https://bugs.llvm.org/show_bug.cgi?id=43550
Bug ID: 43550
Summary: missed DCE optimization after loop unrolling
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Keywords: code-quality
Severity: normal
Priority: P
Component: Common Code Generator Code
Assignee: unassignedb...@nondot.org
Reporter: tras...@gmail.com
CC: llvm-bugs@lists.llvm.org
#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 after everything is unrolled the while loop unnecessarily
remains: https://godbolt.org/z/cA-S8J
Of course it's even more severe with N=8: https://godbolt.org/z/niZPeS
Interestingly if you change the swap line slightly it suddenly starts using
pshufd which seems quite clever: https://godbolt.org/z/5YJnJ1
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