[PATCH] D100264: [SCEV] Don't walk uses of phis without SCEV expression when forgetting

[Nikita Popov via Phabricator via llvm-commits]

nikic created this revision.
nikic added reviewers: mkazantsev, reames.
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I've run into some cases where a large fraction of compile-time is spent invalidating SCEV. One of the causes is forgetLoop(), which walks all values that are def-use reachable from the loop header phis. When invalidating a topmost loop, that might be close to all values in a function. Additionally, it's fairly common for there to not actually be anything to invalidate, but we'll still be performing this walk again and again.

My first thought was that we don't need to continue walking the uses if the current value doesn't have a SCEV expression. However, this isn't quite right, because SCEV construction can skip over values (e.g. for chain of adds, we might only create a SCEV expression for the final value).

What this patch does instead is to only walk the (full) def-use chain of loop phis that have a SCEV expression. If there's no expression for a phi, then we also don't have any dependent expressions to invalidate.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D100264

Files:
  llvm/include/llvm/Analysis/ScalarEvolution.h
  llvm/lib/Analysis/ScalarEvolution.cpp


Index: llvm/lib/Analysis/ScalarEvolution.cpp
===================================================================
--- llvm/lib/Analysis/ScalarEvolution.cpp
+++ llvm/lib/Analysis/ScalarEvolution.cpp
@@ -7020,13 +7020,17 @@
 }
 
 /// Push PHI nodes in the header of the given loop onto the given Worklist.
-static void
-PushLoopPHIs(const Loop *L, SmallVectorImpl<Instruction *> &Worklist) {
+/// Only push PHIs for which a SCEV expression has been cached, otherwise there
+/// cannot be any dependent expressions to invalidate.
+void ScalarEvolution::pushCachedLoopPHIs(
+    const Loop *L, SmallVectorImpl<Instruction *> &Worklist) {
   BasicBlock *Header = L->getHeader();
-
-  // Push all Loop-header PHIs onto the Worklist stack.
-  for (PHINode &PN : Header->phis())
-    Worklist.push_back(&PN);
+  for (PHINode &PN : Header->phis()) {
+    ValueExprMapType::iterator It =
+        ValueExprMap.find_as(static_cast<Value *>(&PN));
+    if (It != ValueExprMap.end())
+      Worklist.push_back(&PN);
+  }
 }
 
 const ScalarEvolution::BackedgeTakenInfo &
@@ -7087,7 +7091,7 @@
   // it handles SCEVUnknown PHI nodes specially.
   if (Result.hasAnyInfo()) {
     SmallVector<Instruction *, 16> Worklist;
-    PushLoopPHIs(L, Worklist);
+    pushCachedLoopPHIs(L, Worklist);
 
     SmallPtrSet<Instruction *, 8> Discovered;
     while (!Worklist.empty()) {
@@ -7210,7 +7214,7 @@
     }
 
     // Drop information about expressions based on loop-header PHIs.
-    PushLoopPHIs(CurrL, Worklist);
+    pushCachedLoopPHIs(CurrL, Worklist);
 
     while (!Worklist.empty()) {
       Instruction *I = Worklist.pop_back_val();
Index: llvm/include/llvm/Analysis/ScalarEvolution.h
===================================================================
--- llvm/include/llvm/Analysis/ScalarEvolution.h
+++ llvm/include/llvm/Analysis/ScalarEvolution.h
@@ -2053,6 +2053,12 @@
   std::tuple<SCEV *, FoldingSetNodeID, void *>
   findExistingSCEVInCache(SCEVTypes SCEVType, ArrayRef<const SCEV *> Ops);
 
+  /// Push PHI nodes in the header of the given loop onto the given Worklist
+  /// if they have a cached SCEV expression. If no expression is cached, then
+  /// there also aren't any dependent expressions to invalidate.
+  void pushCachedLoopPHIs(const Loop *L,
+                          SmallVectorImpl<Instruction *> &Worklist);
+
   FoldingSet<SCEV> UniqueSCEVs;
   FoldingSet<SCEVPredicate> UniquePreds;
   BumpPtrAllocator SCEVAllocator;


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