On Mon, Jan 31, 2022 at 11:07 AM Xi Ruoyao <[email protected]> wrote:
>
> On Mon, 2022-01-31 at 10:35 +0100, Richard Biener wrote:
> > On Sun, Jan 30, 2022 at 7:39 PM Xi Ruoyao via Gcc-patches
> > <[email protected]> wrote:
> > >
> > > 'inf/inf' should raise an invalid operation exception at runtime.
> > > So it
> > > should not be folded during compilation unless -fno-trapping-math is
> > > used.
> >
> > I wonder if it would make sense to handle it similar to
> >
> > inexact = real_arithmetic (&value, code, &d1, &d2);
> > real_convert (&result, mode, &value);
> >
> > /* Don't constant fold this floating point operation if
> > the result has overflowed and flag_trapping_math. */
> > if (flag_trapping_math
> > && MODE_HAS_INFINITIES (mode)
> > && REAL_VALUE_ISINF (result)
> > && !REAL_VALUE_ISINF (d1)
> > && !REAL_VALUE_ISINF (d2))
> > return NULL_TREE;
>
> > thus whenever we fold to NaN but flag_trapping_math is set,
> > do not fold?
>
> Like this?
>
> inexact = real_arithmetic (&value, code, &d1, &d2);
> real_convert (&result, mode, &value);
>
> + /* Don't constant fold this floating point operation if
> + both operands are not NaN but the result is NaN, and
> + flag_trapping_math. Such operations should raise an
> + invalid operation exception. */
> + if (flag_trapping_math
> + && MODE_HAS_NANS (mode)
> + && REAL_VALUE_ISNAN (result)
> + && !REAL_VALUE_ISNAN (d1)
> + && !REAL_VALUE_ISNAN (d2))
> + return NULL_TREE;
> +
> /* Don't constant fold this floating point operation if
> the result has overflowed and flag_trapping_math. */
> if (flag_trapping_math
>
> > We already exclude NaN operands earlier
> > (but unconditionally return NaN even for NaN + 1. though
> > that should raise invalid as well, no?)
>
> qNaN + 1 (or even qNaN + qNaN) should produce a qNaN but should not
> raise any exception. OTOH sNaN + 1 should produce a qNaN and raise an
> invalid operation exception.
Ah, I see. So yes, like that.
Joseph, do you agree?
Thanks,
Richard.
> For sNaN operands we already ruled out the
> const folding earlier:
>
> > /* Don't perform operation if we honor signaling NaNs and
> > either operand is a signaling NaN. */
> > if (HONOR_SNANS (mode)
> > && (REAL_VALUE_ISSIGNALING_NAN (d1)
> > || REAL_VALUE_ISSIGNALING_NAN (d2)))
> > return NULL_TREE;
>
> > I suppose Joseph should know best what the correct semantics
> > are here.
> >
> > Thanks,
> > Richard.
> >
> > > gcc/
> > > PR middle-end/95115
> > > * fold-const.cc (const_binop): Do not fold "inf/inf".
> > >
> > > gcc/testsuite
> > > * gcc.dg/pr95115.c: New test.
> > > ---
> > > gcc/fold-const.cc | 8 ++++++++
> > > gcc/testsuite/gcc.dg/pr95115.c | 25 +++++++++++++++++++++++++
> > > 2 files changed, 33 insertions(+)
> > > create mode 100644 gcc/testsuite/gcc.dg/pr95115.c
> > >
> > > diff --git a/gcc/fold-const.cc b/gcc/fold-const.cc
> > > index fd9c6352d4f..0e99d5a2e3d 100644
> > > --- a/gcc/fold-const.cc
> > > +++ b/gcc/fold-const.cc
> > > @@ -1283,6 +1283,14 @@ const_binop (enum tree_code code, tree arg1,
> > > tree arg2)
> > > && (flag_trapping_math || ! MODE_HAS_INFINITIES (mode)))
> > > return NULL_TREE;
> > >
> > > + /* Don't perform "inf/inf", because it would raise an invalid
> > > + operation exception (IEEE 754 section 7.2 clause d). */
> > > + if (code == RDIV_EXPR
> > > + && flag_trapping_math
> > > + && REAL_VALUE_ISINF (d1)
> > > + && REAL_VALUE_ISINF (d2))
> > > + return NULL_TREE;
> > > +
> > > /* If either operand is a NaN, just return it. Otherwise,
> > > set up
> > > for floating-point trap; we return an overflow. */
> > > if (REAL_VALUE_ISNAN (d1))
> > > diff --git a/gcc/testsuite/gcc.dg/pr95115.c
> > > b/gcc/testsuite/gcc.dg/pr95115.c
> > > new file mode 100644
> > > index 00000000000..46a95dfb698
> > > --- /dev/null
> > > +++ b/gcc/testsuite/gcc.dg/pr95115.c
> > > @@ -0,0 +1,25 @@
> > > +/* { dg-do run } */
> > > +/* { dg-options "-O2 -ftrapping-math" } */
> > > +/* { dg-add-options ieee } */
> > > +/* { dg-require-effective-target fenv_exceptions } */
> > > +
> > > +#include <fenv.h>
> > > +#include <stdlib.h>
> > > +
> > > +double
> > > +x (void)
> > > +{
> > > + double d = __builtin_inf ();
> > > + return d / d;
> > > +}
> > > +
> > > +int
> > > +main (void)
> > > +{
> > > + double r = x ();
> > > + if (!__builtin_isnan (r))
> > > + abort ();
> > > + if (!fetestexcept (FE_INVALID))
> > > + abort ();
> > > + exit (0);
> > > +}
> > > --
> > > 2.35.1
> > >
> > >
>
> --
> Xi Ruoyao <[email protected]>
> School of Aerospace Science and Technology, Xidian University
