https://bugs.llvm.org/show_bug.cgi?id=35448
Bug ID: 35448
Summary: Loop unrolling breaks vectorization
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: Loop Optimizer
Assignee: unassignedb...@nondot.org
Reporter: san...@playingwithpointers.com
CC: llvm-bugs@lists.llvm.org
Perhaps this is already a known issue, but Clang/LLVM trunk does not vectorize
the inner matmult loop unless the "#pragma unroll" is enabled:
void f(int * __restrict__ a, int * __restrict__ b, int * __restrict__ r) {
for (int m = 0; m < 64; m++) {
int c = 0;
// #pragma unroll
for (int i = 0; i < 32; i++) {
c += a[i] * b[m * 32 + i];
}
r[m] = c;
}
}
It looks like the loop unroller fully unrolls the inner loop and the SLP
vectorizer is unable to vectorize as well as the Loop vectorizer would have
vectorized the not-unrolled loop.
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