https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114207
Bug ID: 114207
Summary: Wrong code bug since GCC 12.1
Product: gcc
Version: 12.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: congli at smail dot nju.edu.cn
Target Milestone: ---
The program below shows a wrong code bug, where the correct result should be
"s.a=12, s.b=6" while `-w -O3 -fno-tree-forwprop` prints "s.a=12, s.b=0" or
"s.a=12, s.b=<random>". The bug stems from at least GCC 12.1 to our trunk.
```
#include <stdio.h>
#include <stdint.h>
struct S {
int a, b;
};
__attribute__((noinline))
void foo (struct S *s) {
struct S ss = (struct S) {
.a = s->b,
.b = s->a
};
*s = ss;
}
int main() {
struct S s = {6, 12};
foo(&s);
printf("s.a=%d, s.b=%d\n", s.a, s.b);
return 0;
}
```
Compiler Explorer: https://gcc.godbolt.org/z/8dbMWjsd8
After checking the assembly, we found that `8(%rsp)` (representing s.b) was not
initialized by the constant `6` while it is printed.
