https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128078
Bug ID: 128078
Summary: wrong code at -O1, -O2 and -Os summing (long long) ((b
+ i) * s) in a loop with step 3
Product: gcc
Version: 17.0
Status: UNCONFIRMED
Keywords: wrong-code
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: kyuwoncho18 at gmail dot com
Target Milestone: ---
Target: x86_64-pc-linux-gnu
__attribute__((noinline)) unsigned long long f(int b, int s, int n)
{
unsigned long long z = 0;
for (int i = 0; i < n; i += 3)
z += (unsigned long long)(long long)((b + i) * s);
return z;
}
int main(void)
{
if (f(-2, 1 << 30, 4) != -(1ULL << 30))
__builtin_abort();
}
godbolt: https://godbolt.org/z/eWPaWTG6x
The loop runs for i = 0 and i = 3, so (b + i) * s is -2^31 and then 2^30, both
representable in int, and f returns 2^64 - 2^30 (18446744072635809792). With
-O0, -O3 or -O2 -fno-ivopts the program exits normally; with -O1, -O2 or -Os f
returns 2^64 - 5 * 2^30 (18446744068340842496) and the program aborts.
