[SCEV] A different fix for PR33494

Summary:
I don't think rL309080 is the right fix for PR33494 -- caching ExitLimit only
hides the problem[0].  The real issue is that because of how we forget SCEV
expressions ScalarEvolution::getBackedgeTakenInfo, in the test case for PR33494
computing the backedge for any loop invalidates the trip count for every other
loop.  This effectively makes the SCEV cache useless.

I've instead made the SCEV expression invalidation in
ScalarEvolution::getBackedgeTakenInfo less aggressive to fix this issue.

[0]: One way to think about this is that rL309080 essentially augmented the
backedge-taken-count cache with another equivalent exit-limit cache.  The bug
went away because we were explicitly not clearing the exit-limit cache in
getBackedgeTakenInfo.  But instead of doing all of that, we can just avoid
clearing the backedge-taken-count cache.

Reviewers: mkazantsev, mzolotukhin

Subscribers: mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D39361

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319678 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/include/llvm/Analysis/ScalarEvolution.h b/include/llvm/Analysis/ScalarEvolution.h
index 96309debd84a6111b4b31e4c11be41e337d1bbf7..d60c16158fdba44e63586c7a6b25befd77e5c5f0 100644 (file)
--- a/include/llvm/Analysis/ScalarEvolution.h
+++ b/include/llvm/Analysis/ScalarEvolution.h
@@ -1272,9 +1272,6 @@ private:
   /// function as they are computed.
   DenseMap<const Loop *, BackedgeTakenInfo> PredicatedBackedgeTakenCounts;
 
-  // Cache the calculated exit limits for the loops.
-  DenseMap<ExitLimitQuery, ExitLimit> ExitLimits;
-
   /// This map contains entries for all of the PHI instructions that we
   /// attempt to compute constant evolutions for.  This allows us to avoid
   /// potentially expensive recomputation of these properties.  An instruction
@@ -1426,9 +1423,6 @@ private:
   ExitLimit computeExitLimit(const Loop *L, BasicBlock *ExitingBlock,
                              bool AllowPredicates = false);
 
-  ExitLimit computeExitLimitImpl(const Loop *L, BasicBlock *ExitingBlock,
-                                 bool AllowPredicates = false);
-
   /// Compute the number of times the backedge of the specified loop will
   /// execute if its exit condition were a conditional branch of ExitCond,
   /// TBB, and FBB.
@@ -1668,9 +1662,8 @@ private:
   /// to be a constant.
   Optional<APInt> computeConstantDifference(const SCEV *LHS, const SCEV *RHS);
 
-  /// Drop memoized information computed for S. Only erase Exit Limits info if
-  /// we expect that the operation we have made is going to change it.
-  void forgetMemoizedResults(const SCEV *S, bool EraseExitLimit = true);
+  /// Drop memoized information computed for S.
+  void forgetMemoizedResults(const SCEV *S);
 
   /// Return an existing SCEV for V if there is one, otherwise return nullptr.
   const SCEV *getExistingSCEV(Value *V);

diff --git a/lib/Analysis/ScalarEvolution.cpp b/lib/Analysis/ScalarEvolution.cpp
index e598f2005c308bc8c3667c5000731be3f9a18549..98c2845c1e6fccd3ea3703f24118ab869cbaabde 100644 (file)
--- a/lib/Analysis/ScalarEvolution.cpp
+++ b/lib/Analysis/ScalarEvolution.cpp
@@ -6433,13 +6433,36 @@ ScalarEvolution::getBackedgeTakenInfo(const Loop *L) {
         // own when it gets to that point.
         if (!isa<PHINode>(I) || !isa<SCEVUnknown>(Old)) {
           eraseValueFromMap(It->first);
-          forgetMemoizedResults(Old, false);
+          forgetMemoizedResults(Old);
         }
         if (PHINode *PN = dyn_cast<PHINode>(I))
           ConstantEvolutionLoopExitValue.erase(PN);
       }
 
-      PushDefUseChildren(I, Worklist);
+      // Since we don't need to invalidate anything for correctness and we're
+      // only invalidating to make SCEV's results more precise, we get to stop
+      // early to avoid invalidating too much.  This is especially important in
+      // cases like:
+      //
+      //   %v = f(pn0, pn1) // pn0 and pn1 used through some other phi node
+      // loop0:
+      //   %pn0 = phi
+      //   ...
+      // loop1:
+      //   %pn1 = phi
+      //   ...
+      //
+      // where both loop0 and loop1's backedge taken count uses the SCEV
+      // expression for %v.  If we don't have the early stop below then in cases
+      // like the above, getBackedgeTakenInfo(loop1) will clear out the trip
+      // count for loop0 and getBackedgeTakenInfo(loop0) will clear out the trip
+      // count for loop1, effectively nullifying SCEV's trip count cache.
+      for (auto *U : I->users())
+        if (auto *I = dyn_cast<Instruction>(U)) {
+          auto *LoopForUser = LI.getLoopFor(I->getParent());
+          if (LoopForUser && L->contains(LoopForUser))
+            Worklist.push_back(I);
+        }
     }
   }
 
@@ -6510,12 +6533,6 @@ void ScalarEvolution::forgetLoop(const Loop *L) {
       PushDefUseChildren(I, Worklist);
     }
 
-    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.
@@ -6777,18 +6794,6 @@ ScalarEvolution::computeBackedgeTakenCount(const Loop *L,
 
 ScalarEvolution::ExitLimit
 ScalarEvolution::computeExitLimit(const Loop *L, BasicBlock *ExitingBlock,
-                                  bool AllowPredicates) {
-  ExitLimitQuery Query(L, ExitingBlock, AllowPredicates);
-  auto MaybeEL = ExitLimits.find(Query);
-  if (MaybeEL != ExitLimits.end())
-    return MaybeEL->second;
-  ExitLimit EL = computeExitLimitImpl(L, ExitingBlock, AllowPredicates);
-  ExitLimits.insert({Query, EL});
-  return EL;
-}
-
-ScalarEvolution::ExitLimit
-ScalarEvolution::computeExitLimitImpl(const Loop *L, BasicBlock *ExitingBlock,
                                       bool AllowPredicates) {
   // Okay, we've chosen an exiting block.  See what condition causes us to exit
   // at this block and remember the exit block and whether all other targets
@@ -10682,7 +10687,6 @@ ScalarEvolution::ScalarEvolution(ScalarEvolution &&Arg)
       BackedgeTakenCounts(std::move(Arg.BackedgeTakenCounts)),
       PredicatedBackedgeTakenCounts(
           std::move(Arg.PredicatedBackedgeTakenCounts)),
-      ExitLimits(std::move(Arg.ExitLimits)),
       ConstantEvolutionLoopExitValue(
           std::move(Arg.ConstantEvolutionLoopExitValue)),
       ValuesAtScopes(std::move(Arg.ValuesAtScopes)),
@@ -11097,7 +11101,7 @@ bool ScalarEvolution::ExitLimit::hasOperand(const SCEV *S) const {
 }
 
 void
-ScalarEvolution::forgetMemoizedResults(const SCEV *S, bool EraseExitLimit) {
+ScalarEvolution::forgetMemoizedResults(const SCEV *S) {
   ValuesAtScopes.erase(S);
   LoopDispositions.erase(S);
   BlockDispositions.erase(S);
@@ -11130,13 +11134,6 @@ ScalarEvolution::forgetMemoizedResults(const SCEV *S, bool EraseExitLimit) {
 
   RemoveSCEVFromBackedgeMap(BackedgeTakenCounts);
   RemoveSCEVFromBackedgeMap(PredicatedBackedgeTakenCounts);
-
-  // TODO: There is a suspicion that we only need to do it when there is a
-  // SCEVUnknown somewhere inside S. Need to check this.
-  if (EraseExitLimit)
-    for (auto I = ExitLimits.begin(), E = ExitLimits.end(); I != E; ++I)
-      if (I->second.hasOperand(S))
-        ExitLimits.erase(I);
 }
 
 void ScalarEvolution::addToLoopUseLists(const SCEV *S) {