[PATCH] D120314: [NFC][SCEV] Give hints on why we forget loops

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev created this revision.
mkazantsev added reviewers: nikic, lebedev.ri, reames, fhahn.
Herald added subscribers: javed.absar, hiraditya.
mkazantsev requested review of this revision.
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Following discussion in D120303 <https://reviews.llvm.org/D120303>, introduce API that allows us to
notify SCEV that some loop will be broken or entirely deleted with
all its subloops, and therefore there is no need to store cached data
for them. The conservative fallback is "loop may stay, keep dependencies
that should stay alive".


https://reviews.llvm.org/D120314

Files:
  llvm/include/llvm/Analysis/ScalarEvolution.h
  llvm/lib/Analysis/ScalarEvolution.cpp
  llvm/lib/Transforms/Scalar/LoopDeletion.cpp
  llvm/lib/Transforms/Utils/LoopUtils.cpp


Index: llvm/lib/Transforms/Utils/LoopUtils.cpp
===================================================================
--- llvm/lib/Transforms/Utils/LoopUtils.cpp
+++ llvm/lib/Transforms/Utils/LoopUtils.cpp
@@ -705,7 +705,7 @@
   auto *Header = L->getHeader();
   Loop *OutermostLoop = getOutermostLoop(L);
 
-  SE.forgetLoop(L);
+  SE.forgetLoop(L, ScalarEvolution::ForgetLoopReason::WillBreak);
 
   std::unique_ptr<MemorySSAUpdater> MSSAU;
   if (MSSA)
Index: llvm/lib/Transforms/Scalar/LoopDeletion.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/LoopDeletion.cpp
+++ llvm/lib/Transforms/Scalar/LoopDeletion.cpp
@@ -460,7 +460,7 @@
     // We need to forget the loop before setting the incoming values of the exit
     // phis to undef, so we properly invalidate the SCEV expressions for those
     // phis.
-    SE.forgetLoop(L);
+    SE.forgetLoop(L, ScalarEvolution::ForgetLoopReason::WillDelete);
     // Set incoming value to undef for phi nodes in the exit block.
     for (PHINode &P : ExitBlock->phis()) {
       std::fill(P.incoming_values().begin(), P.incoming_values().end(),
Index: llvm/lib/Analysis/ScalarEvolution.cpp
===================================================================
--- llvm/lib/Analysis/ScalarEvolution.cpp
+++ llvm/lib/Analysis/ScalarEvolution.cpp
@@ -8040,7 +8040,7 @@
   PredicatedSCEVRewrites.clear();
 }
 
-void ScalarEvolution::forgetLoop(const Loop *L) {
+void ScalarEvolution::forgetLoop(const Loop *L, ForgetLoopReason ForgetReason) {
   SmallVector<const Loop *, 16> LoopWorklist(1, L);
   SmallVector<Instruction *, 32> Worklist;
   SmallPtrSet<Instruction *, 16> Visited;
@@ -8068,6 +8068,9 @@
     if (LoopUsersItr != LoopUsers.end()) {
       ToForget.insert(ToForget.end(), LoopUsersItr->second.begin(),
                 LoopUsersItr->second.end());
+      // All subloops will be destroyed, so no need to keep their users.
+      if (ForgetReason == WillDelete)
+        LoopUsers.erase(CurrL);
     }
 
     // Drop information about expressions based on loop-header PHIs.
@@ -8094,6 +8097,9 @@
     LoopWorklist.append(CurrL->begin(), CurrL->end());
   }
   forgetMemoizedResults(ToForget);
+  // This loop will be broken, so no need to store its users.
+  if (ForgetReason == WillBreak)
+    LoopUsers.erase(L);
 }
 
 void ScalarEvolution::forgetTopmostLoop(const Loop *L) {
Index: llvm/include/llvm/Analysis/ScalarEvolution.h
===================================================================
--- llvm/include/llvm/Analysis/ScalarEvolution.h
+++ llvm/include/llvm/Analysis/ScalarEvolution.h
@@ -906,11 +906,32 @@
   // bodies).
   void forgetAllLoops();
 
+  /// Defines the type of invalidation on forgetLoop.
+  /// `Recompute` means that we change some essential facts to the loop (such as
+  /// exiting blocks), but the loop itself may stay in the IR. It is also the
+  /// reason that should be used when we just request forgetLoop to get more
+  /// precise results, or as a safe default option when we don't know whether
+  /// the loop will stay or not.
+  /// `WillBreak` means that we are going to break the loop itself (e.g. break
+  /// its backedge), but its header and some of the contained loops may stay
+  /// reachable. In this case we can safely drop cached information related to
+  /// this immediate loop.
+  /// `WillDelete` means that the header of this loop is no longer reachable,
+  /// and therefore this loop itself and all its contained loops will be
+  /// deleted. In this case we may safely drop cached information related to the
+  /// loop itself and all its subloops recursively.
+  enum ForgetLoopReason {
+    Recompute,
+    WillBreak,
+    WillDelete,
+  };
   /// This method should be called by the client when it has changed a loop in
   /// a way that may effect ScalarEvolution's ability to compute a trip count,
   /// or if the loop is deleted.  This call is potentially expensive for large
-  /// loop bodies.
-  void forgetLoop(const Loop *L);
+  /// loop bodies. The parameter \p WillDelete gives a hint on what will happen
+  /// next to this loop (if we are going to break it or delete with all
+  /// subloops), and this allows us to manage cached data more carefully.
+  void forgetLoop(const Loop *L, ForgetLoopReason ForgetReason = Recompute);
 
   // This method invokes forgetLoop for the outermost loop of the given loop
   // \p L, making ScalarEvolution forget about all this subtree. This needs to


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