Firefox 81 on POWER

Firefox 81 is released. In addition to new themes of dubious colour coordination, media controls now move to keyboards and supported headsets, the built-in JavaScript PDF viewer now supports forms (if we ever get a JIT going this will work a lot better), and there are relatively few developer-relevant changes.

This release heralds the first official change in our standard POWER9 .mozconfig since Fx67. Link-time optimization continues to work well (and in 81 the LTO-enhanced build I'm using now benches about 6% faster than standard -O3 -mcpu=power9), so I'm now making it a standard part of my regular builds with a minor tweak we have to make due to bug 1644409. Build time still about doubles on this dual-8 Talos II and it peaks out at almost 84% of its 64GB RAM during LTO, but the result is worth it.

Unfortunately PGO (profile-guided optimization) still doesn't work right, probably due to bug 1601903. The build system does appear to generate a profile properly, i.e., a controlled browser instance pops up, runs some JavaScript code, does some browser operations and so forth, and I see gcc created .gcda files with all the proper count information, but then the build system can't seem to find them to actually tune the executable. This needs a little more hacking which I might work on as I have free time™. I'd also like to eliminate ac_add_options --disable-release as I suspect it is no longer necessary but I need to do some more thorough testing first.

In any event, reliable LTO at least with the current Fedora 32 toolchain is still continuous progress. I've heard concerns that some distributions are not making functional builds of Firefox for ppc64le (let alone ppc64, which has its own problems), though Fedora is not one of them. Still, if you have issues with your distribution's build and you are not able to build it for yourself, if there is interest I may put up a repo or a download spot for the binaries I use since I consider them reliable. Without further ado, here are the current .mozconfigs that I attest as functional.

Optimized Configuration

export CC=/usr/bin/gcc
export CXX=/usr/bin/g++

mk_add_options MOZ_MAKE_FLAGS="-j24"
ac_add_options --enable-application=browser
ac_add_options --enable-optimize="-O3 -mcpu=power9"
ac_add_options --disable-release
ac_add_options --enable-linker=bfd
ac_add_options --enable-lto=full

#export GN=/uncomment/and/set/path/if/you/haz
export RUSTC_OPT_LEVEL=2

Debug Configuration

export CC=/usr/bin/gcc
export CXX=/usr/bin/g++

mk_add_options MOZ_MAKE_FLAGS="-j24"
ac_add_options --enable-application=browser
ac_add_options --enable-optimize="-Og -mcpu=power9"
ac_add_options --enable-debug
ac_add_options --disable-release
ac_add_options --enable-linker=bfd

#export GN=/uncomment/and/set/path/if/you/haz
export RUSTC_OPT_LEVEL=0