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From: "rguenther at suse dot de" <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 09:53:33 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107608

--- Comment #47 from rguenther at suse dot de <rguenther at suse dot de> ---
On Fri, 27 Jan 2023, xry111 at gcc dot gnu.org wrote:

> 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 exceptions =
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 elide
> > > > 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 ca=
se, and
> > > fix any regression on Glibc side?
> >=20
> > I think we should adjust the testcase with a volatile like I suggested =
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 nasty=
=20
> (x - x) / (x - x) to raise the exception?

Not trivially.  I'd suggest glibc uses a volatile use, like for example

  tem =3D Inf/Inf;
  __asm__ volatile ("" : : "g" (tem));

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.=