[PATCH] D15829: [PGO] Clang Option that enables IR level PGO instrumentation

[Sean Silva via cfe-commits]

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