (the generic compiler driver) gained plugin support. It is now easier to experiment with llvmc2 and build your own tools based on it.
• raw_ostream + formatting
• Recycler + pool allocation stuff?
• ...

If you're already an LLVM user, this section lists some "gotchas" that you may run into upgrading from the previous release.

The LLVM IR generated by llvm-gcc no longer names all instructions. Use the instnamer pass if you want everything named.

The LoadVN and GCSE passes have been removed.

LLVM API Changes:

• ... Attributes changes ...
• The DbgStopPointInst methods getDirectory and getFileName now return Value* instead of strings. These can be converted to strings using llvm::GetConstantStringInfo defined via "llvm/Analysis/ValueTracking.h".
• API change: BinaryOperator::create -> Create (CmpInst, CastInst too)
• Various header files like "llvm/ADT/iterator" were given a .h suffix. Change your code to #include "llvm/ADT/iterator.h" instead.

LLVM is known to work on the following platforms:

• Intel and AMD machines (IA32) running Red Hat Linux, Fedora Core and FreeBSD (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.
• Itanium-based (IA64) machines running Linux and HP-UX.

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 all known problems with the LLVM system, listed by component. As new problems are discovered, they will be added to these sections. 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 MSIL, IA64, Alpha, SPU, and MIPS 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 mingw64 runtime currently due to several bugs due to lack of support for the 'u' inline assembly constraint and X87 floating point inline assembly.
• The X86-64 backend does not yet support the LLVM IR instruction va_arg. Currently, the llvm-gcc front-end supports 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.
• There is a bug in QEMU-ARM (<= 0.9.0) which causes it to incorrectly execute programs compiled with LLVM. Please use more recent versions of QEMU.

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

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

• The Itanium backend is highly experimental, and has a number of known issues. We are looking for a maintainer for the Itanium backend. If you are interested, please contact the llvmdev mailing list.

• 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.

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).

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. Workaround: configure with --disable-bootstrap.
• The c380004, c393010 and cxg2021 ACATS tests fail (c380004 also fails with gcc-4.2 mainline).
• 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.