From: Steffen Nurpmeso <steffen@sdaoden.eu>
Subject: Re: [PATCH] amd64: import optimized memcmp from FreeBSD
To: Christian Schulte <cs@schulte.it>
Cc: tech@openbsd.org
Date: Mon, 02 Dec 2024 20:31:47 +0100

Christian Schulte wrote in
 <ea7ddaf9-29c9-4e68-94c7-d80ca2108658@schulte.it>:
 |On 11/29/24 17:08, Mateusz Guzik wrote:
 |> On Fri, Nov 29, 2024 at 4:49 PM Stuart Henderson <stu@spacehopper.org> \
 |> wrote:
 |>> On 2024/11/29 02:01, Mateusz Guzik wrote:
 |>>> The rep-prefixed cmps is incredibly slow even on modern CPUs.
 |>>>
 |>>> The new implementation uses regular cmp to do it.
 |>>>
 |>>> The code got augmented to account for retguard, otherwise it matches \
 |>>> FreeBSD.
 |>>
 |>> Would that make sense in libc too?
 |> 
 |> this and the other routines are faster than what's in openbsd libc,
 |> but they are not the optimal choice due to lack of simd. definitely an
 |> improvement for the time being.
 |
 |This has been done before. E.g.:
 |
 |<https://cvsweb.openbsd.org/src/lib/libc/arch/amd64/string/strlen.S>
 |
 |Removed nearly 13 years ago in favour of the .c version being faster
 |(gcc builtin back then), then readded based on NetBSD a couple of years
 |later. When reading

Fyi, it originally came from DragonFly:

  https://marc.info/?l=dragonfly-commits&m=132241713812022&w=2

I remember i could not believe it, and tried it out.  It really
was "magic".  (By then i was still "prowd" of my ASM stuff, having
had spent surely hundreds of hours fiddling with optimizations.)
And i do not remember falsely that it had nothing to do with gcc.
I guess(ed) it was CPU internal stuff for "[cld;] repne scasb".

  ...

--steffen
|
|Der Kragenbaer,                The moon bear,
|der holt sich munter           he cheerfully and one by one
|einen nach dem anderen runter  wa.ks himself off
|(By Robert Gernhardt)
|
|And in Fall, feel "The Dropbear Bard"s ball(s).
|
|The banded bear
|without a care,
|Banged on himself for e'er and e'er
|
|Farewell, dear collar bear