[DAGCombine] Simplify ISD::AND in GetDemandedBits.

This helps in cases involving bitfields where an AND is exposed by
legalization.

Differential Revision: https://reviews.llvm.org/D30472

llvm-svn: 297249

1 files changed, 11 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 1abc7049b1c..8bed34497f6 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -7461,6 +7461,9 @@ SDValue DAGCombiner::visitAssertZext(SDNode *N) {
 /// See if the specified operand can be simplified with the knowledge that only
 /// the bits specified by Mask are used.  If so, return the simpler operand,
 /// otherwise return a null SDValue.
+///
+/// (This exists alongside SimplifyDemandedBits because GetDemandedBits can
+/// simplify nodes with multiple uses more aggressively.)
 SDValue DAGCombiner::GetDemandedBits(SDValue V, const APInt &Mask) {
   switch (V.getOpcode()) {
   default: break;
@@ -7496,6 +7499,14 @@ SDValue DAGCombiner::GetDemandedBits(SDValue V, const APInt &Mask) {
         return DAG.getNode(ISD::SRL, SDLoc(V), V.getValueType(),
                            SimplifyLHS, V.getOperand(1));
     }
+    break;
+  case ISD::AND: {
+    // X & -1 -> X (ignoring bits which aren't demanded).
+    ConstantSDNode *AndVal = isConstOrConstSplat(V.getOperand(1));
+    if (AndVal && (AndVal->getAPIntValue() & Mask) == Mask)
+      return V.getOperand(0);
+    break;
+  }
   }
   return SDValue();
 }