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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
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D32667

--- 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=3D32667
>=20
> Rich Felker <bugdal at aerifal dot cx> changed:
>=20
>            What    |Removed                     |Added
> -------------------------------------------------------------------------=
---
>                  CC|                            |bugdal at aerifal dot cx
>=20
> --- 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!
>=20
> 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.
>=20
> 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.=