https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119322
Bug ID: 119322
Summary: Different results between -O1 and -O3
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: egor.pugin at gmail dot com
Target Milestone: ---
gmult2() is a function from Galois/Counter Mode (GCM) for AES and other
ciphers.
This code is not minimized. You can try to make it shorter.
===
using u64 = unsigned long long;
void gmult2(u64 *X, u64 *Y) {
u64 Z[2] = {0, 0};
u64 V[2];
int i, j;
V[0] = X[0];
V[1] = X[1];
for (i = 0; i < 2; i++) {
auto y = Y[i];
for (j = 0; j < 64; j++) {
u64 mask = 0 - (y >> 63);
Z[0] ^= V[0] & mask;
Z[1] ^= V[1] & mask;
auto v1 = (0 - (V[1] & 1)) & 0xE100000000000000ULL;
V[1] >>= 1;
V[1] |= V[0] << 63;
V[0] >>= 1;
V[0] ^= v1;
y <<= 1;
}
}
X[0] = Z[0];
X[1] = Z[1];
}
int main() {
__int128 a{3}, b{4};
gmult2((u64*)&a, (u64*)&b);
return a;
}
===
gcc gives 0 at -O1 and 3 at -O3 since 11 release. So, 11/12/13/14/15 are
affected.
gcc gives 0 at both -O1 and -O3 in version 10.
clang gives 0 at both -O1 and -O3 in version 20 (others were not tested).
godbolt for reference https://godbolt.org/z/5r1Ednf1Y
