https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127345
Bug ID: 127345
Summary: [16/17 Regression] wrong code storing a signed
_BitInt(N) struct
Product: gcc
Version: 17.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: shimizu2486 at gmail dot com
Target Milestone: ---
Live Reproducer: https://godbolt.org/z/Wx378zEnT
Storing a signed _BitInt(N) into a struct member, where N does not fill its
container, gives the wrong value from -O2 up.
$ cat t1.c
#include <stdio.h>
int a1_0;
long in6;
typedef struct { _BitInt(3) f1; } S0;
int main(void) {
long v0 = 0;
S0 s0;
if (!in6) {
for (; a1_0; v0 = 1)
;
s0.f1 = v0 - 1;
}
printf("%llu\n", (unsigned long long)(long long)s0.f1);
}
a1_0 and in6 are zero, so the loop never runs, v0 stays 0, and s0.f1 is -1.
$ gcc -O0 t1.c -o t1 && ./t1
18446744073709551615 (correct)
$ gcc -O2 t1.c -o t1 && ./t1
0 (wrong)
15.3 correct
16.2 wrong
trunk (b5a11752d) wrong
The program does not trigger UB.
