[perf-training] Change profile file pattern string to use %4m instead of %p

[xinxinw1]

Summary

With %p, each test file that we're using to generate profile data will make its own profraw file which is around 60 MB in size. If we have a lot of test files, that quickly uses a lot of space. Use %4m instead to share the profraw files used to store the profile data. We use 4 here based on the default value in https://reviews.llvm.org/source/llvm-github/browse/master/llvm/CMakeLists.txt$604

[xiaobai]

Adding @vsk since he added the code that you're referencing in your summary.

[vsk]

Lgtm.

Separately, if you have the bandwidth to test out the new '%c%m' mode (https://clang.llvm.org/docs/SourceBasedCodeCoverage.html#running-the-instrumented-program / https://reviews.llvm.org/D68351), I'd appreciate any feedback about it. It should use ~4x less space.

[xinxinw1]

@vsk Thanks! The %c mode looks pretty cool but yeah, I don't really have the bandwidth to test it right now

clang/utils/perf-training/lit.cfg

 use_lit_shell = os.environ.get("LIT_USE_INTERNAL_SHELL")
 config.test_format = lit.formats.ShTest(use_lit_shell == "0")
 config.substitutions.append( ('%clang_cpp_skip_driver', ' %s %s %s ' % (cc1_wrapper, config.clang, sysroot_flags)))
 config.substitutions.append( ('%clang_cpp', ' %s --driver-mode=g++ %s ' % (config.clang, sysroot_flags)))
 config.substitutions.append( ('%clang_skip_driver', ' %s %s %s ' % (cc1_wrapper, config.clang, sysroot_flags)))
 config.substitutions.append( ('%clang', ' %s %s ' % (config.clang, sysroot_flags) ) )
 config.substitutions.append( ('%test_root', config.test_exec_root ) )
 
-config.environment['LLVM_PROFILE_FILE'] = 'perf-training-%p.profraw'
+config.environment['LLVM_PROFILE_FILE'] = 'perf-training-%4m.profraw'