2014-07-02 20:27 GMT+04:00 Jakub Jelinek <ja...@redhat.com>: > On Wed, Jul 02, 2014 at 09:21:25AM -0700, Andi Kleen wrote: >> Ilya Enkovich <enkovich....@gmail.com> writes: >> >> > Silvermont processors have penalty for instructions having 4+ bytes of >> > prefixes (including escape bytes in opcode). This situation happens >> > when REX prefix is used in SSE4 instructions. This patch tries to >> > avoid such situation by preferring xmm0-xmm7 usage over xmm8-xmm15 in >> > those instructions. I achieved it by adding new tuning flag and new >> > alternatives affected by tuning. >> >> Why make it a tuning flag? Shouldn't this help unconditionally for code >> size everywhere? Or is there some drawback? > > I don't think it will make code smaller, if you already have some value in > xmm8..xmm15 register, then by not allowing those registers directly on SSE4 > insns just means it reloading and larger code. > > BTW, is that change needed also when emitting AVX insns instead of SSE4?
This is for Silvermont only. It does not have AVX. Ilya > > Jakub
