LLVM 2.8 Release Notes

1. Introduction
2. Sub-project Status Update
3. External Projects Using LLVM 2.8
4. What's New in LLVM 2.8?
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 2.8 release.
You may prefer the LLVM 2.7 Release Notes.

Introduction

This document contains the release notes for the LLVM Compiler Infrastructure, release 2.8. 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 2.8 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 and Objective-C on x86 (32- and 64-bit).

In the LLVM 2.8 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.

In the LLVM 2.8 time-frame,

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.

With the release of LLVM 2.8, ...

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 in LLVM 2.8: Soft float support

DragonEgg: llvm-gcc ported to gcc-4.5

DragonEgg is a port of llvm-gcc to gcc-4.5. Unlike llvm-gcc, which makes many intrusive changes to the underlying gcc-4.2 code, dragonegg in theory does not require any gcc-4.5 modifications whatsoever (currently one small patch is needed). This is thanks to the new gcc plugin architecture, which makes it possible to modify the behaviour of gcc at runtime by loading a plugin, which is nothing more than a dynamic library which conforms to the gcc plugin interface. DragonEgg is a gcc plugin that causes the LLVM optimizers to be run instead of the gcc optimizers, and the LLVM code generators instead of the gcc code generators, just like llvm-gcc. To use it, you add "-fplugin=path/dragonegg.so" to the gcc-4.5 command line, and gcc-4.5 magically becomes llvm-gcc-4.5!

DragonEgg is still a work in progress. Currently C works very well, while C++, Ada and Fortran work fairly well. 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 needs an additional gcc patch).

2.8 status here.

llvm-mc: Machine Code Toolkit

The LLVM Machine Code (aka MC) sub-project of LLVM 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. It is a sub-project of LLVM which provides it with a number of advantages over other compilers that do not have tightly integrated assembly-level tools. For a gentle introduction, please see the Intro to the LLVM MC Project Blog Post.

2.8 status here

External Open Source Projects Using LLVM 2.8

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 2.8.

What's New in LLVM 2.8?

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 Community Changes

In addition to changes to the code, between LLVM 2.7 and 2.8, a number of organization changes have happened:

Major New Features

LLVM 2.8 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:

- LLVM 2.8 changes the internal order of operands in `InvokeInst` and `CallInst`. To be portable across releases, resort to `CallSite` and the high-level accessors, such as `getCalledValue` and `setUnwindDest`.
- You can no longer pass use_iterators directly to cast<> (and similar), because these routines tend to perform costly dereference operations more than once. You have to dereference the iterators yourself and pass them in.
- llvm.memcpy.*, llvm.memset.*, llvm.memmove.* (and possibly other?) intrinsics take an extra parameter now (i1 isVolatile), totaling 5 parameters. If you were creating these intrinsic calls and prototypes yourself (as opposed to using Intrinsic::getDeclaration), you can use UpgradeIntrinsicFunction/UpgradeIntrinsicCall to be portable accross releases. Note that you cannot use Intrinsic::getDeclaration() in a backwards compatible way (needs 2/3 types now, in 2.7 it needed just 1).
- SetCurrentDebugLocation takes a DebugLoc now instead of a MDNode. Change your code to use SetCurrentDebugLocation(DebugLoc::getFromDILocation(...)).
- VISIBILITY_HIDDEN is gone.
- The `RegisterPass` and `RegisterAnalysisGroup` templates are considered deprecated, but continue to function in LLVM 2.8. Clients are strongly advised to use the upcoming `INITIALIZE_PASS()` and `INITIALIZE_AG_PASS()` macros instead.
- SMDiagnostic takes different parameters now. //FIXME: how to upgrade?
- The constructor for the Triple class no longer tries to understand odd triple specifications. Frontends should ensure that they only pass valid triples to LLVM. The Triple::normalize utility method has been added to help front-ends deal with funky triples.
- Some APIs got renamed:
  - llvm_report_error -> report_fatal_error
  - llvm_install_error_handler -> install_fatal_error_handler
  - llvm::DwarfExceptionHandling -> llvm::JITExceptionHandling

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:

- MachO writer works.

X86-32 and X86-64 Target Improvements

New features of the X86 target include:

- The X86 backend now supports holding X87 floating point stack values in registers across basic blocks, dramatically improving performance of code that uses long double, and when targetting CPUs that don't support SSE.

ARM Target Improvements

New features of the ARM target 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 2.7, this section lists some "gotchas" that you may run into upgrading from the previous release.

- .ll file doesn't produce #uses comments anymore, to get them, run a .bc file through "llvm-dis --show-annotations".
- MSIL Backend removed.
- ABCD and SSI passes removed.
- 'Union' LLVM IR feature removed.

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:

- 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 10.4 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.

- 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.
- There have been reports of Solaris and/or OpenSolaris build failures due to an incompatibility in the nm program as well. The nm from binutils does seem to work.

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, SPU, MIPS, PIC16, Blackfin, MSP430, SystemZ and MicroBlaze backends are experimental.
- `llc` "`-filetype=asm`" (the default) is the only supported value for this option. XXX Update me

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 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 and 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).

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.