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From: "xry111 at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/107608] [13 Regression] Failure on
 fold-overflow-1.c and pr95115.c
Date: Fri, 27 Jan 2023 10:02:57 +0000
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X-Bugzilla-Component: tree-optimization
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X-Bugzilla-Keywords: missed-optimization, wrong-code
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107608

--- Comment #48 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
(In reply to rguenther@suse.de from comment #47)
> On Fri, 27 Jan 2023, xry111 at gcc dot gnu.org wrote:
>=20
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107608
> >=20
> > --- Comment #46 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
> > (In reply to Richard Biener from comment #45)
> > > (In reply to Xi Ruoyao from comment #44)
> > > > (In reply to rguenther@suse.de from comment #43)
> >=20
> > > > > If the result is unused then no, GCC will happily elide exception=
s from
> > > > > unused computations like Inexact from the statement
> > > > >=20
> > > > >  1./3.;
> > > > >=20
> > > > > but this has been done before.  What's new is that GCC can now el=
ide
> > > > > some uses (in this case the isnan check is the only use)
> > > >=20
> > > > The should we just change PR95115 to "INVALID" and remove the test =
case, and
> > > > fix any regression on Glibc side?
> > >=20
> > > I think we should adjust the testcase with a volatile like I suggeste=
d above
> > > so we verify that we don't eliminate the computation with a "constant=
" NaN.
> >=20
> > Ok, I'll post a patch.
> >=20
> > Glibc already changed the code from Inf/Inf to (x - x) / (x - x) where =
x=20
> > is not a constant, but I'm wandering if the compiler will attempt to=20
> > optimize out (x - x) / (x - x) later...  Is it possible to provide a=20
> > "__builtin_feraiseexcept" so we'd be able to use it instead of the nast=
y=20
> > (x - x) / (x - x) to raise the exception?
>=20
> Not trivially.  I'd suggest glibc uses a volatile use, like for example
>=20
>   tem =3D Inf/Inf;
>   __asm__ volatile ("" : : "g" (tem));
>=20
> or so to preserve the computation and avoid an actual store to a volatile
> variable.  Though I see at least GCC 7 optimizing the above division
> to a constant, lacking a fix we deployed later.

Currently Glibc returns the produced NaN (and there will be no IPA involved=
 as
the TU only contains one function, and AFAIK Glibc just does not support
building with LTO).  Is it enough to preserve the computation?=