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Introduction

This is a small python-based wrapper around a GCC-compatible compiler to make it easy to build whole-program (or whole-library) LLVM bitcode files. The idea is that it first invokes the compiler as normal to build a real object file. It then invokes a bitcode compiler to generate the corresponding bitcode, recording the location of the bitcode file in an ELF section of the actual object file.

When object files are linked together, the contents of non-special ELF sections are just concatenated (so we don't lose the locations of any of the constituent bitcode files). This package contains an extra utility, extract-bc, to read the contents of this ELF section and link all of the bitcode into a single whole-program bitcode file.

This two-phase build process is slower and more elaborate than normal, but in practice is necessary to be a drop-in replacement for gcc in any build system. Approaches using the LTO framework in gcc and the gold linker plugin work for many cases, but fail in the presence of static libraries in builds. This approach has the distinct advantage of generating working binaries, in case some part of a build process actually requires that.

Currently, this package only works using clang or the dragonegg plugin with gcc 4.5 (with the required patch for dragonegg).

Usage

There are three environment variables that must be set to use this wrapper script:

  • LLVM_COMPILER should be set to 'dragonegg' or 'clang'.
  • LLVM_GCC_PREFIX should be set to the prefix for the version of gcc that should be used with dragonegg. This can be empty if there is no prefix. This variable is not used if $LLVM_COMPILER == clang.
  • LLVM_DRAGONEGG_PLUGIN should be the full path to the dragonegg plugin. This variable is not used if $LLVM_COMPILER == clang.

Once the environment is set up, just use wllvm and wllvm++ as your C and C++ compilers, respectively.

Example

export LLVM_COMPILER=dragonegg
export LLVM_GCC_PREFIX=llvm-
export LLVM_DRAGONEGG_PLUGIN=/unsup/llvm-2.9/lib/dragonegg.so

tar xf pkg-config-0.26.tar.gz
cd pkg-config-0.26
CC=wllvm ./configure
make

# Produces pkg-config.bc
extract-bc pkg-config