https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91986
Bug ID: 91986
Summary: missed loop unrolling optimization
Product: gcc
Version: 9.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: trass3r at gmail dot com
Target Milestone: ---
#include <cstdint>
#include <utility>
const int N = 4;
void bitreverse(int* data)
{
uint32_t j = 0;
for (uint32_t i = 0; i < N; ++i)
{
if (j > i)
std::swap(data[i], data[j]);
uint32_t k = N/2;
while (k <= j)
{
j -= k;
k /= 2;
}
j += k;
}
}
Even with -O3 gcc doesn't fully unroll the loop but still there are quite some
redundant instructions: https://godbolt.org/z/cA-S8J
It's similar with N=8: https://godbolt.org/z/niZPeS
Interestingly if you change the swap line slightly clang suddenly starts using
pshufd which seems quite clever: https://godbolt.org/z/5YJnJ1
