[llvm] goldstein/select and zext (PR #66793)

[Craig Topper via llvm-commits]

================
@@ -6294,6 +6294,37 @@ static SDValue foldAndOrOfSETCC(SDNode *LogicOp, SelectionDAG &DAG) {
   return SDValue();
 }
 
+// Combine `(select c, (X & 1), 0)` -> `(and (zext c), X)`.
+// We canonicalize to the `select` form in the middle end, but the `and` form
+// gets better codegen and all tested targets (arm, x86, riscv)
+static SDValue combineSelectAsExtAnd(SDValue Cond, SDValue T, SDValue F,
+                                     const SDLoc &DL, SelectionDAG &DAG) {
+  const TargetLowering &TLI = DAG.getTargetLoweringInfo();
+  auto *FC = dyn_cast<ConstantSDNode>(F);
+  if (FC == nullptr || !FC->isZero())
+    return SDValue();
+
+  EVT CondVT = Cond.getValueType();
+  ISD::NodeType BoolExtOpc =
+      TLI.getExtendForContent(TLI.getBooleanContents(CondVT));
+  if (BoolExtOpc != ISD::ZERO_EXTEND)
+    return SDValue();
+
+  if (T.getOpcode() != ISD::AND)
+    return SDValue();
+
+  auto *TC1 = dyn_cast<ConstantSDNode>(T.getOperand(1));
+  if (TC1 == nullptr || !TC1->isOne())
----------------
topperc wrote:

!isOneConstant(T)?

https://github.com/llvm/llvm-project/pull/66793