drepper/gcc-passes

gcc plugin to discover optimization passes used during compilation

Discover GCC Optimization Passes

This project is a script which, in conjunction with the included GCC plugin, allows to discover which optimization passes GCC executes for which optimization level. All the levels of -O are supported (0, g, 1, s, 2, 3, fast).

When the script is called with one of the levels as the command line argument it shows all the passes by name, nested according to the part of the compiler they belong too.

When the script is given two parameters the passes used in the invocations according to the two optimization levels are compared and differences highlighted.

As an example see the beginning of the output of the following command:

 $ ./gcc-passes.sh 0
 GIMPLE pass:
     pass *warn_unused_result
     pass omplower
     pass lower
     pass ehopt
     pass eh
     pass cfg
     pass *warn_function_return
     pass ompexp
     pass *build_cgraph_edges
 simple IPA pass:
     pass *free_lang_data
     pass visibility
     pass build_ssa_passes
 GIMPLE pass:
     pass    fixup_cfg
     pass    ssa
     pass    *rebuild_cgraph_edges
 simple IPA pass:
     pass opt_local_passes
 GIMPLE pass:
     pass    fixup_cfg
     pass    *rebuild_cgraph_edges
     pass    local-fnsummary
     pass    einline

This output shows the different types of passes (GIMPLE, IPA, …). This might vary from compiler version to version. What this also shows that even with -O0 some optimization passes are executed.

Author: Ulrich Drepper drepper@gmail.com