Date: Thu, 12 Feb 2015 12:27:35 -0500 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: [PATCH 1/2] x86_64/memset: avoid multiply insn if possible 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. > Signed-off-by: Denys Vlasenko <vda.linux@...glemail.com> > --- > src/string/x86_64/memset.s | 34 +++++++++++++++++++++------------- > 1 file changed, 21 insertions(+), 13 deletions(-) > > diff --git a/src/string/x86_64/memset.s b/src/string/x86_64/memset.s > index 3cc8fcf..523caa0 100644 > --- a/src/string/x86_64/memset.s > +++ b/src/string/x86_64/memset.s > @@ -1,13 +1,12 @@ > .global memset > .type memset,@function > memset: > - movzbl %sil,%esi > - mov $0x101010101010101,%rax > - # 64-bit imul has 3-7 cycles latency, launch early > - imul %rsi,%rax > - > + movzbq %sil,%rax > cmp $16,%rdx > - jb 1f > + jb .Less_than_16 > + test %esi,%esi > + jnz .L_widen_rax # unlikely > +.L_widened: Generally I prefer unnamed labels, but I think you've reached the level where the code is getting confusing without names, so I'm fine with having them here. Once you're done with improvements to the 64-bit version, do you plan to adapt the same principles to the 32-bit version? Or should I or someone else work on that later? Rich
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