[SelectionDAG] Teach GetDemandedBits to look at the known zeros of the LHS when handling ISD::AND

If the LHS has known zeros, then the RHS immediate mask might have been simplified to remove those bits.

This patch adds a call to computeKnownBits to get the known zeroes to handle that possibility. I left an early out to skip the call if all of the demanded bits are set in the mask.

Differential Revision: https://reviews.llvm.org/D58464

llvm-svn: 354514

1 files changed, 7 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 65b738151e6..dd284430645 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -2102,9 +2102,13 @@ SDValue SelectionDAG::GetDemandedBits(SDValue V, const APInt &Mask) {
     break;
   case ISD::AND: {
     // X & -1 -> X (ignoring bits which aren't demanded).
-    ConstantSDNode *AndVal = isConstOrConstSplat(V.getOperand(1));
-    if (AndVal && Mask.isSubsetOf(AndVal->getAPIntValue()))
-      return V.getOperand(0);
+    // Also handle the case where masked out bits in X are known to be zero.
+    if (ConstantSDNode *RHSC = isConstOrConstSplat(V.getOperand(1))) {
+      const APInt &AndVal = RHSC->getAPIntValue();
+      if (Mask.isSubsetOf(AndVal) ||
+          Mask.isSubsetOf(computeKnownBits(V.getOperand(0)).Zero | AndVal))
+        return V.getOperand(0);
+    }
     break;
   }
   case ISD::ANY_EXTEND: {