LLVM 2.6 Release Notes

1. Introduction
2. Sub-project Status Update
3. External Projects Using LLVM 2.6
4. What's New in LLVM 2.6?
5. Installation Instructions
6. Portability and Supported Platforms
7. Known Problems
8. Additional Information

Written by the LLVM Team

Introduction

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.

Sub-project Status Update

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.

Clang: C/C++/Objective-C Frontend Toolkit

The Clang project is an effort to build a set of new 'LLVM native' front-end technologies for the LLVM optimizer and code generator. While Clang is not included in the LLVM 2.6 release, it is continuing to make major strides forward in all areas. Its C and Objective-C parsing and code generation support is now very solid. For example, it is capable of successfully building many real-world applications for X86-32 and X86-64, including the FreeBSD kernel and gcc 4.2. C++ is also making incredible progress, and work on templates has recently started. If you are interested in fast compiles and good diagnostics, we encourage you to try it out by building from mainline and reporting any issues you hit to the Clang front-end mailing list.

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

- Something wonderful!
- Many many bugs are fixed and many features have been added.

Clang Static Analyzer

Previously announced in the 2.4 LLVM release, 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!

VMKit: JVM/CLI Virtual Machine Implementation

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!

External Projects Using LLVM 2.6
Pure

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.

In addition to the usual algebraic data structures, Pure also has MATLAB-style matrices in order to support numeric computations and signal processing in an efficient way. Pure is mainly aimed at mathematical applications right now, but it has been designed as a general purpose language. The dynamic interpreter environment and the C interface make it possible to use it as a kind of functional scripting language for many application areas.

LLVM D Compiler

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

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

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

Rubinius

Rubinius is a new virtual machine for Ruby. It leverages LLVM to dynamically compile Ruby code down to machine code using LLVM's JIT.

What's New in LLVM 2.6?

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.

Major New Features

LLVM 2.6 includes several major new capabilities:

- Something wonderful!

llvm-gcc 4.2 Improvements

LLVM fully supports the llvm-gcc 4.2 front-end, which marries the GCC front-ends and driver with the LLVM optimizer and code generator. It currently includes support for the C, C++, Objective-C, Ada, and Fortran front-ends.

- Something wonderful!

LLVM IR and Core Improvements

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:

- Something wonderful!

Optimizer Improvements

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:

- Something wonderful!

Target Independent Code Generator Improvements

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:

- Something wonderful!

X86-32 and X86-64 Target Improvements

New features of the X86 target include:

- Something wonderful!

PIC16 Target Improvements

New features of the PIC16 target include:

- Something wonderful!

Things not yet supported:

- Floating point.
- Passing/returning aggregate types to and from functions.
- Variable arguments.
- Indirect function calls.
- Interrupts/programs.
- Debug info.

Improvements in LLVMC

New features include:

- Something wonderful!

Major Changes and Removed Features

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.

- Something horrible!

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

- LLVM's global uniquing tables for `Type`s and `Constant`s 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.

Portability and Supported Platforms

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.

Known Problems

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.

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, Alpha, SPU, MIPS, and PIC16 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.
- 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.

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 MIPS back-end

- The O32 ABI is not fully supported.
- 64-bit MIPS targets are not supported yet.

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 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.
- The C backend does not support arbitrary precision integers.

Known problems with the llvm-gcc C front-end

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.

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. Currently only Linux and Darwin targets are supported (both 32 and 64 bit).

Known problems with the llvm-gcc Fortran front-end

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

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. 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.

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.