For the longer term, one possible solution is to make FE based instrumentation only used for coverage testing which can be turned on with -fcoverage-mapping option (currently, -fcoverage-mapping can not be used alone and must be used together with -fprofile-instr-generate). To summarize:
A. Current behavior: ----------------------- 1) -fprofile-instr-generate turns on FE based instrumentation 2) -fprofile-instr-generate -fcoverage-mapping turns on FE based instrumentation and coverage mapping data generation. 3) -fprofile-instr-use=<..> assumes profile data from FE instrumentation. B. Proposed new behavior: -------------------------------- 1) -fprofile-instr-generate turns on IR late instrumentation 2) -fcoverage-mapping turns on FE instrumentation and coverage-mapping 3) -fprofile-instr-generate -fcoverage-mapping result in compiler warning 4) -fprofile-instr-use=<> will automatically determine how to use the profile data. B.2) above can be done today for improved usability. B.1) needs a transition period before the IR based instrumentation becomes stablized (and can be flipped to the default). During the transition period, the behavior of 1) does not change, but a cc1 option can be used to turn on IR instrumentation (as proposed by Sean). In the real longer term, I think IR based instrumentation can also be used for coverage testing too (by disabling some pre-optimizations and pre-inlining needed for PGO purpose) -- but this is a different topic to be discussed. thanks, David On Thu, Jan 21, 2016 at 7:40 PM, Sean Silva <chisophu...@gmail.com> wrote: > silvas added a comment. > > @bogner btw, did you say that at Apple you guys have a requirement of > supporting existing profdata? I.e. users can pass older profdata to a newer > compiler? > > Realistically, it would be nice if our PGO offering defaulted to the IR stuff > (since it seems like it is going to be the focus of optimization work and so > is likely to be our best-performing offering). But if we need to support > profdata across versions of clang, that puts us in a thorny situation because > suddenly `clang -c foo.c > -fprofile-instr-use=old-profdata-from-frontend-instrumentation.profdata` has > a default meaning equivalent to `clang -c foo.c -fir-level-instrumentation > -fprofile-instr-use=old-profdata-from-frontend-instrumentation.profdata` > (setting aside the flag name issue), which is a case we want to error on due > to mismatch between frontend and IR-level instrumentation. > > > http://reviews.llvm.org/D15829 > > > _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
