[PATCH] D61397: [DAGCombiner] Limit number of nodes to explore, to avoid quadratic compile time.

[Florian Hahn via Phabricator via llvm-commits]

fhahn updated this revision to Diff 197761.
fhahn added a comment.

Relax limits to 1000 nodes to explore. Further experimenting showed that those
bigger limits are still sufficient to ensure limiting quadratic compile time.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D61397/new/

https://reviews.llvm.org/D61397

Files:
  llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp


Index: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
===================================================================
--- llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -1764,6 +1764,12 @@
 }
 
 SDValue DAGCombiner::visitTokenFactor(SDNode *N) {
+  // Limit the number of nodes to explore, to avoid quadratic compile times.
+  // The limit helps to limit compile time for basic blocks with a large number
+  // of stores.
+  if (N->getNumOperands() > 1000)
+    return SDValue();
+
   // If N has two operands, where one has an input chain equal to the other,
   // the 'other' chain is redundant.
   if (N->getNumOperands() == 2) {
@@ -14837,9 +14843,15 @@
 
   RootNode = St->getChain().getNode();
 
+  // Limit the number of nodes to explore, to avoid quadratic compile times.
+  // The limit helps to limit compile time for basic blocks with a large number
+  // of stores.
+  unsigned MaxNumberOfNodesToCheck = 1000;
   if (LoadSDNode *Ldn = dyn_cast<LoadSDNode>(RootNode)) {
     RootNode = Ldn->getChain().getNode();
-    for (auto I = RootNode->use_begin(), E = RootNode->use_end(); I != E; ++I)
+    for (auto I = RootNode->use_begin(), E = RootNode->use_end(); I != E; ++I) {
+      if (!MaxNumberOfNodesToCheck--)
+        break;
       if (I.getOperandNo() == 0 && isa<LoadSDNode>(*I)) // walk down chain
         for (auto I2 = (*I)->use_begin(), E2 = (*I)->use_end(); I2 != E2; ++I2)
           if (I2.getOperandNo() == 0)
@@ -14849,15 +14861,21 @@
               if (CandidateMatch(OtherST, Ptr, PtrDiff))
                 StoreNodes.push_back(MemOpLink(OtherST, PtrDiff));
             }
-  } else
-    for (auto I = RootNode->use_begin(), E = RootNode->use_end(); I != E; ++I)
-      if (I.getOperandNo() == 0)
+    }
+  } else {
+    for (auto I = RootNode->use_begin(), E = RootNode->use_end(); I != E; ++I) {
+      if (!MaxNumberOfNodesToCheck--)
+        break;
+      if (I.getOperandNo() == 0) {
         if (StoreSDNode *OtherST = dyn_cast<StoreSDNode>(*I)) {
           BaseIndexOffset Ptr;
           int64_t PtrDiff;
           if (CandidateMatch(OtherST, Ptr, PtrDiff))
             StoreNodes.push_back(MemOpLink(OtherST, PtrDiff));
         }
+      }
+    }
+  }
 }
 
 // We need to check that merging these stores does not cause a loop in
@@ -19978,7 +19996,13 @@
   // Add ST's interval.
   Intervals.insert(0, (St->getMemoryVT().getSizeInBits() + 7) / 8, Unit);
 
+  // Limit the number of chains to explore, to avoid quadratic compile times.
+  // The limit helps to limit compile time for basic blocks with a large number
+  // of stores.
+  unsigned MaxNumberOfChains = 1000;
   while (StoreSDNode *Chain = dyn_cast<StoreSDNode>(STChain->getChain())) {
+    if (!MaxNumberOfChains--)
+      break;
     // If the chain has more than one use, then we can't reorder the mem ops.
     if (!SDValue(Chain, 0)->hasOneUse())
       break;


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