Handle a special case xor undef, undef -> 0. Technically this should be transformed to undef. But this is such a common idiom (misuse) we are going to handle it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@48792 91177308-0d34-0410-b5e6-96231b3b80d8

3 files changed, 15 insertions, 2 deletions

diff --git a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 2229743..e24a294 100644
--- a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -2119,6 +2119,9 @@ SDOperand DAGCombiner::visitXOR(SDNode *N) {
     if (FoldedVOp.Val) return FoldedVOp;
   }
   
+  // fold (xor undef, undef) -> 0. This is a common idiom (misuse).
+  if (N0.getOpcode() == ISD::UNDEF && N1.getOpcode() == ISD::UNDEF)
+    return DAG.getConstant(0, VT);
   // fold (xor x, undef) -> undef
   if (N0.getOpcode() == ISD::UNDEF)
     return N0;
diff --git a/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 05f7edb..ed564b2 100644
--- a/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -2245,6 +2245,12 @@ SDOperand SelectionDAG::getNode(unsigned Opcode, MVT::ValueType VT,
   // Fold a bunch of operators when the RHS is undef. 
   if (N2.getOpcode() == ISD::UNDEF) {
     switch (Opcode) {
+    case ISD::XOR:
+      if (N1.getOpcode() == ISD::UNDEF)
+        // Handle undef ^ undef -> 0 special case. This is a common
+        // idiom (misuse).
+        return getConstant(0, VT);
+      // fallthrough
     case ISD::ADD:
     case ISD::ADDC:
     case ISD::ADDE:
@@ -2258,7 +2264,6 @@ SDOperand SelectionDAG::getNode(unsigned Opcode, MVT::ValueType VT,
     case ISD::SDIV:
     case ISD::UREM:
     case ISD::SREM:
-    case ISD::XOR:
       return N2;       // fold op(arg1, undef) -> undef
     case ISD::MUL: 
     case ISD::AND:
diff --git a/lib/VMCore/ConstantFold.cpp b/lib/VMCore/ConstantFold.cpp
index 762a24a..7b21817 100644
--- a/lib/VMCore/ConstantFold.cpp
+++ b/lib/VMCore/ConstantFold.cpp
@@ -474,9 +474,14 @@ Constant *llvm::ConstantFoldBinaryInstruction(unsigned Opcode,
   // Handle UndefValue up front
   if (isa<UndefValue>(C1) || isa<UndefValue>(C2)) {
     switch (Opcode) {
+    case Instruction::Xor:
+      if (isa<UndefValue>(C1) && isa<UndefValue>(C2))
+        // Handle undef ^ undef -> 0 special case. This is a common
+        // idiom (misuse).
+        return Constant::getNullValue(C1->getType());
+      // Fallthrough
     case Instruction::Add:
     case Instruction::Sub:
-    case Instruction::Xor:
       return UndefValue::get(C1->getType());
     case Instruction::Mul:
     case Instruction::And: