https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96326
Bug ID: 96326
Summary: Incorrect loop optimization at -O3
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: pterjan at gmail dot com
Target Milestone: ---
Found in https://github.com/Sereal/Sereal/issues/229
#include <string.h>
#include <stdio.h>
int main() {
char buf[128];
char *src = buf;
char *op = buf + 3;
int len = 64;
strcpy(buf, "abc");
while (op - src < 8) {
*(unsigned long*)op = *(const unsigned long*)src;
len -= op - src;
op += op - src;
}
while (len > 0) {
*(unsigned long*)op = *(const unsigned long*)src;
src += 8;
op += 8;
len -= 8;
}
printf("%ld\n", strlen(buf));
}
$ gcc -O2 -Wall t.c; ./a.out
68
$ gcc -O3 -Wall t.c; ./a.out
24
Adding a printf in the second loop also fixes it.
