thinlto.md

Extracting Corpora for ThinLTO-enabled Projects

These steps use inlining for size as an example.

Generate the corpus

Pass -thinlto-emit-index-files -save-temps=import to lld, this will output .3.import.bc and .thinlto.bc files in your source directory.

Consolidate the corpus

Run tools/extract_ir.py with --thinlto_build=local and set --obj_base_dir, --output_dir accordingly.

Modify corpus_description.json

Modification of corpus_description.json which is written to output_dir is necessary. An error will be thrown if this is not done.
The global_command_override field in the json needs to be filled with the options to run clang with on each module. These options should be inferred from the lld command that generated the corpus. Most importantly, it should include "-c", some "-O" flag, relevant mllvm flags, and target/arch flags.

Here's an example based on Chrome:

{
  "global_command_override": [
    "-fPIC",
    "-mllvm",
    "-instcombine-lower-dbg-declare=0",
    "-mllvm",
    "-import-instr-limit=5",
    "-march=armv7-a",
    "--target=arm-linux-androideabi23",
    "-no-canonical-prefixes",
    "-O2",
    "-nostdlib++",
    "--sysroot=/path/to/linux-x86_64/sysroot",
    "-c"
  ]
}