2011/8/4 Xinliang David Li <davi...@google.com>: > +Mark who has done size optimization tuning with FDO. > > On Thu, Aug 4, 2011 at 7:05 AM, Mike Hommey <mhom...@mozilla.com> wrote: >> Hi, >> >> We (Mozilla) are trying to get the best of the ARM toolchain for our >> Android build. I recently built an Android Native-code Development Kit >> with GCC 4.6.1 and binutils 2.21.53, instead of GCC 4.4.3 and binutils >> 2.19 that come with the default NDK. >> >> LTO doesn't work at all, I'm getting an ICE that looks like the one from >> bug 41159. >> >> FDO however, works, but sadly, the resulting build is not only quite >> bigger, > > Is this true for both 4.6 and 4.4 gcc? There is a bug in 4.6 that > prevents cold functions from be optimized for size with FDO. The bug > was fixed in trunk recently. > >> it's also slower on some tests (the Sunspider javascript >> benchmark). While we have seen improvements on other tests (most >> notably, the V8 benchmark is much faster) by switching to GCC 4.6 (that >> is, without FDO), FDO doesn't seem to bring anything on the table. It >> even seems to bring performance regression. > > ARM specific performance tuning (with FDO) seems needed. More > parameters (e.g, in inliner related) may need to be made target > dependent. >> >> Note that we do our normal builds with -Os and use -O3 for FDO. As for >> architecture specific flags, we use -marmv7-a -mthumb -mfloat-abi=softfp >> -mfpu=vfp. I've attempted a -O2 build in the past with GCC 4.4 but it >> was both bigger and slower than the -Os builds. >> >> So, it pretty much looks like current aggressive optimizations hit >> current hardware limitations and are slower than builds optimized for >> size. > > Yes, this is very likely. Hardware profiling will be very useful to > help identify the root cause. > >> >> Has there been significant changes to the ARM backend that would justify >> that I try some more with current GCC HEAD? Should I maybe try some more >> with the linaro GCC branch? Are there things we can do to help getting >> better ARM performance? > > It does not hurt to try it :) >
Linaro android toolchain benchmarks: https://wiki.linaro.org/Platform/Android/AndroidToolchainBenchmarking/2011-07 Denis.
