https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124658
Bug ID: 124658
Summary: Explicit register variable bound to eax is corrupted
by function return values under -O2 optimization
Product: gcc
Version: 15.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: 522024330006 at smail dot nju.edu.cn
Target Milestone: ---
To do the quick verification: https://godbolt.org/z/rnv5GWsGY
The program:
```c
#include <stdio.h>
#include <string.h>
int a[256];
unsigned b = 4294967295;
void func(char *f) {
for (int e = 0; e < 2; e++)
b = b ^ a[(f[e]) & 255];
}
char f[56] = "I";
int main() {
for (int c = 0; c < 256; c++) {
a[c] = c;
}
register int g __asm__("eax") = 5;
g++;
func(f);
sprintf(f, "%d\n", g);
func(f);
printf("%d\n", b);
return 0;
}
```
Steps to reproduce:
Compile with GCC: gcc -O0 test.c -o test_O0 && ./test_O0
Compile with GCC: gcc -O2 test.c -o test_O2 && ./test_O2
Compare the output.
Both -O0 and -O2 should have produced the same output, as g is explicitly bound
to a register and its increment should be preserved across the call to func.But
the result of -O2 significantly differs from the -O0.
