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From: "xry111 at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/110622] x87: Miscompilation at O2 level (O1 is working)
Date: Thu, 21 Sep 2023 13:45:47 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110622

--- Comment #15 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
(In reply to Mathieu Malaterre from comment #14)
> (In reply to Andrew Pinski from comment #13)
> > (In reply to Mathieu Malaterre from comment #12)
> > > I am seeing a difference in result (log1p computation) in the range:
> > >=20
> > > 4318952042648305665 - 0x1.0000000000001p-64
> > > 4368493837572636672 - 0x1.002p-53
> > >=20
> > > the other values seems to match expectation of log1p computation.
> >=20
> > But you used excess-precision=3Dfast
> >=20
> > *** This bug has been marked as a duplicate of bug 323 ***
>=20
> AFAIK bug #323 does not mention my trick:
>=20
>   asm volatile("" : "+r"(y.raw[0]) : : "memory");
>=20
> That simple line totally changed the optimizer code generation.

Because in x87 the excessive precision only exists in x87 stack-like regist=
ers.
 The "memory" clobber forces a store and reload for all non-register variab=
les,
thus the value is truncated into a normal double value and the excessive
precision is lost.

There are infinite ways to work around an issue, but it does not mean PR 323
must mention all of them.=