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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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