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
