On Sat, Jul 9, 2016 at 7:39 PM Sean Silva <chisophu...@gmail.com> wrote:
> silvas added a comment. > > In http://reviews.llvm.org/D21823#479418, @davidxl wrote: > > > I should have brought it up earlier, but I forgot. I think a better > (and simpler) proposal is to make -fprofile-generate and -fprofile-use turn > on IR based PGO. > > > > -fprofile-generate and -fprofile-use options were introduced by Diego > (cc'ed) recently for GCC option compatibility. At that time, IR based PGO > was not yet ready, so they were made aliases to FE instrumentation options > -fprofile-instr-generate and -fprofile-instr-use. Now I think it is time > to make it right. The documentation only says that these two options are > gcc compatible options to control profile generation and use, but does not > specify the underlying implementation. It is also likely that Google is the > only user of this option. If a user using this option really want FE > instrumentation, there is always -fprofile-instr-generate to use. It also > more likely that IR instrumentation is what user wants when using GCC > compatible options (as they behave more similarly). > > > This SGTM. > Likewise. > > It may cause some user confusion since there is no obvious distinction > between "profile generate" and "profile instr generate" from a user > perspective. But we can avoid that with improved documentation. > > My main concern is that the existing documentation already says that > -fprofile-generate behaves identically to -fprofile-instr-generate > http://clang.llvm.org/docs/UsersManual.html#cmdoption-fprofile-generate > However, I think it is reasonable to change this behavior (and of course > the documentation), as users of this option are likely using it for > compatibility with GCC and so they likely don't care about the specifics of > clang FEPGO. > We probably want to at least leave a small note in the documentation > regarding this change of behavior. > I think the change in behaviour is needed. Better to make a clean break now that it is early enough in the PGO life cycle. Adding a note about it will help current users. Diego.
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