https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96007
Bug ID: 96007
Summary: -O2 miscompiles memcmp an object with a string literal
containing '\0'
Product: gcc
Version: 9.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: jerryfromearth at gmail dot com
Target Milestone: ---
Hi,
While comparing an object to a literal string that contains "\0" (e.g.
memcmp((&u128), "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0", 16)), -O2 produces
incorrect result, while -O1 and -O0 produces correct result.
This affects at least gcc 9.2, 9.3 and 10.1.
gcc 9.1 is fine.
```
$ cat test.c
#include
#include
#include
typedef struct tagMYUINT128
{
unsigned long u64High;
unsigned long u64Low;
} MYUINT128, *PMYUINT128;
int main(int argc, char **argv) {
MYUINT128 u128 = {0};
char arr[] = "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0";
printf("All results are expected to be non-zero.\n");
// Test A, u128={0x0, 0x1234}
printf("Test A. u128={0x0, 0x1234}\n");
u128.u64High = 0x0;
u128.u64Low = 0x1234;
// Comparing to a literal string is broken.
printf("memcmp1 result=%d\n", memcmp((&u128),
"\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0", 16));
printf("memcmp2 result=%d\n", __builtin_memcmp((&u128),
"\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0", 16));
// Comparing to a array pointer works.
printf("memcmp3 result=%d\n", memcmp((&u128), arr, 16));
printf("memcmp4 result=%d\n", __builtin_memcmp((&u128), arr, 16));
return 0;
}
$ gcc -O2 -o test test.c && ./test
All results are expected to be non-zero.
Test A. u128={0x0, 0x1234}
memcmp1 result=0
memcmp2 result=0
memcmp3 result=52
memcmp4 result=52
$ gcc -O1 -o test test.c && ./test
All results are expected to be non-zero.
Test A. u128={0x0, 0x1234}
memcmp1 result=52
memcmp2 result=52
memcmp3 result=52
memcmp4 result=52
```
godbolt link: https://godbolt.org/z/9M3L8h (it contains more tests)
I looked into the list of bugs fixed in 9.2 and found this probably relevant
issue:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90892
Cheers,
Jiarui Hong
