[SelectionDAG] Optional handling of UNDEF elements in matchUnaryPredicate

Now that SimplifyDemandedBits/SimplifyDemandedVectorElts are simplifying vector elements, we're seeing more constant BUILD_VECTOR containing UNDEFs.

This patch provides opt-in handling of UNDEF elements in matchUnaryPredicate, passing NULL instead of the ConstantSDNode* argument.

I've updated SelectionDAG::simplifyShift to demonstrate its use.

Differential Revision: https://reviews.llvm.org/D55819

llvm-svn: 349616

1 files changed, 13 insertions, 4 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index d69e50d9007..a1ce77455e9 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -269,15 +269,24 @@ bool ISD::allOperandsUndef(const SDNode *N) {
 }
 
 bool ISD::matchUnaryPredicate(SDValue Op,
-                              std::function<bool(ConstantSDNode *)> Match) {
+                              std::function<bool(ConstantSDNode *)> Match,
+                              bool AllowUndefs) {
+  // FIXME: Add support for scalar UNDEF cases?
   if (auto *Cst = dyn_cast<ConstantSDNode>(Op))
     return Match(Cst);
 
+  // FIXME: Add support for vector UNDEF cases?
   if (ISD::BUILD_VECTOR != Op.getOpcode())
     return false;
 
   EVT SVT = Op.getValueType().getScalarType();
   for (unsigned i = 0, e = Op.getNumOperands(); i != e; ++i) {
+    if (AllowUndefs && Op.getOperand(i).isUndef()) {
+      if (!Match(nullptr))
+        return false;
+      continue;
+    }
+
     auto *Cst = dyn_cast<ConstantSDNode>(Op.getOperand(i));
     if (!Cst || Cst->getValueType(0) != SVT || !Match(Cst))
       return false;
@@ -6967,11 +6976,11 @@ SDValue SelectionDAG::simplifyShift(SDValue X, SDValue Y) {
     return X;
 
   // shift X, C >= bitwidth(X) --> undef
-  // All vector elements must be too big to avoid partial undefs.
+  // All vector elements must be too big (or undef) to avoid partial undefs.
   auto isShiftTooBig = [X](ConstantSDNode *Val) {
-    return Val->getAPIntValue().uge(X.getScalarValueSizeInBits());
+    return !Val || Val->getAPIntValue().uge(X.getScalarValueSizeInBits());
   };
-  if (ISD::matchUnaryPredicate(Y, isShiftTooBig))
+  if (ISD::matchUnaryPredicate(Y, isShiftTooBig, true))
     return getUNDEF(X.getValueType());
 
   return SDValue();