LLVM 2.7 Release Notes

1. Introduction
2. Sub-project Status Update
3. External Projects Using LLVM 2.7
4. What's New in LLVM 2.7?
5. Installation Instructions
6. Portability and Supported Platforms
7. Known Problems
8. Additional Information

Written by the LLVM Team

# These are in-progress notes for the upcoming LLVM 2.7 release.
You may prefer the LLVM 2.6 Release Notes.

Introduction

This document contains the release notes for the LLVM Compiler Infrastructure, release 2.7. 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 2.7 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 ...

In the LLVM 2.7 time-frame, the Clang team has made many improvements:

- ...

Clang Static Analyzer

Previously announced in the 2.4, 2.5, and 2.6 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 2.7 time-frame, the analyzer core has ...

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 ?? builds with LLVM 2.7 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 ...

llvm-mc: Machine Code Toolkit

The LLVM Machine Code (MC) Toolkit project is ...

External Open Source Projects Using LLVM 2.7

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 2.7.

Rubinius

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

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

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 ??? and later have been tested and are known to work with LLVM 2.7 (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

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 2.7?

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 2.7 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

- The JIT now defaults to compiling eagerly to avoid a race condition in the lazy JIT. Clients that still want the lazy JIT can switch it on by calling `ExecutionEngine::DisableLazyCompilation(false)`.
- It is now possible to create more than one JIT instance in the same process. These JITs can generate machine code in parallel, although you still have to obey the other threading restrictions.

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:

- Variable arguments.
- Interrupts/programs.

ARM Target Improvements

New features of the ARM target include:

- ...

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 2.6, this section lists some "gotchas" that you may run into upgrading from the previous release.

- The LLVM interpreter now defaults to not using `libffi` even if you have it installed. This makes it more likely that an LLVM built on one system will work when copied to a similar system. To use `libffi`, configure with `--enable-libffi`.

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

- `ModuleProvider` has been removed and its methods moved to `Module` and `GlobalValue`. Most clients can remove uses of `ExistingModuleProvider`, replace `getBitcodeModuleProvider` with `getLazyBitcodeModule`, and pass their `Module` to functions that used to accept `ModuleProvider`. Clients who wrote their own `ModuleProvider`s will need to derive from `GVMaterializer` instead and use `Module::setMaterializer` to attach it to a `Module`.
- `GhostLinkage` has given up the ghost. `GlobalValue`s that have not yet been read from their backing storage have the same linkage they will have after being read in. Clients must replace calls to `GlobalValue::hasNotBeenReadFromBitcode` with `GlobalValue::isMaterializable`.
- FIXME: Debug info has been totally redone. Add pointers to new APIs. Substantial caveats about compatibility of .ll and .bc files.
- The `llvm/Support/DataTypes.h` header has moved to `llvm/System/DataTypes.h`.
- The `isInteger`, `isIntOrIntVector`, `isFloatingPoint`, `isFPOrFPVector` and `isFPOrFPVector` methods have been renamed `isIntegerTy`, `isIntOrIntVectorTy`, `isFloatingPointTy`, `isFPOrFPVectorTy` and `isFPOrFPVectorTy` respectively.

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 x86 and AMD64 machines running Solaris 10, OpenSolaris 0906.
- 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.

- The llvm-gcc bootstrap will fail with some versions of binutils (e.g. 2.15) with a message of "`Error: can not do 8 byte pc-relative relocation`" when building C++ code. We intend to fix this on mainline, but a workaround is to upgrade to binutils 2.17 or later.
- 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/x86-64 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, PIC16, Blackfin, MSP430 and SystemZ backends are experimental.
- The `llc` "`-filetype=asm`" (the default) is the only supported value for this option. The ELF writer is experimental.
- The implementation of Andersen's Alias Analysis has many known bugs.

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

- Support for the Advanced SIMD (Neon) instruction set is still incomplete and not well tested. Some features may not work at all, and the code quality may be poor in some cases.
- 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 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

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.

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.