[llvm] [DAGCombiner] Inverse transform `(select c, (and X, 1), 0)` -> `(and (zext c), X)` (PR #66793)

[Simon Pilgrim via llvm-commits]

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@@ -6299,6 +6299,35 @@ 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();
+  if (!isNullConstant(F))
+    return SDValue();
+
+  EVT CondVT = Cond.getValueType();
+  ISD::NodeType BoolExtOpc =
+      TLI.getExtendForContent(TLI.getBooleanContents(CondVT));
+  if (BoolExtOpc != ISD::ZERO_EXTEND)
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RKSimon wrote:

Why not just use TLI.getBooleanContents(CondVT) != ZeroOrOneBooleanContent?

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