https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128626
Bug ID: 128626
Summary: wrong code at -O2/-O3: vectorizer ignores
anti-dependence between int read and float store to
malloc storage
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);
int n = 4;
int main(void) {
int *p = malloc(4 * (4 + 1));
for (int i = 0; i <= 4; i++) p[i] = i;
unsigned s = 0;
for (int k = 0; k < n; k++) {
((float *)p)[k] = 1;
s += p[k + 1];
}
printf("%u\n", s);
}
$ gcc -O0 t.c && ./a.out
10
$ gcc -O3 t.c && ./a.out
3196059652
Expected: 10. Each iteration first stores a float to element k and then reads
the int at element k+1, which is not yet overwritten. The sum is 1+2+3+4 = 10.
-O3 reads the float bits of 1.0f.
Suspected cause: vect_analyze_data_ref_dependence: the positive-distance
anti-dependence is dropped because TBAA says int and float do not alias (rule
from PR60012, 2014). The storage is malloc storage, so the dynamic type changes
by the store.
Avoided by: -fno-tree-loop-vectorize, -fno-strict-aliasing.
Known to fail: 8.5.0, 10.5.0, 11.4.0, 12.5.0, 13.4.0, 14.3.0, 15.2.0, 16.2.0,
17.0.
-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
