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From: "rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/32667] block copy with exact overlap is expanded as memcpy
Date: Wed, 22 Nov 2023 07:33:23 +0000 [thread overview]
Message-ID: <bug-32667-4-w5qf1ZpdF0@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-32667-4@http.gcc.gnu.org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=32667
--- Comment #25 from rguenther at suse dot de <rguenther at suse dot de> ---
On Tue, 21 Nov 2023, bugdal at aerifal dot cx wrote:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=32667
>
> Rich Felker <bugdal at aerifal dot cx> changed:
>
> What |Removed |Added
> ----------------------------------------------------------------------------
> CC| |bugdal at aerifal dot cx
>
> --- Comment #24 from Rich Felker <bugdal at aerifal dot cx> ---
> If the copy is such that gcc is happy to emit an external call to memcpy for
> it, there is no significant size or performance cost to emitting a branch
> checking for equality before making the call, and performing this branch would
> greatly optimize the (maybe rare in the caller, maybe not) case of
> self-assignment!
>
> On the other hand, expecting the libc memcpy to make this check greatly
> pessimizes every reasonable small use of memcpy with a gratuitous branch for
> what is undefined behavior and should never appear in any valid program.
>
> Fix it on the compiler side please.
The only reasonable fix on the compiler side is to never emit memcpy
but always use memmove. The pessimization of that compared to the
non-existing "real" issue with calling memcpy with exact overlap
is the reason for the non-action.
next prev parent reply other threads:[~2023-11-22 7:33 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <bug-32667-4@http.gcc.gnu.org/bugzilla/>
2011-12-05 17:00 ` [Bug middle-end/32667] builtin operator= generates memcpy with overlapping memory regions lu_zero at gentoo dot org
2011-12-05 20:38 ` mans at mansr dot com
2011-12-06 8:46 ` [Bug middle-end/32667] block copy with exact overlap is expanded as memcpy rguenth at gcc dot gnu.org
2015-02-12 5:38 ` pinskia at gcc dot gnu.org
2015-02-12 9:00 ` rguenth at gcc dot gnu.org
2015-02-12 9:42 ` jakub at gcc dot gnu.org
2015-09-28 18:18 ` hugh at mimosa dot com
2021-06-09 18:50 ` public at timruffing dot de
2021-06-10 6:36 ` rguenther at suse dot de
2023-01-05 8:53 ` pinskia at gcc dot gnu.org
2023-01-05 9:19 ` nyh at math dot technion.ac.il
2023-01-06 18:48 ` lopresti at gmail dot com
2023-10-28 10:04 ` post+gcc at ralfj dot de
2023-11-21 15:57 ` bugdal at aerifal dot cx
2023-11-22 7:33 ` rguenther at suse dot de [this message]
2023-11-22 14:57 ` bugdal at aerifal dot cx
2023-11-22 15:06 ` jakub at gcc dot gnu.org
2023-11-22 19:07 ` bugdal at aerifal dot cx
2023-11-23 0:00 ` lopresti at gmail dot com
2023-11-23 7:44 ` post+gcc at ralfj dot de
2023-11-23 7:58 ` rguenth at gcc dot gnu.org
2023-11-23 8:03 ` fw at gcc dot gnu.org
2023-11-23 8:08 ` rguenther at suse dot de
2023-11-23 8:32 ` cvs-commit at gcc dot gnu.org
2023-11-23 8:39 ` jakub at gcc dot gnu.org
2023-11-23 13:54 ` bugdal at aerifal dot cx
2023-11-23 13:57 ` bugdal at aerifal dot cx
2023-11-23 14:41 ` public at timruffing dot de
2023-11-23 15:00 ` public at timruffing dot de
2023-11-23 15:01 ` sjames at gcc dot gnu.org
2023-11-23 15:29 ` post+gcc at ralfj dot de
2023-11-23 18:55 ` bugdal at aerifal dot cx
2023-11-23 19:04 ` post+gcc at ralfj dot de
2023-11-23 20:27 ` bugdal at aerifal dot cx
2023-11-24 8:19 ` public at timruffing dot de
2023-11-24 9:29 ` rguenth at gcc dot gnu.org
2023-11-24 10:09 ` jakub at gcc dot gnu.org
2023-11-24 19:08 ` post+gcc at ralfj dot de
2023-11-27 14:12 ` post+gcc at ralfj dot de
2023-11-27 23:32 ` joseph at codesourcery dot com
2023-11-28 7:17 ` post+gcc at ralfj dot de
2023-11-28 7:20 ` post+gcc at ralfj dot de
2023-11-28 10:58 ` rguenther at suse dot de
2023-11-28 11:03 ` rguenth at gcc dot gnu.org
2023-11-28 11:07 ` fw at gcc dot gnu.org
2024-01-04 12:52 ` rearnsha at gcc dot gnu.org
2024-01-04 13:22 ` bugdal at aerifal dot cx
2024-01-04 13:37 ` jakub at gcc dot gnu.org
2024-01-04 14:28 ` rearnsha at gcc dot gnu.org
2024-01-04 14:29 ` bugdal at aerifal dot cx
2024-01-04 14:41 ` rearnsha at gcc dot gnu.org
2024-01-04 14:44 ` bugdal at aerifal dot cx
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