This document contains the release notes for the LLVM Compiler Infrastructure, release 2.9. 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 2.9 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 is an LLVM front end for the C, C++, and Objective-C languages. Clang aims to provide a better user experience through expressive diagnostics, a high level of conformance to language standards, fast compilation, and low memory use. Like LLVM, Clang provides a modular, library-based architecture that makes it suitable for creating or integrating with other development tools. Clang is considered a production-quality compiler for C, Objective-C, C++ and Objective-C++ on x86 (32- and 64-bit), and for darwin/arm targets.
In the LLVM 2.9 time-frame, the Clang team has made many improvements in C, C++ and Objective-C support. C++ support is now generally rock solid, has been exercised on a broad variety of code, and has several new C++'0x features implemented (such as rvalue references and variadic templates). LLVM 2.9 has also brought in a large range of bug fixes and minor features (e.g. __label__ support), and is much more compatible with the Linux Kernel.
If Clang rejects your code but another compiler accepts it, please take a look at the language compatibility guide to make sure this is not intentional or a known issue.
DragonEgg is a gcc plugin that replaces GCC's optimizers and code generators with LLVM's. Currently it requires a patched version of gcc-4.5. The plugin can target the x86-32 and x86-64 processor families and has been used successfully on the Darwin, FreeBSD and Linux platforms. The Ada, C, C++ and Fortran languages work well. The plugin is capable of compiling plenty of Obj-C, Obj-C++ and Java but it is not known whether the compiled code actually works or not!
The 2.9 release has the following notable changes:
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).
In the LLVM 2.9 timeframe, compiler_rt has had several minor changes for better ARM support, and a fairly major license change. All of the code in the compiler-rt project is now dual licensed under MIT and UIUC license, which allows you to use compiler-rt in applications without the binary copyright reproduction clause. If you prefer the LLVM/UIUC license, you are free to continue using it under that license as well.
LLDB is a brand new member of the LLVM umbrella of projects. LLDB is a next generation, high-performance debugger. It is built as a set of reusable components which highly leverage existing libraries in the larger LLVM Project, such as the Clang expression parser, the LLVM disassembler and the LLVM JIT.
libc++ is another new member of the LLVM family. It is an implementation of the C++ standard library, written from the ground up to specifically target the forthcoming C++'0X standard and focus on delivering great performance.
In the LLVM 2.9 timeframe, libc++ has had numerous bugs fixed, and is now being co-developed with Clang's C++'0x mode.
Like compiler_rt, libc++ is now dual licensed under the MIT and UIUC license, allowing it to be used more permissively.
LLBrowse is an interactive viewer for LLVM modules. It can load any LLVM module and displays its contents as an expandable tree view, facilitating an easy way to inspect types, functions, global variables, or metadata nodes. It is fully cross-platform, being based on the popular wxWidgets GUI toolkit.
The VMKit project is an implementation of a Java Virtual Machine (Java VM or JVM) that uses LLVM for static and just-in-time compilation. As of LLVM 2.9, VMKit now supports generational garbage collectors. The garbage collectors are provided by the MMTk framework, and VMKit can be configured to use one of the numerous implemented collectors of MMTk.
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 2.9.
Crack aims to provide the ease of development of a scripting language with the performance of a compiled language. The language derives concepts from C++, Java and Python, incorporating object-oriented programming, operator overloading and strong typing.
TCE is a toolset for designing application-specific processors (ASP) based on the Transport triggered architecture (TTA). The toolset provides a complete co-design flow from C/C++ programs down to synthesizable VHDL and parallel program binaries. Processor customization points include the register files, function units, supported operations, and the interconnection network.
TCE uses Clang and LLVM for C/C++ language support, target independent optimizations and also for parts of code generation. It generates new LLVM-based code generators "on the fly" for the designed TTA processors and loads them in to the compiler backend as runtime libraries to avoid per-target recompilation of larger parts of the compiler chain.
PinaVM is an open source, SystemC front-end. Unlike many other front-ends, PinaVM actually executes the elaboration of the program analyzed using LLVM's JIT infrastructure. It later enriches the bitcode with SystemC-specific information.
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. The interpreter uses LLVM as a backend to JIT-compile Pure programs to fast native code. 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 interface to C and other programming languages (including the ability to load LLVM bitcode modules, and inline C, C++, Fortran and Faust code in Pure programs if the corresponding LLVM-enabled compilers are installed).
Pure version 0.47 has been tested and is known to work with LLVM 2.9 (and continues to work with older LLVM releases >= 2.5).
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.
OpenJDK 7 b112, IcedTea6 1.9 and IcedTea7 1.13 and later have been tested and are known to work with LLVM 2.9 (and continue to work with older LLVM releases >= 2.6 as well).
GHC is an open source, state-of-the-art programming suite for Haskell, a standard lazy functional programming language. It includes an optimizing static compiler generating good code for a variety of platforms, together with an interactive system for convenient, quick development.
In addition to the existing C and native code generators, GHC 7.0 now supports an LLVM code generator. GHC supports LLVM 2.7 and later.
Polly is a project that aims to provide advanced memory access optimizations to better take advantage of SIMD units, cache hierarchies, multiple cores or even vector accelerators for LLVM. Built around an abstract mathematical description based on Z-polyhedra, it provides the infrastructure to develop advanced optimizations in LLVM and to connect complex external optimizers. In its first year of existence Polly already provides an exact value-based dependency analysis as well as basic SIMD and OpenMP code generation support. Furthermore, Polly can use PoCC(Pluto) an advanced optimizer for data-locality and parallelism.
Rubinius is an environment for running Ruby code which strives to write as much of the 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 deoptimization are all used to remove dynamism from ruby execution and increase performance.
FAUST is a compiled language for real-time audio signal processing. The name FAUST stands for Functional AUdio STream. Its programming model combines two approaches: functional programming and block diagram composition. In addition with the C, C++, JAVA output formats, the Faust compiler can now generate LLVM bitcode, and works with LLVM 2.7-2.9.
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 2.9 includes several major new capabilities:
LLVM IR has several new features for better support of new targets and that expose new optimization opportunities:
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:
unsigned long t = a+b; if (t < a) ...into:
addq %rdi, %rbx jno LBB0_2
The LLVM Machine Code (aka MC) subsystem was created to solve a number of problems in the realm of assembly, disassembly, object file format handling, and a number of other related areas that CPU instruction-set level tools work in.
For more information, please see the Intro to the LLVM MC Project Blog Post.
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:
New features and major changes in the X86 target include:
New features of the ARM target include:
If you're already an LLVM user or developer with out-of-tree changes based on LLVM 2.8, 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:
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.
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 C backend has numerous problems and is not being actively maintained. Depending on it for anything serious is not advised.
LLVM 2.9 will be the last release of llvm-gcc.
llvm-gcc is generally very stable for the C family of languages. 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).
Fortran support generally works, but there are still several unresolved bugs in Bugzilla. Please see the tools/gfortran component for details. Note that llvm-gcc is missing major Fortran performance work in the frontend and library that went into GCC after 4.2. If you are interested in Fortran, we recommend that you consider using dragonegg instead.
The llvm-gcc 4.2 Ada compiler has basic functionality, but is no longer being actively maintained. If you are interested in Ada, we recommend that you consider using dragonegg instead.
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.