[llvm] [SDAG] Allow folding stack slots into sincos/frexp in more cases (PR #118117)

[Matt Arsenault via llvm-commits]

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@@ -2474,6 +2474,45 @@ SDValue SelectionDAG::getPartialReduceAdd(SDLoc DL, EVT ReducedTy, SDValue Op1,
   return Subvectors[0];
 }
 
+/// Given a store node \p StoreNode, return true if it is safe to fold that node
+/// into \p FPNode, which expands to a library call with output pointers.
+static bool CanFoldStoreIntoFPLibCall(StoreSDNode *StoreNode, SDNode *FPNode) {
+  SmallVector<const SDNode *, 8> Worklist;
+  SmallVector<const SDNode *, 8> DeferredNodes;
+  SmallPtrSet<const SDNode *, 16> Visited;
+
+  // Skip FPNode use by StoreNode (that's the use we want to fold into FPNode).
+  for (SDValue Op : StoreNode->ops())
+    if (Op.getNode() != FPNode)
+      Worklist.push_back(Op.getNode());
+
+  while (!Worklist.empty()) {
+    const SDNode *Node = Worklist.pop_back_val();
+    auto [_, Inserted] = Visited.insert(Node);
+    if (!Inserted)
+      continue;
+
+    // Reached the FPNode (would result in a cycle).
+    // OR Reached CALLSEQ_START (would result in nested call sequences).
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arsenm wrote:

Needing to special case calls feels wrong. Can you just use reachesChainWithoutSideEffects? 

https://github.com/llvm/llvm-project/pull/118117