at least spidermonkey 1.8.5 forces -O3 when building when it should not. probably applies to more SLOTs though. build log: https://bugs.gentoo.org/attachment.cgi?id=422482
So it does.. fixed now: commit b20ee3f5f1087779c266f250aebedd42fb0b6a3a Author: Ian Stakenvicius <axs@gentoo.org> Date: Wed Jan 13 12:31:28 2016 -0500 dev-lang/spidermonkey-1.8.5: honour CFLAGS, disable forced -O3 optimization Bug: http://bugs.gentoo.org/571762