On September 13, 2017 6:24:21 PM GMT+02:00, Jan Hubicka <hubi...@ucw.cz> wrote: >> >I don't see static profile prediction to be very useful here to find >> >"really >> >hot code" - neither in current implementation or future. The problem >of >> >-O2 is that we kind of know that only 10% of code somewhere matters >for >> >performance but we have no way to reliably identify it. >> >> It's hard to do better than statically look at (ipa) loop depth. But >shouldn't that be good enough? > >Only if you assume that you have whole program and understand indirect >calls. >There are some stats on this here >http://ieeexplore.ieee.org/document/717399/ > >It shows that propagating static profile across whole progrma (which is >just >tiny bit more fancy than counting loop depth) sort of work >statistically. I >really do not have very high hopes of this reliably working in >production >compiler. We already have PRs for single function benchmark where deep >loop >nest is used ininitialization or so and the actual hard working part >has small >loop nest & gets identified as cold. > >As soon as you start propagating in whole program context, such local >mistakes >will become more comon.
Heh, I would just make loop nests hot without globally making anything cold because of that. Basically sth like optimistic ipa profile propagation. Richard. >> >> > >> >I would make sense to have less agressive vectoriazaoitn at -O2 and >> >more at >> >-Ofast/-O3. >> >> We tried that but the runtime effects were not offsetting the compile >time cost. > >Yep, i remember that. > >Honza
