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Introduction to WLLVM

This project, WLLVM, provides tools for building whole-program (or whole-library) LLVM bitcode files from an unmodified C or C++ source package. It currently runs on *nix platforms such as Linux, FreeBSD, and Mac OS X.

WLLVM provides python-based compiler wrappers that work in two steps. The wrappers first invoke the compiler as normal. Then, for each object file, they call a bitcode compiler to produce LLVM bitcode. The wrappers also store the location of the generated bitcode file in a dedicated section of the object file. When object files are linked together, the contents of the dedicated sections are concatenated (so we don't lose the locations of any of the constituent bitcode files). After the build completes, one can use an WLLVM utility to read the contents of the dedicated section and link all of the bitcode into a single whole-program bitcode file. This utility works for both executable and native libraries.

This two-phase build process is necessary to be a drop-in replacement for gcc or g++ in any build system. Using the LTO framework in gcc and the gold linker plugin works in many cases, but fails in the presence of static libraries in builds. WLLVM's approach has the distinct advantage of generating working binaries, in case some part of a build process requires that.

WLLVM works with either clang or the gcc dragonegg plugin.

Usage

WLLVM includes four python executables: wllvm for compiling C code and wllvm++ for compiling C++, an auxiliary tool extract-bc for extracting the bitcode from a build product (object file, executable, library or archive), and a sanity checker, wllvm-sanity-checker for detecting configuration oversights.

Three environment variables must be set to use these wrappers:

  • LLVM_COMPILER should be set to either 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.

In addition to the above environment variables the following can be optionally used:

  • LLVM_CC_NAME can be set if your clang compiler is not called clang but something like clang-3.7. Similarly LLVM_CXX_NAME can be used to describe what the C++ compiler is called. Note that in these sorts of cases, the environment variable LLVM_COMPILER should still be set to clang not clang-3.7 etc. We also pay attention to the environment variables LLVM_LINK_NAME and LLVM_AR_NAME in an analagous way, since they too get adorned with suffixes in various Linux distributions.
  • LLVM_COMPILER_PATH can be set to the absolute path to the folder that contains the compiler and other LLVM tools such as llvm-link to be used. This prevents searching for the compiler in your PATH environment variable. This can be useful if you have different versions of clang on your system and you want to easily switch compilers without tinkering with your PATH variable. Example LLVM_COMPILER_PATH=/home/user/llvm_and_clang/Debug+Asserts/bin.
  • WLLVM_CONFIGURE_ONLY can be set to anything. If it is set, wllvm and wllvm++ behave like a normal C or C++ compiler. They do not produce bitcode. Setting WLLVM_CONFIGURE_ONLY may prevent configuration errors caused by the unexpected production of hidden bitcode files.

Documentation

More detailed documentation as well as some tutorials can be found here:

https://github.com/SRI-CSL/whole-program-llvm