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Current PGO profile counts are not context sensitive. The branch probabilities for the inlined functions are kept the same for all call-sites, and they might be very different from the actual branch probabilities. These suboptimal profiles can greatly affect some downstream optimizations, in particular for the machine basic block placement optimization. In this patch, we propose to have a post-inline PGO instrumentation/use pass, which we called Context Sensitive PGO (CSPGO). For the users who want the best possible performance, they can perform a second round of PGO instrument/use on the top of the regular PGO. They will have two sets of profile counts. The first pass profile will be manly for inline, indirect-call promotion, and CGSCC simplification pass optimizations. The second pass profile is for post-inline optimizations and code-gen optimizations. A typical usage: // Regular PGO instrumentation and generate pass1 profile. > clang -O2 -fprofile-generate source.c -o gen > ./gen > llvm-profdata merge default.*profraw -o pass1.profdata // CSPGO instrumentation. > clang -O2 -fprofile-use=pass1.profdata -fcs-profile-generate -o gen2 > ./gen2 // Merge two sets of profiles > llvm-profdata merge default.*profraw pass1.profdata -o profile.profdata // Use the combined profile. Pass manager will invoke two PGO use passes. > clang -O2 -fprofile-use=profile.profdata -o use This change touches many components in the compiler. The reviewed patch (D54175) will committed in phrases. Differential Revision: https://reviews.llvm.org/D54175 llvm-svn: 354930 |
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