[llvm] [SelectionDAG] Expand CLMUL of zero-extended all-ones operand via parallel prefix XOR (PR #207339)

[Simon Pilgrim via llvm-commits]

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@@ -8909,16 +8909,30 @@ SDValue TargetLowering::expandCLMUL(SDNode *Node, SelectionDAG &DAG) const {
       }
     }
 
-    // Special case: clmul(X, ~0) is equivalent to a "parallel prefix XOR" or
-    // "bitwise parity" operation.
-    if (isAllOnesOrAllOnesSplat(Y)) {
-      SDValue R = X;
-      for (unsigned I = 1; I < BW; I <<= 1) {
-        SDValue ShAmt = DAG.getShiftAmountConstant(I, VT, DL);
-        SDValue Shifted = DAG.getNode(ISD::SHL, DL, VT, R, ShAmt);
-        R = DAG.getNode(ISD::XOR, DL, VT, R, Shifted);
+    // Special case: clmul(X, Y) where Y is a known constant (splat) that forms
+    // a contiguous block of trailing ones (e.g. i8 0xFF, i8 0x0F, ...). In this
+    // special case, clmul(X, Y) is equivalent to a "parallel prefix XOR" or
+    // "bitwise parity" operation on X, optionally surrounded by truncation
+    // and zero-extension.
+    if (auto *C = isConstOrConstSplat(Y, /*AllowUndefs=*/true)) {
+      APInt YVal = C->getAPIntValue();
----------------
RKSimon wrote:

```suggestion
      const APInt &YVal = C->getAPIntValue();
```

https://github.com/llvm/llvm-project/pull/207339