[llvm] [X86][AVX] Fix handling of out-of-bounds shift amounts in AVX2 vector logical shift nodes #83840 (PR #86922)

[Simon Pilgrim via llvm-commits]

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@@ -45612,6 +45612,30 @@ static SDValue combineSelect(SDNode *N, SelectionDAG &DAG,
     }
   }
 
+  // Exploits AVX2 VSHLV/VSRLV instructions for efficient unsigned vector shifts
+  // with out-of-bounds clamping.
+
+  // Unlike general shift instructions (SHL/SRL), AVX2's VSHLV/VSRLV handle
+  // shift amounts exceeding the element bitwidth. VSHLV clamps the amount to
+  // bitwidth-1 for unsigned shifts, effectively performing a maximum left shift
+  // of bitwidth-1 positions. Similarly, VSRLV returns zero for unsigned shifts
+  // exceeding bitwidth-1, achieving a maximum right shift of bitwidth-1.
+  if (N->getOpcode() == ISD::VSELECT &&
+      (LHS.getOpcode() == ISD::SRL || LHS.getOpcode() == ISD::SHL) &&
+      supportedVectorVarShift(VT, Subtarget, LHS.getOpcode())) {
+    APInt SV;
+    if (Cond.getOpcode() == ISD::SETCC &&
+        Cond.getOperand(0) == LHS.getOperand(1) &&
+        cast<CondCodeSDNode>(Cond.getOperand(2))->get() == ISD::SETULT &&
+        ISD::isConstantSplatVector(Cond.getOperand(1).getNode(), SV) &&
+        ISD::isConstantSplatVectorAllZeros(RHS.getNode()) &&
+        SV == VT.getScalarSizeInBits()) {
+      return DAG.getNode(
+          LHS.getOpcode() == ISD::SRL ? X86ISD::VSRLV : X86ISD::VSHLV, DL,
+          LHS->getVTList(), LHS.getOperand(0), LHS.getOperand(1));
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RKSimon wrote:

LHS->getVTList() can be replaced with VT

https://github.com/llvm/llvm-project/pull/86922