[DAGCombiner] Fix infinite loop in vector mul/shl combining

We have the following DAGCombiner transformations:
 (mul (shl X, c1), c2) -> (mul X, c2 << c1)
 (mul (shl X, C), Y) -> (shl (mul X, Y), C)
 (shl (mul x, c1), c2) -> (mul x, c1 << c2)
Usually the constant shift is optimised by SelectionDAG::getNode when it is
constructed, by SelectionDAG::FoldConstantArithmetic, but when we're dealing
with vectors and one of those vector constants contains an undef element
FoldConstantArithmetic does not fold and we enter an infinite loop.

Fix this by making FoldConstantArithmetic use getNode to decide how to fold each
vector element, the same as FoldConstantVectorArithmetic does, and rather than
adding the constant shift to the work list instead only apply the transformation
if it's already been folded into a constant, as if it's not we're going to loop
endlessly. Additionally add missing NoOpaques to one of those transformations,
which I noticed when writing the tests for this.

Differential Revision: https://reviews.llvm.org/D26605

llvm-svn: 287766

2 files changed, 15 insertions, 18 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 4890441e37c..b3813ba82ce 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -2157,11 +2157,11 @@ SDValue DAGCombiner::visitMUL(SDNode *N) {
 
   // (mul (shl X, c1), c2) -> (mul X, c2 << c1)
   if (N0.getOpcode() == ISD::SHL &&
-      isConstantOrConstantVector(N1) &&
-      isConstantOrConstantVector(N0.getOperand(1))) {
+      isConstantOrConstantVector(N1, /* NoOpaques */ true) &&
+      isConstantOrConstantVector(N0.getOperand(1), /* NoOpaques */ true)) {
     SDValue C3 = DAG.getNode(ISD::SHL, SDLoc(N), VT, N1, N0.getOperand(1));
-    AddToWorklist(C3.getNode());
-    return DAG.getNode(ISD::MUL, SDLoc(N), VT, N0.getOperand(0), C3);
+    if (isConstantOrConstantVector(C3))
+      return DAG.getNode(ISD::MUL, SDLoc(N), VT, N0.getOperand(0), C3);
   }
 
   // Change (mul (shl X, C), Y) -> (shl (mul X, Y), C) when the shift has one
@@ -4714,8 +4714,8 @@ SDValue DAGCombiner::visitSHL(SDNode *N) {
       isConstantOrConstantVector(N1, /* No Opaques */ true) &&
       isConstantOrConstantVector(N0.getOperand(1), /* No Opaques */ true)) {
     SDValue Shl = DAG.getNode(ISD::SHL, SDLoc(N1), VT, N0.getOperand(1), N1);
-    AddToWorklist(Shl.getNode());
-    return DAG.getNode(ISD::MUL, SDLoc(N), VT, N0.getOperand(0), Shl);
+    if (isConstantOrConstantVector(Shl))
+      return DAG.getNode(ISD::MUL, SDLoc(N), VT, N0.getOperand(0), Shl);
   }
 
   if (N1C && !N1C->isOpaque())
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index f2bb3795a2d..71c83ae5c0f 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -3498,25 +3498,22 @@ SDValue SelectionDAG::FoldConstantArithmetic(unsigned Opcode, const SDLoc &DL,
   EVT SVT = VT.getScalarType();
   SmallVector<SDValue, 4> Outputs;
   for (unsigned I = 0, E = BV1->getNumOperands(); I != E; ++I) {
-    ConstantSDNode *V1 = dyn_cast<ConstantSDNode>(BV1->getOperand(I));
-    ConstantSDNode *V2 = dyn_cast<ConstantSDNode>(BV2->getOperand(I));
-    if (!V1 || !V2) // Not a constant, bail.
-      return SDValue();
-
-    if (V1->isOpaque() || V2->isOpaque())
-      return SDValue();
+    SDValue V1 = BV1->getOperand(I);
+    SDValue V2 = BV2->getOperand(I);
 
     // Avoid BUILD_VECTOR nodes that perform implicit truncation.
-    // FIXME: This is valid and could be handled by truncating the APInts.
+    // FIXME: This is valid and could be handled by truncation.
     if (V1->getValueType(0) != SVT || V2->getValueType(0) != SVT)
       return SDValue();
 
     // Fold one vector element.
-    std::pair<APInt, bool> Folded = FoldValue(Opcode, V1->getAPIntValue(),
-                                              V2->getAPIntValue());
-    if (!Folded.second)
+    SDValue ScalarResult = getNode(Opcode, DL, SVT, V1, V2);
+
+    // Scalar folding only succeeded if the result is a constant or UNDEF.
+    if (!ScalarResult.isUndef() && ScalarResult.getOpcode() != ISD::Constant &&
+        ScalarResult.getOpcode() != ISD::ConstantFP)
       return SDValue();
-    Outputs.push_back(getConstant(Folded.first, DL, SVT));
+    Outputs.push_back(ScalarResult);
   }
 
   assert(VT.getVectorNumElements() == Outputs.size() &&