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From: "rguenth 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 07:35:39 +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 #45 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Xi Ruoyao from comment #44)
> (In reply to rguenther@suse.de from comment #43)
> > On Thu, 19 Jan 2023, xry111 at gcc dot gnu.org wrote:
> >=20
> > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107608
> > >=20
> > > --- Comment #42 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
> > > (In reply to Richard Biener from comment #41)
> > > > We could fix the testcase with
> > > >=20
> > > > diff --git a/gcc/testsuite/gcc.dg/pr95115.c b/gcc/testsuite/gcc.dg/=
pr95115.c
> > > > index 69c4f83250c..09273e445d2 100644
> > > > --- a/gcc/testsuite/gcc.dg/pr95115.c
> > > > +++ b/gcc/testsuite/gcc.dg/pr95115.c
> > > > @@ -17,6 +17,7 @@ int
> > > >  main (void)
> > > >  {
> > > >    double r =3D x ();
> > > > +  volatile double rr =3D r;
> > > >    if (!__builtin_isnan (r))
> > > >         abort ();
> > > >    if (!fetestexcept (FE_INVALID))
> > > >=20
> > > > that preserves optimizing the isnan check but also preserves the co=
mputation
> > > > and checks the non-propagation of a NaN.
> > >=20
> > > Hmm, so it means we cannot rely on Inf / Inf to raise an exception?  =
Then we
> > > need to fix Glibc...
> >=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 case, =
and
> fix any regression on Glibc side?

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.=