https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114347
Bug ID: 114347
Summary: wrong constant folding when casting __bf16 to int
Product: gcc
Version: 13.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: eggert at cs dot ucla.edu
Target Milestone: ---
This is gcc (GCC) 13.2.1 20231205 (Red Hat 13.2.1-6) on Fedora 39. I found this
bug when looking into a GNU coreutils report <https://bugs.gnu.org/69770>
originally reported against Clang (Clang has a different bug).
Compile and run this program:
__bf16 x = 257.0bf16;
int
main (void)
{
return (int) x != (int) 257.0bf16;
}
with "gcc -O2 v.c; ./a.out; echo $?". This prints "1"; it should print "0".
The problem is that GCC constant-folds '(int) 257.0bf16' to 257. This is
incorrect, as 257.0bf16 is exactly equal to 256.0bf16, due to rounding when the
constant is parsed. The expression '(int) x' correctly yields 256 at runtime,
and 256 is not equal to the 257 incorrectly yielded by the constant folding.
