[llvm] [ISel] Introduce llvm.clmul intrinsic (PR #168731)

[Craig Topper via llvm-commits]

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@@ -11440,6 +11420,28 @@ SDValue DAGCombiner::visitSRL(SDNode *N) {
   if (SDValue AVG = foldShiftToAvg(N, DL))
     return AVG;
 
+  // Fold clmul(zext(x), zext(y)) >> (BW - 1 | BW) -> clmul(r|h)(x, y).
+  if (VT.getScalarSizeInBits() % 2)
+    return SDValue();
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topperc wrote:

Don't early out. Inevitably someone will add code below this without noticing this. Either wrap the whole thing with an if or more move it to a function like combineShiftToMULH.

https://github.com/llvm/llvm-project/pull/168731