Limit the recursion depth of SelectionDAG::isSplatValue()

This method previously always recursively checked both the left-hand
side and right-hand side of binary operations for splatted (broadcast)
vector values to determine if the parent DAG node is a splat.

Like several other SelectionDAG methods, limit the recursion depth to
MaxRecursionDepth (6). This prevents stack overflow.
See also https://issuetracker.google.com/173785481

Patch by Nicolas Capens. Thanks!

Differential Revision: https://reviews.llvm.org/D92421

Author: bogner

llvm/include/llvm/CodeGen/SelectionDAG.h

@@ -1829,7 +1829,8 @@ class SelectionDAG {
   /// for \p DemandedElts.
   ///
   /// NOTE: The function will return true for a demanded splat of UNDEF values.
-  bool isSplatValue(SDValue V, const APInt &DemandedElts, APInt &UndefElts);
+  bool isSplatValue(SDValue V, const APInt &DemandedElts, APInt &UndefElts,
+                    unsigned Depth = 0);
 
   /// Test whether \p V has a splatted value.
   bool isSplatValue(SDValue V, bool AllowUndefs = false);

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -2355,13 +2355,16 @@ bool SelectionDAG::MaskedValueIsAllOnes(SDValue V, const APInt &Mask,
 /// sense to specify which elements are demanded or undefined, therefore
 /// they are simply ignored.
 bool SelectionDAG::isSplatValue(SDValue V, const APInt &DemandedElts,
-                                APInt &UndefElts) {
+                                APInt &UndefElts, unsigned Depth) {
   EVT VT = V.getValueType();
   assert(VT.isVector() && "Vector type expected");
 
   if (!VT.isScalableVector() && !DemandedElts)
     return false; // No demanded elts, better to assume we don't know anything.
 
+  if (Depth >= MaxRecursionDepth)
+    return false; // Limit search depth.
+
   // Deal with some common cases here that work for both fixed and scalable
   // vector types.
   switch (V.getOpcode()) {
@@ -2376,8 +2379,8 @@ bool SelectionDAG::isSplatValue(SDValue V, const APInt &DemandedElts,
     APInt UndefLHS, UndefRHS;
     SDValue LHS = V.getOperand(0);
     SDValue RHS = V.getOperand(1);
-    if (isSplatValue(LHS, DemandedElts, UndefLHS) &&
-        isSplatValue(RHS, DemandedElts, UndefRHS)) {
+    if (isSplatValue(LHS, DemandedElts, UndefLHS, Depth + 1) &&
+        isSplatValue(RHS, DemandedElts, UndefRHS, Depth + 1)) {
       UndefElts = UndefLHS | UndefRHS;
       return true;
     }
@@ -2386,7 +2389,7 @@ bool SelectionDAG::isSplatValue(SDValue V, const APInt &DemandedElts,
   case ISD::TRUNCATE:
   case ISD::SIGN_EXTEND:
   case ISD::ZERO_EXTEND:
-    return isSplatValue(V.getOperand(0), DemandedElts, UndefElts);
+    return isSplatValue(V.getOperand(0), DemandedElts, UndefElts, Depth + 1);
   }
 
   // We don't support other cases than those above for scalable vectors at
@@ -2441,7 +2444,7 @@ bool SelectionDAG::isSplatValue(SDValue V, const APInt &DemandedElts,
     unsigned NumSrcElts = Src.getValueType().getVectorNumElements();
     APInt UndefSrcElts;
     APInt DemandedSrcElts = DemandedElts.zextOrSelf(NumSrcElts).shl(Idx);
-    if (isSplatValue(Src, DemandedSrcElts, UndefSrcElts)) {
+    if (isSplatValue(Src, DemandedSrcElts, UndefSrcElts, Depth + 1)) {
       UndefElts = UndefSrcElts.extractBits(NumElts, Idx);
       return true;
     }