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Date: Thu, 16 Sep 2021 18:17:17 -0400
From: Chris Kennelly <ckennelly@...gle.com>
To: libc-coord@...ts.openwall.com
Cc: GNU C Library <libc-alpha@...rceware.org>, gcc@....gnu.org
Subject: Re: Add new ABI '__memcmpeq()' to libc
On Thu, Sep 16, 2021 at 5:50 PM enh <enh@...gle.com> wrote:
> plus testing for _equality_ can (as mentioned earlier) have slightly
> different properties from the three-way comparator behavior of
> bcmp()/memcmp().
>
llvm-libc's implementation only returns the boolean, though.
The mem* functions are extremely sensitive to instruction cache effects, so
having 3 unique implementations (__memcmpeq, bcmp, memcmp) that do similar,
but subtly different things can be a hidden performance cost--one that is
hard to demonstrate with a microbenchmark. Our experience developing
optimized mem* routines ended up showing better performance in actual
applications when we accepted seemingly worse microbenchmark performance by
optimizing for code footprint instead (more extensive notes for mem* in
general
<https://storage.googleapis.com/pub-tools-public-publication-data/pdf/4f7c3da72d557ed418828823a8e59942859d677f.pdf>
and
memcmp specifically (section 4.4)
<https://storage.googleapis.com/pub-tools-public-publication-data/pdf/e52f61fd2c51e8962305120548581efacbc06ffc.pdf>
).
The alternative would be to alias (as the NOTES suggest as a possible
implementation), but I think that raises James' question of why not just
use bcmp? Dependencies on non-boolean implementations of bcmp seem
rare--namely, I haven't actually seen one.
> On Thu, Sep 16, 2021 at 2:43 PM Joseph Myers <joseph@...esourcery.com>
> wrote:
>
>> On Thu, 16 Sep 2021, James Y Knight wrote:
>>
>> > Wouldn't it be far simpler to just un-deprecate bcmp?
>>
>> The aim is to have something to which calls can be generated in all
>> standards modes. bcmp has never been part of ISO C; there's nothing to
>> undeprecate there.
>
>
>> --
>> Joseph S. Myers
>> joseph@...esourcery.com
>>
>
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