[PATCH] D120648: [DAGCombine] fold (bswap(srl (bswap c), x)) -> (shl c, x)

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added inline comments.


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Comment at: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:9727
 
+  // fold (bswap(srl (bswap c), x)) -> (shl c, x)
+  if (N0->getOpcode() == ISD::SRL && N0.hasOneUse()) {
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Comment at: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:9729-9730
+  if (N0->getOpcode() == ISD::SRL && N0.hasOneUse()) {
+    auto *ShAmt = dyn_cast<ConstantSDNode>(N0.getOperand(1));
+    if (ShAmt && ShAmt->getZExtValue() % 8 == 0) {
+      SDValue BSwap = N0->getOperand(0);
----------------
I'm guessing it isn't worth it to instead check with the knownbits that low 3 bits are zeros?


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D120648/new/

https://reviews.llvm.org/D120648