LLVM 2.9 Release Notes

1. Introduction
2. Sub-project Status Update
3. External Projects Using LLVM 2.9
4. What's New in LLVM 2.9?
5. Installation Instructions
6. Known Problems
7. Additional Information

Written by the LLVM Team

# These are in-progress notes for the upcoming LLVM 2.9 release.
You may prefer the LLVM 2.8 Release Notes.

Introduction

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.

Sub-project Status Update

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

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:

Clang Static Analyzer

The Clang Static Analyzer project is an effort to use static source code analysis techniques to automatically find bugs in C and Objective-C programs (and hopefully C++ in the future!). The tool is very good at finding bugs that occur on specific paths through code, such as on error conditions.

The LLVM 2.9 release...

DragonEgg: llvm-gcc ported to gcc-4.5
NOTE: This should be written to be self-contained without referencing llvm-gcc.

DragonEgg is a port of llvm-gcc to gcc-4.5. Unlike llvm-gcc, dragonegg in theory does not require any gcc-4.5 modifications whatsoever (currently one small patch is needed) thanks to the new gcc plugin architecture. DragonEgg is a gcc plugin that makes gcc-4.5 use the LLVM optimizers and code generators instead of gcc's, just like with llvm-gcc.

DragonEgg is still a work in progress, but it is able to compile a lot of code, for example all of gcc, LLVM and clang. Currently Ada, C, C++ and Fortran work well, while all other languages either don't work at all or only work poorly. For the moment only the x86-32 and x86-64 targets are supported, and only on linux and darwin (darwin may need additional gcc patches).

The 2.9 release has the following notable changes:

VMKit: JVM Virtual Machine Implementation

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. UPDATE.

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. NEW: MIT License as well. New in LLVM 2.9, UPDATE

LLDB: Low Level Debugger

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.

LLDB is in early development and not included as part of the LLVM 2.9 release, UPDATE!

libc++: C++ Standard Library

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.

As of the LLVM 2.9 release, UPDATE!

KLEE: A Symbolic Execution Virtual Machine

KLEE 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 some algorithms.

UPDATE!

External Open Source Projects Using LLVM 2.9

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.

What's New in LLVM 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.

Major New Features

LLVM 2.9 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:

- TBAA.
- LTO has been improved to use MC for parsing inline asm and now can build large programs like Firefox 4 on both OS X and Linux.

MC Level Improvements

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.

- MC is now used by default for ELF systems on x86 and x86-64.
- MC supports and CodeGen uses the `.loc` directives for producing line number debug info. This produces more compact line tables.
- MC supports the `.cfi_*` directives for producing DWARF frame information, but it is still not used by CodeGen by default.
- COFF support?

For more information, please see the Intro to the LLVM MC Project Blog Post.

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:

FastISel for ARM.

X86-32 and X86-64 Target Improvements

New features and major changes in the X86 target include:

ARM Target Improvements

New features of the ARM target include:

Major Changes and Removed Features

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:

Development Infrastructure Changes

This section lists changes to the LLVM development infrastructure. This mostly impacts users who actively work on LLVM or follow development on mainline, but may also impact users who leverage the LLVM build infrastructure or are interested in LLVM qualification.

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.

- The Alpha, Blackfin, CellSPU, MicroBlaze, MSP430, MIPS, PTX, SystemZ and XCore backends are experimental.
- `llc` "`-filetype=obj`" is experimental on all targets other than darwin-i386 and darwin-x86_64. FIXME: Not true on ELF anymore?

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'.
- 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 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, 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.

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

- 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 numerous problems and is not being actively maintained. Depending on it for anything serious is not advised.

- 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 front-end

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.

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.