I have enabled this use flag and the output of emerge --info shows it has been recognised in USE but not in FCFLAGs/CFLAGs;
grep USE.*custom ff* ffafter-cust-opt.txt:USE="custom-optimization dbus gmp-autoupdate gtk2 hwaccel jemalloc3 jit pulseaudio -bindist -custom-cflags -debug -hardened (-neon) (-pgo) (-selinux) -startup-notification (-system-cairo) -system-harfbuzz -system-icu -system-jpeg -system-libevent -system-libvpx -system-sqlite -test -wifi" ffbefore-cust-opt.txt:USE="dbus gmp-autoupdate gtk2 hwaccel jemalloc3 jit pulseaudio -bindist -custom-cflags -custom-optimization -debug -hardened (-neon) (-pgo) (-selinux) -startup-notification (-system-cairo) -system-harfbuzz -system-icu -system-jpeg -system-libevent -system-libvpx -system-sqlite -test -wifi" grep CFLAGS ff* ffafter-cust-opt.txt:CFLAGS="-march=amdfam10 -mcx16 -msahf -mpopcnt -mabm -O2 -pipe" ffafter-cust-opt.txt:FCFLAGS="-O2 -pipe" ffafter-cust-opt.txt:CFLAGS="-march=amdfam10 -pipe" ffbefore-cust-opt.txt:CFLAGS="-march=amdfam10 -mcx16 -msahf -mpopcnt -mabm -O2 -pipe" ffbefore-cust-opt.txt:FCFLAGS="-O2 -pipe" ffbefore-cust-opt.txt:CFLAGS="-march=amdfam10 -pipe" I assumed that enabling custom-optimization would let the optimization setting from CFLAGS in make.conf pass through unfiltered, but that's not the case. What else do i need to do to make it work?
