3 files changed, 32 insertions, 13 deletions

diff --git a/llvm/include/llvm/CodeGen/SelectionDAGNodes.h b/llvm/include/llvm/CodeGen/SelectionDAGNodes.h
index f8248b84533..41407f15654 100644
--- a/llvm/include/llvm/CodeGen/SelectionDAGNodes.h
+++ b/llvm/include/llvm/CodeGen/SelectionDAGNodes.h
@@ -74,6 +74,10 @@ namespace ISD {
   /// ISD::SCALAR_TO_VECTOR node or a BUILD_VECTOR node where only the low
   /// element is not an undef.
   bool isScalarToVector(const SDNode *N);
+
+  /// allOperandsUndef - Return true if the node has at least one operand
+  /// and all operands of the specified node are ISD::UNDEF.
+  bool allOperandsUndef(const SDNode *N);
 }  // end llvm:ISD namespace
 
 //===----------------------------------------------------------------------===//
diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 1e87d5184f2..0e5ecd5d2dc 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -7598,6 +7598,11 @@ SDValue DAGCombiner::visitBUILD_VECTOR(SDNode *N) {
   unsigned NumInScalars = N->getNumOperands();
   DebugLoc dl = N->getDebugLoc();
   EVT VT = N->getValueType(0);
+
+  // A vector built entirely of undefs is undef.
+  if (ISD::allOperandsUndef(N))
+    return DAG.getUNDEF(VT);
+
   // Check to see if this is a BUILD_VECTOR of a bunch of values
   // which come from any_extend or zero_extend nodes. If so, we can create
   // a new BUILD_VECTOR using bit-casts which may enable other BUILD_VECTOR
@@ -7605,12 +7610,11 @@ SDValue DAGCombiner::visitBUILD_VECTOR(SDNode *N) {
   // using shuffles.
   EVT SourceType = MVT::Other;
   bool AllAnyExt = true;
-  bool AllUndef = true;
+
   for (unsigned i = 0; i != NumInScalars; ++i) {
     SDValue In = N->getOperand(i);
     // Ignore undef inputs.
     if (In.getOpcode() == ISD::UNDEF) continue;
-    AllUndef = false;
 
     bool AnyExt  = In.getOpcode() == ISD::ANY_EXTEND;
     bool ZeroExt = In.getOpcode() == ISD::ZERO_EXTEND;
@@ -7638,9 +7642,6 @@ SDValue DAGCombiner::visitBUILD_VECTOR(SDNode *N) {
     AllAnyExt &= AnyExt;
   }
 
-  if (AllUndef)
-    return DAG.getUNDEF(VT);
-
   // In order to have valid types, all of the inputs must be extended from the
   // same source type and all of the inputs must be any or zero extend.
   // Scalar sizes must be a power of two.
@@ -7817,14 +7818,7 @@ SDValue DAGCombiner::visitCONCAT_VECTORS(SDNode *N) {
     return N->getOperand(0);
 
   // Check if all of the operands are undefs.
-  bool AllUndef = true;
-  for (unsigned i = 0; i < N->getNumOperands(); ++i)
-    if (N->getOperand(i).getOpcode() != ISD::UNDEF) {
-      AllUndef = false;
-      break;
-    }
-
-  if (AllUndef)
+  if (ISD::allOperandsUndef(N))
     return DAG.getUNDEF(N->getValueType(0));
 
   return SDValue();
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index a8dce2176fb..3947648e6a7 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -219,6 +219,22 @@ bool ISD::isScalarToVector(const SDNode *N) {
   return true;
 }
 
+/// allOperandsUndef - Return true if the node has at least one operand
+/// and all operands of the specified node are ISD::UNDEF.
+bool ISD::allOperandsUndef(const SDNode *N) {
+  // Return false if the node has no operands.
+  // This is "logically inconsistent" with the definition of "all" but
+  // is probably the desired behavior.
+  if (N->getNumOperands() == 0)
+    return false;
+
+  for (unsigned i = 0, e = N->getNumOperands(); i != e ; ++i)
+    if (N->getOperand(i).getOpcode() != ISD::UNDEF)
+      return false;
+
+  return true;
+}
+
 /// getSetCCSwappedOperands - Return the operation corresponding to (Y op X)
 /// when given the operation for (X op Y).
 ISD::CondCode ISD::getSetCCSwappedOperands(ISD::CondCode Operation) {
@@ -2674,6 +2690,11 @@ SDValue SelectionDAG::getNode(unsigned Opcode, DebugLoc DL, EVT VT,
     if (N1 == N2) return N1;
     break;
   case ISD::CONCAT_VECTORS:
+    // Concat of UNDEFs is UNDEF.
+    if (N1.getOpcode() == ISD::UNDEF &&
+        N2.getOpcode() == ISD::UNDEF)
+      return getUNDEF(VT);
+
     // A CONCAT_VECTOR with all operands BUILD_VECTOR can be simplified to
     // one big BUILD_VECTOR.
     if (N1.getOpcode() == ISD::BUILD_VECTOR &&