[ScalarEvolution] Refactor forgetLoop() to improve performance

forgetLoop() has pretty bad performance because it goes over
the same instructions over and over again in particular when
nested loop are involved.
The refactoring changes the function to a not-recursive function
and reusing the allocation for data-structures and the Visited
set.

NFCI

Differential Revision: https://reviews.llvm.org/D37659

llvm-svn: 312920

Author: Kariddi

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -6353,61 +6353,66 @@ ScalarEvolution::getBackedgeTakenInfo(const Loop *L) {
 void ScalarEvolution::forgetLoop(const Loop *L) {
   // Drop any stored trip count value.
   auto RemoveLoopFromBackedgeMap =
-      [L](DenseMap<const Loop *, BackedgeTakenInfo> &Map) {
+      [](DenseMap<const Loop *, BackedgeTakenInfo> &Map, const Loop *L) {
         auto BTCPos = Map.find(L);
         if (BTCPos != Map.end()) {
           BTCPos->second.clear();
           Map.erase(BTCPos);
         }
       };
 
-  RemoveLoopFromBackedgeMap(BackedgeTakenCounts);
-  RemoveLoopFromBackedgeMap(PredicatedBackedgeTakenCounts);
+  SmallVector<const Loop *, 16> LoopWorklist(1, L);
+  SmallVector<Instruction *, 32> Worklist;
+  SmallPtrSet<Instruction *, 16> Visited;
 
-  // Drop information about predicated SCEV rewrites for this loop.
-  for (auto I = PredicatedSCEVRewrites.begin();
-       I != PredicatedSCEVRewrites.end();) {
-    std::pair<const SCEV *, const Loop *> Entry = I->first;
-    if (Entry.second == L)
-      PredicatedSCEVRewrites.erase(I++);
-    else
-      ++I;
-  }
+  // Iterate over all the loops and sub-loops to drop SCEV information.
+  while (!LoopWorklist.empty()) {
+    auto *CurrL = LoopWorklist.pop_back_val();
 
-  // Drop information about expressions based on loop-header PHIs.
-  SmallVector<Instruction *, 16> Worklist;
-  PushLoopPHIs(L, Worklist);
-
-  SmallPtrSet<Instruction *, 8> Visited;
-  while (!Worklist.empty()) {
-    Instruction *I = Worklist.pop_back_val();
-    if (!Visited.insert(I).second)
-      continue;
+    RemoveLoopFromBackedgeMap(BackedgeTakenCounts, CurrL);
+    RemoveLoopFromBackedgeMap(PredicatedBackedgeTakenCounts, CurrL);
 
-    ValueExprMapType::iterator It =
-      ValueExprMap.find_as(static_cast<Value *>(I));
-    if (It != ValueExprMap.end()) {
-      eraseValueFromMap(It->first);
-      forgetMemoizedResults(It->second);
-      if (PHINode *PN = dyn_cast<PHINode>(I))
-        ConstantEvolutionLoopExitValue.erase(PN);
+    // Drop information about predicated SCEV rewrites for this loop.
+    for (auto I = PredicatedSCEVRewrites.begin();
+         I != PredicatedSCEVRewrites.end();) {
+      std::pair<const SCEV *, const Loop *> Entry = I->first;
+      if (Entry.second == CurrL)
+        PredicatedSCEVRewrites.erase(I++);
+      else
+        ++I;
     }
 
-    PushDefUseChildren(I, Worklist);
-  }
+    // Drop information about expressions based on loop-header PHIs.
+    PushLoopPHIs(CurrL, Worklist);
 
-  for (auto I = ExitLimits.begin(); I != ExitLimits.end(); ++I) {
-    auto &Query = I->first;
-    if (Query.L == L)
-      ExitLimits.erase(I);
-  }
+    while (!Worklist.empty()) {
+      Instruction *I = Worklist.pop_back_val();
+      if (!Visited.insert(I).second)
+        continue;
 
-  // Forget all contained loops too, to avoid dangling entries in the
-  // ValuesAtScopes map.
-  for (Loop *I : *L)
-    forgetLoop(I);
+      ValueExprMapType::iterator It =
+          ValueExprMap.find_as(static_cast<Value *>(I));
+      if (It != ValueExprMap.end()) {
+        eraseValueFromMap(It->first);
+        forgetMemoizedResults(It->second);
+        if (PHINode *PN = dyn_cast<PHINode>(I))
+          ConstantEvolutionLoopExitValue.erase(PN);
+      }
+
+      PushDefUseChildren(I, Worklist);
+    }
 
-  LoopPropertiesCache.erase(L);
+    for (auto I = ExitLimits.begin(); I != ExitLimits.end(); ++I) {
+      auto &Query = I->first;
+      if (Query.L == CurrL)
+        ExitLimits.erase(I);
+    }
+
+    LoopPropertiesCache.erase(CurrL);
+    // Forget all contained loops too, to avoid dangling entries in the
+    // ValuesAtScopes map.
+    LoopWorklist.append(CurrL->begin(), CurrL->end());
+  }
 }
 
 void ScalarEvolution::forgetValue(Value *V) {