On Wed, Oct 7, 2015 at 9:11 AM, Maxim Ostapenko <m.ostape...@partner.samsung.com> wrote: > Hi, > > when testing OpenSSL performance, I found out that sometimes PGO-built > binaries can actually introduce performance regressions. We could identify > affected object files and disable PGO for them by simply removing > corresponding .gcda file. However, even if profile data is not presented, > GCC doesn't switch back -fprofile-use dependent optimizations (e.g. > -fbranch-probabilities, -fvpt, etc). This may also lead to performance > degradations. > > The issue had already raised quite time ago > (https://gcc.gnu.org/ml/gcc-patches/2009-09/msg02119.html), but for some > reasons wasn't discussed. > > Here a draft patch that disables -fprofile-use related optimizations if > profile data wasn't found (perhaps it makes sense to introduce a special > switch for this?). Does this look reasonable?
I thought if profile is not present, then branch probabilities goes back to the original heuristics? Which option is really causing the performance degradation here? Also I think your patch is very incomplete as someone could use -frename-registers with -fprofile-use and then you just turned it off. Thanks, Andrew Pinski > > Thanks, > -Maxim
