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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/109484] [Wrong Code][inline-asm] output operands
 overlap with output
Date: Wed, 12 Apr 2023 11:07:30 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109484

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to =E3=80=82 from comment #2)
> (In reply to Richard Biener from comment #1)
> > but you clobber 'temp' early and fail to indicate that so GCC allocates=
 the
> > same register as part of the "+m" output.
>=20
> The requirements you describe are not reflected in the documentation. The
> document only says that `GCC assumpts that the assembler code consumes its
> inputs before producing outputs`, and this code fits the assumption. Firs=
t,
> it reads the input from %1, then write the output to %0, then write the
> output to %1. No outputs happend before inputs.

You first write to 'temp' and then read from it.  The wording applies to the
assigned register / address, _not_ to the C variables mapped.

Note I'm not an expert here and I wonder if an output operand is the
appropriate
way to create a scratch register for arbitrary use.=