Date: Thu, 12 Feb 2015 20:26:27 +0100 From: Denys Vlasenko <vda.linux@...glemail.com> To: musl <musl@...ts.openwall.com> Subject: Re: [PATCH 1/2] x86_64/memset: avoid multiply insn if possible On Thu, Feb 12, 2015 at 6:27 PM, Rich Felker <dalias@...c.org> wrote: > On Thu, Feb 12, 2015 at 06:17:02PM +0100, Denys Vlasenko wrote: >> memset is very, very often called with fill=0, >> and 64-bit imul is expensive on many CPUs. >> Avoid it if fill=0. >> >> Also avoid multiply on "short memset" codepath if possible, >> and when we do need it, use 32-bit one, which is cheaper on many CPUs. > > I'd actually like to extend the "short" range up to at least 32 bytes > using two 8-byte writes for the middle, unless the savings from using > 32-bit imul instead of 64-bit are sufficient to justify 4 4-byte > writes for the middle. On the cpu I tested on, the difference is 11 > cycles vs 32 cycles for non-rep path versus rep path at size 32. The short path causes mixed feelings in me. On one hand, it's elegant in a contrived way. On the other hand, multiple overlaying stores must be causing hell in store unit. I'm thinking, maybe there's a faster way to do that. Can you post your memset testsuite?
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