[llvm] [X86] SimplifyDemandedBitsForTargetNode: add X86ISD::BZHI handling (PR #177881)

[Simon Pilgrim via llvm-commits]

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@@ -45456,6 +45440,63 @@ bool X86TargetLowering::SimplifyDemandedBitsForTargetNode(
 
     break;
   }
+  case X86ISD::BZHI: {
+    SDValue Op0 = Op.getOperand(0); // src
+    SDValue Op1 = Op.getOperand(1); // mask
+
+    // Rule 1: Only the bottom 8 bits of the mask are required.
+    // Track an upper bound on mask[7:0] so we can apply Rule 2.
+    uint64_t MaxMask8 = 255;
+
+    if (auto *Cst1 = dyn_cast<ConstantSDNode>(Op1)) {
+      // NOTE: SimplifyDemandedBits won't do this for constants.
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RKSimon wrote:

why does this matter?

https://github.com/llvm/llvm-project/pull/177881