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Re: [PATCH, i386] Add prefixes avoidance tuning for silvermont target
- From: Andi Kleen <andi at firstfloor dot org>
- To: Ilya Enkovich <enkovich dot gnu at gmail dot com>
- Cc: gcc-patches at gcc dot gnu dot org
- Date: Wed, 02 Jul 2014 09:21:25 -0700
- Subject: Re: [PATCH, i386] Add prefixes avoidance tuning for silvermont target
- Authentication-results: sourceware.org; auth=none
- References: <20140702103536 dot GA1827 at msticlxl57 dot ims dot intel dot com>
Ilya Enkovich <firstname.lastname@example.org> 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?
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