[TargetLowering] use isSubsetOf in SimplifyDemandedBits; NFCI

This is the DAG equivalent of https://reviews.llvm.org/D32255 , 
which will hopefully be committed again. The functionality
(preferring a 'not' op) is already here in the DAG, so this is
just intended to be a clean-up and performance improvement.

llvm-svn: 302087

1 files changed, 14 insertions, 18 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index 2d39ecd9779..60de36d04a3 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -659,7 +659,7 @@ bool TargetLowering::SimplifyDemandedBits(SDValue Op,
     // Output known-1 are known to be set if set in either the LHS | RHS.
     Known.One |= Known2.One;
     break;
-  case ISD::XOR:
+  case ISD::XOR: {
     if (SimplifyDemandedBits(Op.getOperand(1), NewMask, Known, TLO, Depth+1))
       return true;
     assert((Known.Zero & Known.One) == 0 && "Bits known to be one AND zero?");
@@ -704,28 +704,24 @@ bool TargetLowering::SimplifyDemandedBits(SDValue Op,
       }
     }
 
-    // If the RHS is a constant, see if we can simplify it.
-    // for XOR, we prefer to force bits to 1 if they will make a -1.
-    // If we can't force bits, try to shrink the constant.
-    if (ConstantSDNode *C = isConstOrConstSplat(Op.getOperand(1))) {
-      APInt Expanded = C->getAPIntValue() | (~NewMask);
-      // If we can expand it to have all bits set, do it.
-      if (Expanded.isAllOnesValue()) {
-        if (Expanded != C->getAPIntValue()) {
-          EVT VT = Op.getValueType();
-          SDValue New = TLO.DAG.getNode(Op.getOpcode(), dl,VT, Op.getOperand(0),
-                                        TLO.DAG.getConstant(Expanded, dl, VT));
-          return TLO.CombineTo(Op, New);
-        }
-        // If it already has all the bits set, nothing to change
-        // but don't shrink either!
-      } else if (ShrinkDemandedConstant(Op, NewMask, TLO)) {
-        return true;
+    // If the RHS is a constant, see if we can change it. Don't alter a -1
+    // constant because that's a 'not' op, and that is better for combining and
+    // codegen.
+    ConstantSDNode *C = isConstOrConstSplat(Op.getOperand(1));
+    if (C && !C->isAllOnesValue()) {
+      if (NewMask.isSubsetOf(C->getAPIntValue())) {
+        // We're flipping all demanded bits. Flip the undemanded bits too.
+        SDValue New = TLO.DAG.getNOT(dl, Op.getOperand(0), Op.getValueType());
+        return TLO.CombineTo(Op, New);
       }
+      // If we can't turn this into a 'not', try to shrink the constant.
+      if (ShrinkDemandedConstant(Op, NewMask, TLO))
+        return true;
     }
 
     Known = std::move(KnownOut);
     break;
+  }
   case ISD::SELECT:
     if (SimplifyDemandedBits(Op.getOperand(2), NewMask, Known, TLO, Depth+1))
       return true;