[Bug c/104389] New: HUGE_VAL * 0.0 is no longer a NaN

["vstinner at redhat dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104389

            Bug ID: 104389
           Summary: HUGE_VAL * 0.0 is no longer a NaN
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vstinner at redhat dot com
  Target Milestone: ---

Created attachment 52352
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52352&action=edit
Reproducer: huge_val_0_nan.c

The Python project uses "HUGE_VAL * 0" to get a Not-a-Number (NaN) double
value. Pseudo-code:

#define Py_NAN (Py_HUGE_VAL * 0.)
double my_nan(void)
{ return Py_NAN; }

Until Fedora gcc-12.0.1-0.4.fc36.x86_64
(03182470d2d2b272f06500184acab6b8ed78d8ad), it worked well.

With Fedora gcc-12.0.1-0.5.fc36.x86_64
(fb6057a2be99e071993fb54a5d338ab0febba8ff), my_nan() now returns zero (0.0).

Try attached huge_val_0_nan.c.

gcc-12.0.1-0.5.fc36.x86_64 output:
---
(1) Compute HUGE_VAL * 0 in the C compiler
isnan? 0
bytes: [0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]

(2) Compute HUGE_VAL * 0 at runtime
isnan? 1
bytes: [0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xfffffff8, 0xffffffff]
---

gcc-11.2.1-7.fc35.x86_64 output:
---
(1) Compute HUGE_VAL * 0 in the C compiler
isnan? 1
bytes: [0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xfffffff8, 0x7f]

(2) Compute HUGE_VAL * 0 at runtime
isnan? 1
bytes: [0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xfffffff8, 0xffffffff]
---

In Python, Py_NAN is defined as "#define Py_NAN (Py_HUGE_VAL * 0.)" at:

https://github.com/python/cpython/blob/bf95ff91f2c1fc5a57190491f9ccdc63458b089e/Include/pymath.h#L60

FYI diff of the Fedora package between -4 and -5, the most important part if
the Git commit which changed ;-)

https://src.fedoraproject.org/rpms/gcc/c/398138b5f3809856b98eb4241b3c9f70da3fce9b?branch=rawhide