[TargetLowering] Fix propagation of undefs in zero extension ops (PR40091)

As described on PR40091, we have several places where zext (and zext_vector_inreg) fold an undef input into an undef output. For zero extensions this is incorrect as the output should guarantee to least have the new upper bits set to zero.

SimplifyDemandedVectorElts is the worst offender (and its the most likely to cause new undefs to appear) but DAGCombiner's tryToFoldExtendOfConstant has a similar issue.

Thanks to @dmgreen for catching this.

Differential Revision: https://reviews.llvm.org/D55883

llvm-svn: 349625

2 files changed, 23 insertions, 4 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 7a22caf9c8b..10cce7813dc 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -8064,10 +8064,15 @@ static SDValue tryToFoldExtendOfConstant(SDNode *N, const TargetLowering &TLI,
   unsigned NumElts = VT.getVectorNumElements();
   SDLoc DL(N);
 
-  for (unsigned i=0; i != NumElts; ++i) {
-    SDValue Op = N0->getOperand(i);
-    if (Op->isUndef()) {
-      Elts.push_back(DAG.getUNDEF(SVT));
+  // For zero-extensions, UNDEF elements still guarantee to have the upper
+  // bits set to zero.
+  bool IsZext =
+      Opcode == ISD::ZERO_EXTEND || Opcode == ISD::ZERO_EXTEND_VECTOR_INREG;
+
+  for (unsigned i = 0; i != NumElts; ++i) {
+    SDValue Op = N0.getOperand(i);
+    if (Op.isUndef()) {
+      Elts.push_back(IsZext ? DAG.getConstant(0, DL, SVT) : DAG.getUNDEF(SVT));
       continue;
     }
 
diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index 4c551d5b231..7053e596237 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -1848,6 +1848,13 @@ bool TargetLowering::SimplifyDemandedVectorElts(
       return true;
     KnownZero = SrcZero.zextOrTrunc(NumElts);
     KnownUndef = SrcUndef.zextOrTrunc(NumElts);
+
+    if (Op.getOpcode() == ISD::ZERO_EXTEND_VECTOR_INREG) {
+      // zext(undef) upper bits are guaranteed to be zero.
+      if (DemandedElts.isSubsetOf(KnownUndef))
+        return TLO.CombineTo(Op, TLO.DAG.getConstant(0, SDLoc(Op), VT));
+      KnownUndef.clearAllBits();
+    }
     break;
   }
   case ISD::OR:
@@ -1892,6 +1899,13 @@ bool TargetLowering::SimplifyDemandedVectorElts(
     if (SimplifyDemandedVectorElts(Op.getOperand(0), DemandedElts, KnownUndef,
                                    KnownZero, TLO, Depth + 1))
       return true;
+
+    if (Op.getOpcode() == ISD::ZERO_EXTEND) {
+      // zext(undef) upper bits are guaranteed to be zero.
+      if (DemandedElts.isSubsetOf(KnownUndef))
+        return TLO.CombineTo(Op, TLO.DAG.getConstant(0, SDLoc(Op), VT));
+      KnownUndef.clearAllBits();
+    }
     break;
   default: {
     if (Op.getOpcode() >= ISD::BUILTIN_OP_END) {