[SCEV][NFC] Reduce memory footprint & compile time via DFS refactoring

Current implementations of DFS in SCEV check unique-visited of traversed
values on pop, and not on push. As result, the same value may be pushed
multiple times just to be thrown away when popped. These operations are
meaningless and only waste time and increase memory footprint of the
worklist.

This patch reworks the DFS strategy to check uniqueness before push.
Should be NFC.

Differential Revision: https://reviews.llvm.org/D111774
Reviewed By: nikic, reames

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -4410,24 +4410,24 @@ const SCEV *ScalarEvolution::getPointerBase(const SCEV *V) {
 }
 
 /// Push users of the given Instruction onto the given Worklist.
-static void
-PushDefUseChildren(Instruction *I,
-                   SmallVectorImpl<Instruction *> &Worklist) {
+static void PushDefUseChildren(Instruction *I,
+                               SmallVectorImpl<Instruction *> &Worklist,
+                               SmallPtrSetImpl<Instruction *> &Visited) {
   // Push the def-use children onto the Worklist stack.
-  for (User *U : I->users())
-    Worklist.push_back(cast<Instruction>(U));
+  for (User *U : I->users()) {
+    auto *UserInsn = cast<Instruction>(U);
+    if (Visited.insert(UserInsn).second)
+      Worklist.push_back(UserInsn);
+  }
 }
 
 void ScalarEvolution::forgetSymbolicName(Instruction *PN, const SCEV *SymName) {
   SmallVector<Instruction *, 16> Worklist;
-  PushDefUseChildren(PN, Worklist);
-
   SmallPtrSet<Instruction *, 8> Visited;
   Visited.insert(PN);
+  Worklist.push_back(PN);
   while (!Worklist.empty()) {
     Instruction *I = Worklist.pop_back_val();
-    if (!Visited.insert(I).second)
-      continue;
 
     auto It = ValueExprMap.find_as(static_cast<Value *>(I));
     if (It != ValueExprMap.end()) {
@@ -4453,7 +4453,7 @@ void ScalarEvolution::forgetSymbolicName(Instruction *PN, const SCEV *SymName) {
       }
     }
 
-    PushDefUseChildren(I, Worklist);
+    PushDefUseChildren(I, Worklist, Visited);
   }
 }
 
@@ -7370,13 +7370,15 @@ bool ScalarEvolution::isBackedgeTakenCountMaxOrZero(const Loop *L) {
 }
 
 /// Push PHI nodes in the header of the given loop onto the given Worklist.
-static void
-PushLoopPHIs(const Loop *L, SmallVectorImpl<Instruction *> &Worklist) {
+static void PushLoopPHIs(const Loop *L,
+                         SmallVectorImpl<Instruction *> &Worklist,
+                         SmallPtrSetImpl<Instruction *> &Visited) {
   BasicBlock *Header = L->getHeader();
 
   // Push all Loop-header PHIs onto the Worklist stack.
   for (PHINode &PN : Header->phis())
-    Worklist.push_back(&PN);
+    if (Visited.insert(&PN).second)
+      Worklist.push_back(&PN);
 }
 
 const ScalarEvolution::BackedgeTakenInfo &
@@ -7437,9 +7439,8 @@ ScalarEvolution::getBackedgeTakenInfo(const Loop *L) {
   // it handles SCEVUnknown PHI nodes specially.
   if (Result.hasAnyInfo()) {
     SmallVector<Instruction *, 16> Worklist;
-    PushLoopPHIs(L, Worklist);
-
     SmallPtrSet<Instruction *, 8> Discovered;
+    PushLoopPHIs(L, Worklist, Discovered);
     while (!Worklist.empty()) {
       Instruction *I = Worklist.pop_back_val();
 
@@ -7551,12 +7552,10 @@ void ScalarEvolution::forgetLoop(const Loop *L) {
     }
 
     // Drop information about expressions based on loop-header PHIs.
-    PushLoopPHIs(CurrL, Worklist);
+    PushLoopPHIs(CurrL, Worklist, Visited);
 
     while (!Worklist.empty()) {
       Instruction *I = Worklist.pop_back_val();
-      if (!Visited.insert(I).second)
-        continue;
 
       ValueExprMapType::iterator It =
           ValueExprMap.find_as(static_cast<Value *>(I));
@@ -7567,7 +7566,7 @@ void ScalarEvolution::forgetLoop(const Loop *L) {
           ConstantEvolutionLoopExitValue.erase(PN);
       }
 
-      PushDefUseChildren(I, Worklist);
+      PushDefUseChildren(I, Worklist, Visited);
     }
 
     LoopPropertiesCache.erase(CurrL);
@@ -7589,14 +7588,12 @@ void ScalarEvolution::forgetValue(Value *V) {
 
   // Drop information about expressions based on loop-header PHIs.
   SmallVector<Instruction *, 16> Worklist;
-  Worklist.push_back(I);
-
   SmallPtrSet<Instruction *, 8> Visited;
+  Worklist.push_back(I);
+  Visited.insert(I);
+
   while (!Worklist.empty()) {
     I = Worklist.pop_back_val();
-    if (!Visited.insert(I).second)
-      continue;
-
     ValueExprMapType::iterator It =
       ValueExprMap.find_as(static_cast<Value *>(I));
     if (It != ValueExprMap.end()) {
@@ -7606,7 +7603,7 @@ void ScalarEvolution::forgetValue(Value *V) {
         ConstantEvolutionLoopExitValue.erase(PN);
     }
 
-    PushDefUseChildren(I, Worklist);
+    PushDefUseChildren(I, Worklist, Visited);
   }
 }