https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128133
Bug ID: 128133
Summary: wrong code at -O2 and -O3 converting an unsigned
_BitInt(17) array to unsigned int
Product: gcc
Version: 17.0
Status: UNCONFIRMED
Keywords: wrong-code
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: kyuwoncho18 at gmail dot com
Target Milestone: ---
Target: x86_64-pc-linux-gnu
#include <stdio.h>
__attribute__((noinline))
void convert(unsigned *dst, unsigned _BitInt(17) *src)
{
for (int i = 0; i < 4; ++i)
dst[i] = src[i];
}
int main(void)
{
unsigned _BitInt(17) src[4];
unsigned dst[4];
__builtin_memset(src, 0xff, sizeof src);
convert(dst, src);
printf("%u\n", dst[0]);
}
godbolt: https://godbolt.org/z/hsh9bbb5c
On x86-64, memset sets all 17 value bits of each src element to 1, so
conversion to unsigned int must produce 131071 regardless of the padding bits.
With -O2 or -O3 this prints 4294967295; with -O0, -O1, -Os or -O2
-fno-tree-loop-vectorize it prints the correct 131071.
