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


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), 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.

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 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. Clang currently compiles typical Objective-C code 3x faster than GCC and compiles C code about 30% faster than GCC at -O0 -g (which is when the most pressure is on the frontend).

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:

Clang Static Analyzer

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 checks to find bugs that occur on a specific path within a program.

In the LLVM 2.6 time-frame, the analyzer core has undergone several important improvements and cleanups and now includes a new Checker interface that is intended to eventually serve as a basis for domain-specific checks. Further, in addition to generating HTML files for reporting analysis results, the analyzer can now also emit bug reports in a structured XML format that is intended to be easily readable by other programs.

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 Machine (Microsoft .NET is an implementation of the CLI) using LLVM for static and just-in-time compilation.

VMKit version 0.26 builds with LLVM 2.6 and you can find it on its web page. The release includes bug fixes, cleanup and new features. The major changes are:

compiler-rt: Compiler Runtime Library

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.

KLEE: Symbolic Execution and Automatic Test Case Generator

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.

DragonEgg: GCC-4.5 as an LLVM frontend

The goal of DragonEgg is to make gcc-4.5 act like llvm-gcc without requiring any gcc modifications whatsoever. DragonEgg is a shared library (dragonegg.so) that is loaded by gcc at runtime. It uses the new gcc plugin architecture to disable the GCC optimizers and code generators, and schedule the LLVM optimizers and code generators (or direct output of LLVM IR) instead. Currently only Linux and Darwin are supported, and only on x86-32 and x86-64. It should be easy to add additional unix-like architectures and other processor families. In theory it should be possible to use DragonEgg with any language supported by gcc, however only C and Fortran work well for the moment. Ada and C++ work to some extent, while Java, Obj-C and Obj-C++ are so far entirely untested. Since gcc-4.5 has not yet been released, neither has DragonEgg. To build DragonEgg you will need to check out the development versions of gcc, llvm and DragonEgg from their respective subversion repositories, and follow the instructions in the DragonEgg README.

llvm-mc: Machine Code Toolkit

The LLVM Machine Code (MC) Toolkit project is a (very early) effort to build better tools for dealing with machine code, object file formats, etc. The idea is to be able to generate most of the target specific details of assemblers and disassemblers from existing LLVM target .td files (with suitable enhancements), and to build infrastructure for reading and writing common object file formats. One of the first deliverables is to build a full assembler and integrate it into the compiler, which is predicted to substantially reduce compile time in some scenarios.

In the LLVM 2.6 timeframe, the MC framework has grown to the point where it can reliably parse and pretty print (with some encoding information) a darwin/x86 .s file successfully, and has the very early phases of a Mach-O assembler in progress. Beyond the MC framework itself, major refactoring of the LLVM code generator has started. The idea is to make the code generator reason about the code it is producing in a much more semantic way, rather than a textual way. For example, the code generator now uses MCSection objects to represent section assignments, instead of text strings that print to .section directives.

MC is an early and ongoing project that will hopefully continue to lead to many improvements in the code generator and build infrastructure useful for many other situations.

External Open Source Projects Using LLVM 2.6

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 2.6.


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).

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 bug fixes and better x86-64 support. This has allowed some major improvements in LDC, getting it 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.


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

IcedTea Java Virtual Machine Implementation

IcedTea provides a harness to build OpenJDK using only free software build tools and to provide replacements for the not-yet free parts of OpenJDK. One of the extensions that IcedTea provides is a new JIT compiler named Shark which uses LLVM to provide native code generation without introducing processor-dependent code.

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:

LLVM IR and Core Improvements

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

Optimizer Improvements

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:

Interpreter and JIT Improvements
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:

X86-32 and X86-64 Target Improvements

New features of the X86 target include:

PIC16 Target Improvements

New features of the PIC16 target include:

Things not yet supported:

ARM Target Improvements

New features of the ARM target include:

These features are still somewhat experimental and subject to change. The Neon intrinsics, in particular, may change in future releases of LLVM. ARMv7 support has progressed a lot on top of tree since 2.6 branched.

Other Target Specific Improvements

New features of other targets include:

New Useful APIs

This release includes a number of new APIs that are used internally, which may also be useful for external clients.

Other Improvements and New Features

Other miscellaneous features include:

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.

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

Portability and Supported Platforms

LLVM is known to work on the following platforms:

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.

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.

Known problems with the X86 back-end
Known problems with the PowerPC back-end
Known problems with the ARM back-end
Known problems with the SPARC back-end
Known problems with the MIPS back-end
Known problems with the Alpha back-end
Known problems with the C back-end
Known problems with the llvm-gcc C front-end

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.

Known problems with the llvm-gcc Fortran front-end
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.
Known problems with the O'Caml bindings

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.

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.

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