LLVM 1.5 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 1.5. 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 1.5 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 sixth public release of the LLVM Compiler Infrastructure.

LLVM 1.5 is known to correctly compile a wide range of C and C++ programs, includes bug fixes for those problems found since the 1.4 release, and includes a large number of new features and enhancements, described below.

New Features in LLVM 1.5
New Native Code Generators

This release includes new native code generators for Alpha, IA-64, and SPARC-V8 (32-bit SPARC). These code generators are still beta quality, but are progressing rapidly. The Alpha backend is implemented with an eye towards being compatible with the widely used SimpleScalar simulator.

New Instruction Selector Framework

This release includes a new framework for building instruction selectors, which has long been the hardest part of building a new LLVM target. This framework handles a lot of the mundane (but easy to get wrong) details of writing the instruction selector, such as generating efficient code for getelementptr instructions, promoting small integer types to larger types (e.g. for RISC targets with one size of integer registers), expanding 64-bit integer operations for 32-bit targets, etc. Currently, the X86, PowerPC, Alpha, and IA-64 backends use this framework. The SPARC backends will be migrated when time permits.

New Support for Per-Function Calling Conventions

LLVM 1.5 adds supports for per-function calling conventions. Traditionally, the LLVM code generators match the native C calling conventions for a target. This is important for compatibility, but is not very flexible. This release allows custom calling conventions to be established for functions, and defines three target-independent conventions (C call, fast call, and cold call) which may be supported by code generators. When possible, the LLVM optimizer promotes C functions to use the "fastcc" convention, allowing the use of more efficient calling sequences (e.g., parameters are passed in registers in the X86 target).

Targets may now also define target-specific calling conventions, allowing LLVM to fully support calling convention altering options (e.g. GCC's -mregparm flag) and well-defined target conventions (e.g. stdcall and fastcall on X86).

New Support for Proper Tail Calls

The release now includes support for proper tail calls, as required to implement languages like Scheme. Tail calls make use of two features: custom calling conventions (described above), which allow the code generator to use a convention where the caller deallocates its stack before it returns. The second feature is a flag on the call instruction, which indicates that the callee does not access the caller's stack frame (indicating that it is acceptable to deallocate the caller stack before invoking the callee). LLVM proper tail calls run on the system stack (as do normal calls), supports indirect tail calls, tail calls with arbitrary numbers of arguments, tail calls where the callee requires more argument space than the caller, etc. The only case not supported are varargs calls, but that could be added if desired.

To ensure a call is interpreted as a tail call, a front-end must mark functions as "fastcc", mark calls with the 'tail' marker, and follow the call with a return of the called value (or void). The optimizer and code generator attempt to handle more general cases, but the simple case will always work if the code generator supports tail calls. Here is an example:

    fastcc int %bar(int %X, int(double, int)* %FP) {       ; fastcc
        %Y = tail call fastcc int %FP(double 0.0, int %X)  ; tail, fastcc
        ret int %Y

In LLVM 1.5, the X86 code generator is the only target that has been enhanced to support proper tail calls (other targets will be enhanced in future). Further, because this support was added very close to the release, it is disabled by default. Pass -enable-x86-fastcc to llc to enable it (this will be enabled by default in the next release). The example above compiles to:

        sub ESP, 8                   # Callee uses more space than the caller
        mov ECX, DWORD PTR [ESP + 8] # Get the old return address
        mov DWORD PTR [ESP + 4], 0   # First half of 0.0
        mov DWORD PTR [ESP + 8], 0   # Second half of 0.0
        mov DWORD PTR [ESP], ECX     # Put the return address where it belongs
        jmp EDX                      # Tail call "FP"

With fastcc on X86, the first two integer arguments are passed in EAX/EDX, the callee pops its arguments off the stack, and the argument area is always a multiple of 8 bytes in size.

Other New Features
  1. LLVM now includes an Interprocedural Sparse Conditional Constant Propagation pass, named -ipsccp, which is run by default at link-time.
  2. LLVM 1.5 is now about 15% faster than LLVM 1.4 and its core data structures use about 30% less memory.
  3. Support for Microsoft Visual Studio is improved, and now documented. Most LLVM tools build natively with Visual C++ now.
  4. Configuring LLVM to build a subset of the available targets is now implemented, via the --enable-targets= option.
  5. LLVM can now create native shared libraries with 'llvm-gcc ... -shared -Wl,-native' (or with -Wl,-native-cbe).
  6. LLVM now supports a new "llvm.prefetch " intrinsic, and llvm-gcc now supports __builtin_prefetch.
  7. LLVM now supports intrinsics for bit counting and llvm-gcc now implements the GCC __builtin_popcount, __builtin_ctz, and __builtin_clz builtins.
  8. LLVM now mostly builds on HP-UX with the HP aCC Compiler.
  9. The LLVM X86 backend can now emit Cygwin-compatible .s files.
  10. LLVM now includes workarounds in the code generator generator which reduces the likelyhood of GCC hitting swap during optimized builds.
  11. The LLVM Transformation Visualizer (llvm-tv) project has been updated to work with LLVM 1.5.
  12. Nightly tester output is now archived on the llvm-testresults mailing list.
Code Quality Improvements in LLVM 1.5
  1. The new -simplify-libcalls pass improves code generated for well-known library calls. The pass optimizes calls to many of the string, memory, and standard I/O functions (e.g. replace the calls with simpler/faster calls) when possible, given information known statically about the arguments to the call.
  2. The -globalopt pass now promotes non-address-taken static globals that are only accessed in main to SSA registers.
  3. Loops with trip counts based on array pointer comparisons (e.g. "for (i = 0; &A[i] != &A[n]; ++i) ...") are optimized better than before, which primarily helps iterator-intensive C++ code.
  4. The optimizer now eliminates simple cases where redundant conditions exist between neighboring blocks.
  5. The reassociation pass (which turns (1+X+3) into (X+1+3) among other things), is more aggressive and intelligent.
  6. The -prune-eh pass now detects no-return functions in addition to the no-unwind functions it did before.
  7. The -globalsmodref alias analysis generates more precise results in some cases.
Code Generator Improvements in LLVM 1.5
  1. The code generator now can provide and use information about commutative two-address instructions when performing register allocation.
  2. The code generator now tracks function live-in registers explicitly, instead of requiring the target to generate 'implicit defs' at the entry to a function.
  3. The code generator can lower integer division by a constant to multiplication by a magic constant and multiplication by a constant into shift/add sequences.
  4. The code generator compiles fabs/fneg/sin/cos/sqrt to assembly instructions when possible.
  5. The PowerPC backend generates better code in many cases, making use of FMA instructions and the recording ("dot") forms of various PowerPC instructions.
Significant Bugs Fixed in LLVM 1.5

Bugs fixed in the LLVM Core:

  1. [dse] DSE deletes stores that are partially overwritten by smaller stores
  2. [instcombine] miscompilation of setcc or setcc in one case
  3. Transition code for LLVM 1.0 style varargs was removed from the .ll file parser. LLVM 1.0 bytecode files are still supported.

Code Generator Bugs:

  1. [cbackend] Logical constant expressions (and/or/xor) not implemented.
  2. [cbackend] C backend does not respect 'volatile'.
  3. The JIT sometimes miscompiled globals and constant pool entries for 64-bit integer constants on 32-bit hosts.
  4. The C backend should no longer produce code that crashes ICC 8.1.

Bugs in the C/C++ front-end:

  1. [llvmgcc] llvm-gcc incorrectly rejects some constant initializers involving the addresses of array elements
  2. [llvm-g++] Crash compiling anonymous union
  3. [llvm-g++] Do not use dynamic initialization where static init will do
  4. [llvmgcc] Field offset miscalculated for some structure fields following bit fields
  5. [llvm-g++] Temporary lifetimes incorrect for short circuit logical operations
  6. [llvm-gcc] Crash compiling bitfield <-> aggregate assignment
  7. [llvm-g++] Error compiling virtual function thunk with an unnamed argument
  8. [llvm-gcc] Crash on certain C99 complex number routines
  9. [llvm-g++] Crash using placement new on an array type
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: 2005/06/29 16:22:25 $