LLVM 1.6 Release Notes
  1. Introduction
  2. What's New?
  3. Installation Instructions
  4. Portability and Supported Platforms
  5. Known Problems
  6. Additional Information

Written by the LLVM Team


This document contains the release notes for the LLVM compiler infrastructure, release 1.6. Here we describe the status of LLVM, including any known problems and major improvements from the previous release. The most up-to-date version of this document can be found on the LLVM 1.6 web site. If you are not reading this on the LLVM web pages, you should probably go there because this document may be updated after the release.

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 CVS or the main LLVM web page, this document applies to the next release, not the current one. To see the release notes for the current or previous releases, see the releases page.

What's New?

This is the seventh public release of the LLVM Compiler Infrastructure. This release incorporates a large number of enhancements and additions (primarily in the code generator), which combine to improve the quality of the code generated by LLVM by up to 30% in some cases. This release is also the first release to have first-class support for Mac OS X: all of the major bugs have been shaken out and it is now as well supported as Linux on X86.

New Features in LLVM 1.6
Instruction Selector Generation from Target Description

LLVM now includes support for auto-generating large portions of the instruction selectors from target descriptions. This allows us to write patterns in the target .td file, instead of writing lots of nasty C++ code. Most of the PowerPC instruction selector is now generated from the PowerPC target description files and other targets are adding support that will be live for LLVM 1.7.

For example, here are some patterns used by the PowerPC backend. A floating-point multiply then subtract instruction (FMSUBS):

(set F4RC:$FRT, (fsub (fmul F4RC:$FRA, F4RC:$FRC), F4RC:$FRB))

Exclusive-or by 16-bit immediate (XORI):

(set GPRC:$dst, (xor GPRC:$src1, immZExt16:$src2))

Exclusive-or by 16-bit immediate shifted right 16-bits (XORIS):

(set GPRC:$dst, (xor GPRC:$src1, imm16Shifted:$src2))

With these definitions, we teach the code generator how to combine these two instructions to xor an abitrary 32-bit immediate with the following definition. The first line specifies what to match (a xor with an arbitrary immediate) the second line specifies what to produce:

def : Pat<(xor GPRC:$in, imm:$imm),
          (XORIS (XORI GPRC:$in, (LO16 imm:$imm)), (HI16 imm:$imm))>;

Instruction Scheduling Support

Instruction selectors using the refined instruction selection framework can now use a simple pre-pass scheduler included with LLVM 1.6. This scheduler is currently simple (cannot be configured much by the targets), but will be extended in the future.

Code Generator Subtarget Support

It is now straight-forward to parameterize a target implementation, and provide a mapping from CPU names to sets of target parameters. LLC now supports a -mcpu=cpu option that lets you choose a subtarget by CPU name: use "llvm-as < /dev/null | llc -march=XXX -mcpu=help" to get a list of supported CPUs for target "XXX". It also provides a -mattr=+attr1,-attr2 option that can be used to control individual features of a target (the previous command will list available features as well).

This functionality is nice when you want tell LLC something like "compile to code that is specialized for the PowerPC G5, but doesn't use altivec code. In this case, using "llc -march=ppc32 -mcpu=g5 -mattr=-altivec".

Other New Features
  1. The JIT now uses mutexes to protect its internal data structures. This allows multi-threaded programs to be run from the JIT or interpreter without corruption of the internal data structures. See PR418 and PR540 for the details.
  2. LLVM on Win32 no longer requires sed, flex, or bison when compiling with Visual C++.
  3. The llvm-test suite can now use the NAG Fortran to C compiler to compile SPEC FP programs if available (allowing us to test all of SPEC'95 & 2000).
  4. When bugpoint is grinding away and the user hits ctrl-C, it now gracefully stops and gives what it has reduced so far, instead of giving up completely. In addition, the JIT debugging mode of bugpoint is much faster.
  5. LLVM now includes Xcode project files in the llvm/Xcode directory.
  6. LLVM now supports Mac OS X on Intel.
  7. LLVM now builds cleanly with GCC 4.1.
Code Quality Improvements in LLVM 1.6
  1. The -globalopt pass can now statically evaluate C++ static constructors when they are simple enough. For example, it can now statically initialize "struct X { int a; X() : a(4) {} } g;".
  2. The Loop Strength Reduction pass has been completely rewritten, is far more aggressive, and is turned on by default in the RISC targets. On PPC, we find that it often speeds up programs from 10-40% depending on the program.
  3. The code produced when exception handling is enabled is far more efficient in some cases, particularly on Mac OS X.
Code Generator Improvements in LLVM 1.6
  1. The Alpha backend is substantially more stable and robust than in LLVM 1.5. For example, it now fully supports varargs functions. The Alpha backend also now features beta JIT support.
  2. The code generator contains a new component, the DAG Combiner. This allows us to optimize lowered code (e.g. after 64-bit operations have been lowered to use 32-bit registers on 32-bit targets) and do fine-grained bit-twiddling optimizations for the backend.
  3. The SelectionDAG infrastructure is far more capable and mature, able to handle many new target peculiarities in a target-independent way.
  4. The default register allocator is now far faster on some testcases, particularly on targets with a large number of registers (e.g. IA64 and PPC).
Significant Bugs Fixed in LLVM 1.6
  1. A vast number of bugs have been fixed in the PowerPC backend and in llvm-gcc when configured for Mac OS X (particularly relating to ABI issues). For example: PR449, PR594, PR603, PR609, PR630, PR643, and several others without bugzilla bugs.
  2. Several bugs in tail call support have been fixed.
  3. configure does not correctly detect gcc version on cygwin.
  4. Many many other random bugs have been fixed. Query our bugzilla with a target of 1.6 for more information.
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 all known problems with the LLVM system, listed by component. As new problems are discovered, they will be added to these sections. 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 LLVM Core
Known problems with the C front-end

If you run into GCC extensions which have not been included in any of these lists, please let us know (also including whether or not they work).

Known problems with the C++ front-end

For this release, the C++ front-end is considered to be fully tested and works for a number of non-trivial programs, including LLVM itself.

Known problems with the C back-end
Known problems with the X86 back-end
Known problems with the PowerPC back-end
Known problems with the SparcV9 back-end
Known problems with the Alpha back-end
Known problems with the IA64 back-end
Known problems with the SPARC-V8 back-end
Additional Information

A wide variety of additional information is available on the LLVM web page, including documentation and publications describing algorithms and components implemented in LLVM. The web page also contains versions of the API documentation which is up-to-date with the CVS 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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Last modified: $Date: 2005/11/08 20:06:34 $