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

[Simon Pilgrim via llvm-commits]

================
@@ -8962,16 +8962,29 @@ 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);
-      }
-      return R;
+    // Special case: clmul(X, Y) where Y is a known constant (splat) that forms
+    // a contiguous block of trailing ones whose length N is a power of two
+    // (e.g. i8 0xFF, i8 0x0F, ...) or equal to the operand width. In this
+    // special case, clmul(X, Y) is equivalent to a "parallel prefix XOR" or
+    // "bitwise parity" operation on X.
+    //
+    // Note: This special currently dose NOT apply when the mask is neither a
+    // power of two nor equal to the operand width because the loop inside
+    // behaves as if the mask was bit-ceiled, and "undoing" the XOR with parts
+    // of that CLMUL is a recursive problem (e.g. CLMUL with a 20-bit mask
+    // requires correction XOR with CLMUL with 12-bit mask).
+    if (auto *C = isConstOrConstSplat(Y, /*AllowUndefs=*/true)) {
+      const APInt &YVal = C->getAPIntValue();
+      unsigned N = YVal.countr_one();
+      if (YVal.isMask() && (isPowerOf2_32(N) || N == BW)) {
----------------
RKSimon wrote:

```suggestion
      if (YVal.isAllOnes() || (YVal.isMask() && isPowerOf2_32(N))) {
```

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