From: Ralf Jung <post@ralfj.de>
To: Alexander Monakov <amonakov@ispras.ru>
Cc: Paul Eggert <eggert@cs.ucla.edu>,
Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>,
libc-alpha <libc-alpha@sourceware.org>
Subject: Re: Support for memcpy with equal source and destination
Date: Mon, 27 Nov 2023 13:34:36 +0100 [thread overview]
Message-ID: <feafb774-7495-4e3b-80a5-3d08786c612b@ralfj.de> (raw)
In-Reply-To: <089bc099-39ab-1a30-eea2-ebb74e489a8d@ispras.ru>
Hi all,
>> This probably needs benchmark to determine on which side the branch is less
>> expensive overall? Or a dedicated memcpy variant that allows src==dest, as has
>> been brought up elsewhere.
>
> Please note that GCC does not use memcpy for "sufficiently small" structure
> copies at -O2, as it's faster to emit the necessary loads+stores inline.
>
> The threshold for "sufficiently small" varies with target and compiler version;
> for instance, it is "above 64 bytes" for 32-bit arm and "above 8192 bytes" for
> x86_64 with current trunk (it also depends on default -march/-mtune, etc.).
Wow, 8 kilobytes?!?
> So on x86 at least adding such a branch in memcpy is not a practical choice.
Sorry, how does that follow from GCC not using memcpy for small copies?
Kind regards,
Ralf
>
> Alexander
next prev parent reply other threads:[~2023-11-27 12:34 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-11-23 12:14 Adhemerval Zanella Netto
2023-11-25 7:48 ` Paul Eggert
2023-11-25 8:20 ` Ralf Jung
2023-11-25 17:11 ` Paul Eggert
2023-11-27 11:15 ` Ralf Jung
2023-11-27 11:46 ` Alexander Monakov
2023-11-27 12:34 ` Ralf Jung [this message]
2023-11-27 14:25 ` Alexander Monakov
2023-11-27 14:38 Wilco Dijkstra
2023-11-27 14:45 ` Ralf Jung
2023-11-27 14:53 ` Zack Weinberg
2023-11-27 15:02 ` enh
2023-11-27 19:28 Aaron Peter Bachmann
2023-11-27 19:39 ` Paul Eggert
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