[DAG] Bound loop dependence check in merge optimization.

The loop dependence check looks for dependencies between store merge
candidates not captured by the chain sub-DAG doing a check of
predecessors which may be very large. Conservatively bound number of
nodes checked for compilation time. (Resolves PR34326).

Landing on behalf of Nirav Dave to unblock the 5.0.0 release.

Differential Revision: https://reviews.llvm.org/D37220

llvm-svn: 312022

1 files changed, 17 insertions, 5 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 306fd6f4984..db87f8a6275 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -12780,25 +12780,37 @@ void DAGCombiner::getStoreMergeCandidates(
         }
 }
 
-// We need to check that merging these stores does not cause a loop
-// in the DAG. Any store candidate may depend on another candidate
+// We need to check that merging these stores does not cause a loop in
+// the DAG. Any store candidate may depend on another candidate
 // indirectly through its operand (we already consider dependencies
 // through the chain). Check in parallel by searching up from
 // non-chain operands of candidates.
+
 bool DAGCombiner::checkMergeStoreCandidatesForDependencies(
     SmallVectorImpl<MemOpLink> &StoreNodes, unsigned NumStores) {
+
+  // FIXME: We should be able to truncate a full search of
+  // predecessors by doing a BFS and keeping tabs the originating
+  // stores from which worklist nodes come from in a similar way to
+  // TokenFactor simplfication.
+
   SmallPtrSet<const SDNode *, 16> Visited;
   SmallVector<const SDNode *, 8> Worklist;
-  // search ops of store candidates
+  unsigned int Max = 8192;
+  // Search Ops of store candidates.
   for (unsigned i = 0; i < NumStores; ++i) {
     SDNode *n = StoreNodes[i].MemNode;
     // Potential loops may happen only through non-chain operands
     for (unsigned j = 1; j < n->getNumOperands(); ++j)
       Worklist.push_back(n->getOperand(j).getNode());
   }
-  // search through DAG. We can stop early if we find a storenode
+  // Search through DAG. We can stop early if we find a store node.
   for (unsigned i = 0; i < NumStores; ++i) {
-    if (SDNode::hasPredecessorHelper(StoreNodes[i].MemNode, Visited, Worklist))
+    if (SDNode::hasPredecessorHelper(StoreNodes[i].MemNode, Visited, Worklist,
+                                     Max))
+      return false;
+    // Check if we ended early, failing conservatively if so.
+    if (Visited.size() >= Max)
       return false;
   }
   return true;