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
> <gcc-patches@gcc.gnu.org> 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. 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 <xry...@mengyan1223.wang>
School of Aerospace Science and Technology, Xidian University
