[PATCH] D123409: [AArch64] Use PerfectShuffle costs in AArch64TTIImpl::getShuffleCost

[Sam Tebbs via Phabricator via llvm-commits]

samtebbs added inline comments.


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Comment at: llvm/lib/Target/AArch64/AArch64PerfectShuffle.h:6590
+static unsigned getPerfectShuffleCost(llvm::ArrayRef<int> M) {
+  assert(M.size() == 4 && "Expected a 4 entry perfect shuffle");
+
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Why do we have to limit this to 4x16 or 4x32 shuffles?


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Comment at: llvm/lib/Target/AArch64/AArch64PerfectShuffle.h:6602-6609
+  unsigned PFIndexes[4];
+  for (unsigned i = 0; i != 4; ++i) {
+    assert(M[i] < 8 && "Expected a maximum entry of 8 for shuffle mask");
+    if (M[i] < 0)
+      PFIndexes[i] = 8;
+    else
+      PFIndexes[i] = M[i];
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A comment about what is being done here would be beneficial.


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Comment at: llvm/lib/Target/AArch64/AArch64PerfectShuffle.h:6611-6617
+  // Compute the index in the perfect shuffle table.
+  unsigned PFTableIndex = PFIndexes[0] * 9 * 9 * 9 + PFIndexes[1] * 9 * 9 +
+                          PFIndexes[2] * 9 + PFIndexes[3];
+  unsigned PFEntry = PerfectShuffleTable[PFTableIndex];
+  // And extract the cost from the upper bits. The cost is encoded as Cost-1.
+  unsigned Cost = (PFEntry >> 30) + 1;
+  return Cost;
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A bit more elaboration here would be nice too.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D123409/new/

https://reviews.llvm.org/D123409