On Thu, Aug 04, 2011 at 05:16:25PM +0200, Richard Guenther wrote: > -fprofile-use enables quite some optimizations that are even off for -O3 > which are -funroll-loops and -fpeel-loops, -ftracer and -funswitch-loops. > Those will all be increasing code-size (hopefully only for hot code pieces > though). > > Did you try using FDO with -Os? FDO should make hot code parts > optimized similar to -O3 but leave other pieces optimized for size. > Using FDO with -O3 gives you the opposite, cold portions optimized > for size while the rest is optimized for speed.
Yes I initially did, and the results were quite similar, in fact (that is, regression on Sunspider ; I hadn't checked V8 by then) I however don't see the difference between the two cases you describe, except if hot and cold parts combined is not the whole. Experimentation shows that FDO -Os and FDO -O3 are different, though. Jan Hubicka also commented on my blog where I mentioned these FDO issues. Since I think it makes more sense to have the discussion happening in one place only (and this list is more appropriate), I'll put answers to his questions below: > I never actually looked on PDO on ARM, but the slowdowns should not > really happen. Would be possible to analyse this bit further (i.e. > figure out what slows down) so we could fix it for 4.7? The android packages are linked from my blog, and they can be unzipped. For convenience to the list readers, here is a link to the files: http://people.mozilla.org/~mhommey/pgo/ The .nightly.apk file is a build from our build bots (standard NDK, GCC 4.4). The .gcc4.6.apk file is a GCC 4.6 -Os build. The pgo.apk file is a GCC 4.6 -O3 FDO build. The corresponding source code is: http://hg.mozilla.org/mozilla-central/file/1dddaeb1366b Please tell me if you need more data, like object files, unstripped libraries, etc. > Also do you get any warnings on profile mismatches? Perhaps something > is wrong to the degree that the relevant part of profile gets > misapplied. I don't get any warning on profile mismatches. I only get a "few" missing gcda files warning, but that's expected. Cheers, Mike
