/llvm-function-split

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How to use the split utility

The split utility operates on a LLVM bitcode file and it splits all of the functions in it into separate (.bc) files.

Obtaining the final "fat" LLVM bitcode file can, theoretically, be done in multiple ways. The recommended way is to use CapableVMs fork of "Go Whole Program LLVM", gllvm. For example, to get the bitcode from a generic Makefile project:

  • Install gllvm: go get github.com/capablevms/gllvm/cmd/...

  • Build with GLLVM:

GCLANG=$HOME/go/bin/gclang
export GET_BC=$HOME/go/bin/get-bc
CC=$GCLANG LLVM_COMPILER_PATH=path/to/llvm/bin make
# or CC=$GCLANG ./configure if you are feeling lucky
  • Extract the bitcode:

Finally, the split utility can be launched with:

./split-llvm-extract path/to/binary.bc -o outdir

To run the version of the binary just split, it is necessary to join the parts (in outdir) together as shared libraries. A "joiner" Makefile can be found here.

Steps specific to CHERI cross-compilation can be found in .buildbot.sh.

Other (old and not maintained) variants

There are 2 other utilities that reside in this repository. The firstone shows how splitting can be done using the llvm API instead of using llvm-extract. The other utility focuses on finding functions which are not public and should be. It also tries to find ways to split the bitcode files while preserving the linkage of the functions. The 2 previously presented splitters are not meant for general purpose use, and are kept as a showcase of what is possible.