[PATCH] D60756: [NFCI] Replace linear scan with bisection.

[Nick Lewycky via Phabricator via llvm-commits]

nicholas created this revision.
Herald added subscribers: llvm-commits, hiraditya.
Herald added a project: LLVM.
nicholas added a parent revision: D60755: [NFCI] Improve efficiency of isPotentiallyReachableFromManyDomTree..

Instead of storing each DomTreeNode that we've visited and checking whether our next block is dominated by any of them, store only the blocks which aren't dominated by other blocks in  the visited set, and use bisection on the dfs-in number to identify the only block in the set that might be dominating.


https://reviews.llvm.org/D60756

Files:
  llvm/lib/Analysis/CFG.cpp


Index: llvm/lib/Analysis/CFG.cpp
===================================================================
--- llvm/lib/Analysis/CFG.cpp
+++ llvm/lib/Analysis/CFG.cpp
@@ -130,17 +130,52 @@
 
   const Loop *StopLoop = NodeToBlock(StopNode);
 
+  DT.updateDFSNumbers();
+
   // Limit the number of blocks we visit. The goal is to avoid run-away compile
   // times on large CFGs without hampering sensible code. Arbitrarily chosen.
   unsigned Limit = 32;
-  SmallPtrSet<const DomTreeNode *, 32> Visited;
+  SmallVector<const DomTreeNode *, 32> Visited;
+
+  // Insert a DomTreeNode into Visited. Coalesces redundant DomTreeNodes.
+  // Returns true iff the insertion modified Visited.
+  auto Insert = [&](const DomTreeNode *N) {
+    auto StartIt = lower_bound(
+        Visited, N, [](const DomTreeNode *LHS, const DomTreeNode *RHS) {
+          return LHS->getDFSNumIn() < RHS->getDFSNumIn();
+        });
+    if (StartIt != Visited.end() && DT.dominates(*StartIt, N))
+      return false;
+
+    // Can't be anything to coalesce if our dfs-in number is largest.
+    if (StartIt == Visited.end()) {
+      Visited.push_back(N);
+      return true;
+    }
+
+    // Linear scan to find ranges to coalesce into N.
+    ++StartIt;
+    auto EndIt = StartIt;
+    for (; EndIt != Visited.end(); ++EndIt)
+      if (!DT.dominates(N, *EndIt))
+        break;
+
+    // Nothing to coalesce, just add the new entry.
+    if (StartIt == EndIt) {
+      Visited.insert(StartIt, N);
+      return true;
+    }
+
+    // We have one or more to coalesce. Reuse one of the slots in the vector
+    // and erase the rest.
+    *StartIt = N;
+    Visited.erase(++StartIt, EndIt);
+    return true;
+  };
 
   do {
     DomTreeNode *DTN = Worklist.pop_back_val();
-
-    if (any_of(Visited, [&](const DomTreeNode *VisitedNode) {
-            return DT.dominates(VisitedNode, DTN);
-        }))
+    if (!Insert(DTN))
       continue;
 
     if (const Loop *Outer = NodeToBlock(DTN)) {
@@ -152,8 +187,6 @@
       DTN = DT.getNode(Outer->getHeader());
     }
 
-    Visited.insert(DTN);
-
     if (DT.dominates(DTN, StopNode))
       return true;
 


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