LLVM 2.3 Release Notes

1. Introduction
2. What's New?
3. Installation Instructions
4. Portability and Supported Platforms
5. Known Problems
6. Additional Information

Written by the LLVM Team

Introduction

This document contains the release notes for the LLVM compiler infrastructure, release 2.3. Here we describe the status of LLVM, including major improvements from the previous release and any 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.

What's New?

This is the fourteenth public release of the LLVM Compiler Infrastructure. It includes a large number of features and refinements from LLVM 2.2.

Major Changes in LLVM 2.3

LLVM 2.2 was the last LLVM release to support llvm-gcc 4.0 and llvm-upgrade. llvm-gcc 4.0 has been replaced with llvm-gcc 4.2. llvm-upgrade was useful for upgrading LLVM 1.9 files to LLVM 2.x syntax, but you can always use a previous LLVM release to do this. One nice impact of this is that the LLVM regressionn test suite no longer depends on llvm-upgrade, which makes it run faster.

LLVM 2.3 renames the LLVMBuilder and LLVMFoldingBuilder classes to IRBuilder.

Major New Features

LLVM 2.3 includes several major new capabilities:

- Multiple Return Value Support

- LLVM 2.3 includes a complete reimplementation of the "llvmc" tool. It is designed to overcome several problems of the original llvmc and to provide a superset of the features as the 'gcc' driver.

  The main features of llvmc2 is:

  - Extended handling of command line options and smart rules for dispatching them to different tools.
  - Flexible (and extensible) rules for defining different tools.
  - The different intermediate steps performed by tools are represented as edged in the abstract graph.
  The 'language' for driver behaviour definition is tablegen and thus it's relatively easy to add new features.
- The definition of driver is transformed into set of C++ classes, thus no runtime interpretation is needed.

- Reimplemented LTO interface in C.
- kaleidoscope tutorial in ocaml.
llvm-gcc 4.2 Improvements and Clang

LLVM 2.3 fully supports llvm-gcc 4.2 front-end.

llvm-gcc 4.2 includes numerous fixes to better support the Objective-C front-end. Objective-C now works very well on Mac OS/X.

llvm-gcc 4.2 includes many other fixes which improve conformance with the relevant parts of the GCC testsuite.

The clang project is an effort to build a set of new 'llvm native' front-end technologies for the LLVM optimizer and code generator. Currently, its C and Objective-C support is maturing nicely, and it has advanced source-to-source analysis and transformation capabilities. If you are interested in building source-level tools for C and Objective-C (and eventually C++), you should take a look. However, note that clang is not an official part of the LLVM 2.3 release. If you are interested in this project, please see its web site.

LLVM Core Improvements

New features include:

Common linkage? Atomic operation support, Alpha, X86, X86-64, PowerPC. "__sync_synchronize"

Code Generator Improvements

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:

- MemOperand in the code generator: describe me!.
- i128 support and APInt through most of codegen.
- Several compile time speedups for code with large basic blocks.

Optimizer Improvements

In addition to a huge array of bug fixes and minor performance tweaks, the LLVM 2.3 optimizers support a few major enhancements:

- Loop index set splitting on by default: describe me.
- LLVM includes a new memcpy optimization pass which optimizes out dead memcpy calls, unneeded copies of aggregates, and handles the return slot optimization. The LLVM optimizer now notices long sequences of consequtive stores and merges them into memcpy's where profitable.

X86/X86-64 Specific Improvements

New target-specific features include:

- llvm-gcc's X86-64 ABI conformance is far improved, particularly in the area of passing and returning structures by value. llvm-gcc Compiled code now interoperates very well on X86-64 systems with other compilers.
- The LLVM X86 backend now supports the support SSE 4.1 instruction set, and the llvm-gcc 4.2 front-end supports the SSE 4.1 compiler builtins.
- The X86 backend now does a number of optimizations that aim to avoid converting numbers back and forth from SSE registers to the X87 floating point stack.
- The X86 backend supports stack realignment, which is particularly useful for vector code on OS's without 16-byte aligned stacks.
- The X86 backend now supports the "sseregparm" options in GCC, which allow functions to be tagged as passing floating point values in SSE registers.
- Trampolines (taking the address of a nested function) now work on Linux/x86-64.
- `__builtin_prefetch` is now compiled into the appropriate prefetch instructions instead of being ignored.

Other Target Specific Improvements

New target-specific features include:

- The LLVM C backend now supports vectors code.

Other Improvements

New features include:

- LLVM now builds with GCC 4.3.
- llvm2cpp tool was moved into llc, use llc -march=cpp

Portability and Supported Platforms

LLVM is known to work on the following platforms:

- Intel and AMD machines 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 8.
- Alpha-based machines running Debian GNU/Linux.
- Itanium-based 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.

Known Problems

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.

Experimental features included with this release

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.

Known problems with the X86 back-end

- 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.

Known problems with the PowerPC back-end

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

Known problems with the ARM back-end

- 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.

Known problems with the SPARC back-end

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

Known problems with the Alpha back-end

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

Known problems with the IA64 back-end

- C++ programs are likely to fail on IA64, as calls to `setjmp` are made where the argument is not 16-byte aligned, as required on IA64. (Strictly speaking this is not a bug in the IA64 back-end; it will also be encountered when building C++ programs using the C back-end.)
- The C++ front-end does not use IA64 ABI compliant layout of v-tables. In particular, it just stores function pointers instead of function descriptors in the vtable. This bug prevents mixing C++ code compiled with LLVM with C++ objects compiled by other C++ compilers.
- There are a few ABI violations which will lead to problems when mixing LLVM output with code built with other compilers, particularly for floating-point programs.
- Defining vararg functions is not supported (but calling them is ok).
- The Itanium backend has bitrotted somewhat.

Known problems with the C back-end

- 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.

Known problems with the llvm-gcc C front-end
Bugs

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, which 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.

Known problems with the llvm-gcc C++ front-end

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, including x86-64 darwin. This works when linking to a libstdc++ compiled by GCC. It is supported on x86-64 linux, but that is disabled by default in this release.

Known problems with the llvm-gcc Ada front-end
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 and c393010 ACATS tests fail (c380004 also fails with gcc-4.2 mainline).
- Many 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.

Known problems with the llvm-gcc Fortran front-end

- The llvm-gcc 4.2 gfortran front-end supports a broad range of Fortran code, but does not support EQUIVALENCE yet.

Additional Information

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.