https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116487
Bug ID: 116487
Summary: Miscompile with different optimization flags
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: yunboni at smail dot nju.edu.cn
Target Milestone: ---
Created attachment 59004
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=59004&action=edit
The preprocessed file when using -O0
When I compiled this code with -O0 and -O1 flags, it generated different
outputs:
```c
#include <stdint.h>
#include <stdio.h>
uint32_t c[];
uint32_t d = 3;
int8_t a;
void e(uint8_t b) { d = d >> 8 ^ c[(d ^ b) & 255]; }
void f(uint32_t g) {
e(g);
e(0);
e(0);
e(0);
}
void l(uint32_t g, char *, int) { f(g); }
void m(uint32_t crc, int) { printf("checksum = %X\n", crc); }
int8_t b() {
int64_t bf;
int32_t bg = 76;
int8_t *bh = &a;
*bh = bg ^= bf || 0;
}
int main() {
int bx, i, j;
uint32_t crc;
uint32_t k = 1;
i = 0;
for (; i < 256; i++) {
crc = i;
for (j = 8; j; j--)
if (crc & 1)
crc = crc >> 1 ^ k;
else
crc >>= 1;
c[i] = crc;
}
b();
l(a, "", bx);
m(d ^ 2, bx);
}
```
It can also be found in https://godbolt.org/z/sscqo3GqP
My gcc version is gcc version 15.0.0 20240826 (experimental) (GCC).
I built this with ../configure --disable-multilib --disable-bootstrap
--enable-languages=c,c++ --prefix=/path/to/gcc.
My system type is x86_64-pc-linux-gnu.
Commands for triggering the miscompilation:
1. gcc -O0 case.c -o case-O0 -Wfatal-errors -w
2. gcc -O1 case.c -o case-O1 -Wfatal-errors -w
The outputs are:
1. When using -O0: checksum = 2
2. When using -O1: checksum = 3
Notes:
I tried other flags, the output is checksum = 3.
