[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

1 files changed, 26 insertions, 29 deletions

diff --git a/lib/Analysis/ScalarEvolution.cpp b/lib/Analysis/ScalarEvolution.cpp
index e598f2005c3..98c2845c1e6 100644
--- 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) {