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Re: [PATCH, i386] Add prefixes avoidance tuning for silvermont target

2014-07-02 20:27 GMT+04:00 Jakub Jelinek <>:
> On Wed, Jul 02, 2014 at 09:21:25AM -0700, Andi Kleen wrote:
>> Ilya Enkovich <> 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.


>         Jakub

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