https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128637
Bug ID: 128637
Summary: [14/15/16/17 Regression] wrong code at -O2: FRE gives
a memcpy read a TBAA-based value of a typed load that
does not execute since r10-1692
Product: gcc
Version: 17.0
Status: UNCONFIRMED
Keywords: alias, wrong-code
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: jeffchen006 at gmail dot com
Target Milestone: ---
int printf(const char *, ...);
void *malloc(unsigned long);
void *memset(void *, int, unsigned long);
void *memcpy(void *, const void *, unsigned long);
int x;
long r;
int main(void) {
unsigned char *b = malloc(8);
memset(b, 0, 8);
*(long long *)b = 1;
b[4] = 1;
if (x)
r = *(long *)b;
long long t;
memcpy(&t, b, 8);
printf("%llx\n", t);
}
$ gcc -O0 t.c && ./a.out
100000001
$ gcc -O2 t.c && ./a.out
100000000
Expected: 100000001. b holds long long 1 and then b[4] = 1, so the 8 bytes are
0x100000001. The long load under if (x) does not run (x = 0). -O2 gives the
memcpy read the memset value for the low word.
Suspected cause: SCCVN: the value of a typed load (found with TBAA) is reused
for a ref-all (memcpy) read at the same VUSE (first bad 38988cbf9eb, 2019,
PR83518). Related: PR101641.
Avoided by: -fno-tree-fre, -fno-strict-aliasing.
Known to fail: 10.5.0, 11.4.0, 12.5.0, 13.4.0, 14.3.0, 15.2.0, 16.2.0, 17.0.
Known to work: 8.5.0.
-O0 -fsanitize=undefined,address is clean.
gcc version 17.0.0 20261111 (experimental) (a2a11abfad3), x86_64-pc-linux-gnu
Configured with: --enable-languages=c,c++ --disable-multilib
--disable-bootstrap --enable-checking=release
Related: PR101641, PR83518.
