LPM: Simplify how passes mark loops for deletion. NFC

When a pass removes a loop it currently has to reach up into the
LPPassManager's internals to update the state of the iteration over
loops. This reverse dependency results in a pretty awkward interplay
of the LPPassManager and its Passes.

Here, we change this to instead keep track of when a loop has become
"unlooped" in the Loop objects themselves, then the LPPassManager can
check this and manipulate its own state directly. This opens the door
to allow most of the loop passes to work without a backreference to
the LPPassManager.

I've kept passes calling the LPPassManager::deleteLoopFromQueue API
now so I could put an assert in to prove that this is NFC, but a later
pass will update passes just to preserve the LoopInfo directly and
stop referencing the LPPassManager completely.

llvm-svn: 255720

llvm/include/llvm/Analysis/LoopInfo.h

@@ -73,6 +73,10 @@ class LoopBase {
 
   SmallPtrSet<const BlockT*, 8> DenseBlockSet;
 
+  /// Indicator that this loops has been "unlooped", so there's no loop here
+  /// anymore.
+  bool IsUnloop = false;
+
   LoopBase(const LoopBase<BlockT, LoopT> &) = delete;
   const LoopBase<BlockT, LoopT>&
     operator=(const LoopBase<BlockT, LoopT> &) = delete;
@@ -150,6 +154,13 @@ public:
     return Blocks.size();
   }
 
+  /// Mark this loop as having been unlooped - the last backedge was removed and
+  /// we no longer have a loop.
+  void markUnlooped() { IsUnloop = true; }
+
+  /// Return true if this no longer represents a loop.
+  bool isUnloop() const { return IsUnloop; }
+
   /// isLoopExiting - True if terminator in the block can branch to another
   /// block that is outside of the current loop.
   ///
@@ -661,8 +672,9 @@ public:
 
   /// updateUnloop - Update LoopInfo after removing the last backedge from a
   /// loop--now the "unloop". This updates the loop forest and parent loops for
-  /// each block so that Unloop is no longer referenced, but the caller must
-  /// actually delete the Unloop object.
+  /// each block so that Unloop is no longer referenced, but does not actually
+  /// delete the Unloop object. Generally, the loop pass manager should manage
+  /// deleting the Unloop.
   void updateUnloop(Loop *Unloop);
 
   /// replacementPreservesLCSSAForm - Returns true if replacing From with To

llvm/include/llvm/Analysis/LoopPass.h

@@ -155,7 +155,6 @@ public:
 
 private:
   std::deque<Loop *> LQ;
-  bool skipThisLoop;
   LoopInfo *LI;
   Loop *CurrentLoop;
 };

llvm/lib/Analysis/LoopInfo.cpp

@@ -630,14 +630,8 @@ LoopInfo::LoopInfo(const DominatorTreeBase<BasicBlock> &DomTree) {
   analyze(DomTree);
 }
 
-/// updateUnloop - The last backedge has been removed from a loop--now the
-/// "unloop". Find a new parent for the blocks contained within unloop and
-/// update the loop tree. We don't necessarily have valid dominators at this
-/// point, but LoopInfo is still valid except for the removal of this loop.
-///
-/// Note that Unloop may now be an empty loop. Calling Loop::getHeader without
-/// checking first is illegal.
 void LoopInfo::updateUnloop(Loop *Unloop) {
+  Unloop->markAsUnloop();
 
   // First handle the special case of no parent loop to simplify the algorithm.
   if (!Unloop->getParentLoop()) {

llvm/lib/Analysis/LoopPass.cpp

@@ -58,37 +58,14 @@ char LPPassManager::ID = 0;
 
 LPPassManager::LPPassManager()
   : FunctionPass(ID), PMDataManager() {
-  skipThisLoop = false;
   LI = nullptr;
   CurrentLoop = nullptr;
 }
 
 /// Delete loop from the loop queue and loop hierarchy (LoopInfo).
 void LPPassManager::deleteLoopFromQueue(Loop *L) {
-
+  assert(CurrentLoop == L && "deleting a loop that is not being operated on");
   LI->updateUnloop(L);
-
-  // Notify passes that the loop is being deleted.
-  deleteSimpleAnalysisLoop(L);
-
-  // If L is current loop then skip rest of the passes and let
-  // runOnFunction remove L from LQ. Otherwise, remove L from LQ now
-  // and continue applying other passes on CurrentLoop.
-  if (CurrentLoop == L)
-    skipThisLoop = true;
-
-  delete L;
-
-  if (skipThisLoop)
-    return;
-
-  for (std::deque<Loop *>::iterator I = LQ.begin(),
-         E = LQ.end(); I != E; ++I) {
-    if (*I == L) {
-      LQ.erase(I);
-      break;
-    }
-  }
 }
 
 // Inset loop into loop nest (LoopInfo) and loop queue (LQ).
@@ -204,9 +181,7 @@ bool LPPassManager::runOnFunction(Function &F) {
   // Walk Loops
   while (!LQ.empty()) {
 
-    CurrentLoop  = LQ.back();
-    skipThisLoop = false;
-
+    CurrentLoop = LQ.back();
     // Run all passes on the current Loop.
     for (unsigned Index = 0; Index < getNumContainedPasses(); ++Index) {
       LoopPass *P = getContainedPass(Index);
@@ -226,11 +201,15 @@ bool LPPassManager::runOnFunction(Function &F) {
 
       if (Changed)
         dumpPassInfo(P, MODIFICATION_MSG, ON_LOOP_MSG,
-                     skipThisLoop ? "<deleted>" :
-                                    CurrentLoop->getHeader()->getName());
+                     CurrentLoop->isUnloop()
+                         ? "<deleted>"
+                         : CurrentLoop->getHeader()->getName());
       dumpPreservedSet(P);
 
-      if (!skipThisLoop) {
+      if (CurrentLoop->isUnloop()) {
+        // Notify passes that the loop is being deleted.
+        deleteSimpleAnalysisLoop(CurrentLoop);
+      } else {
         // Manually check that this loop is still healthy. This is done
         // instead of relying on LoopInfo::verifyLoop since LoopInfo
         // is a function pass and it's really expensive to verify every
@@ -249,12 +228,12 @@ bool LPPassManager::runOnFunction(Function &F) {
 
       removeNotPreservedAnalysis(P);
       recordAvailableAnalysis(P);
-      removeDeadPasses(P,
-                       skipThisLoop ? "<deleted>" :
-                                      CurrentLoop->getHeader()->getName(),
+      removeDeadPasses(P, CurrentLoop->isUnloop()
+                              ? "<deleted>"
+                              : CurrentLoop->getHeader()->getName(),
                        ON_LOOP_MSG);
 
-      if (skipThisLoop)
+      if (CurrentLoop->isUnloop())
         // Do not run other passes on this loop.
         break;
     }
@@ -262,11 +241,13 @@ bool LPPassManager::runOnFunction(Function &F) {
     // If the loop was deleted, release all the loop passes. This frees up
     // some memory, and avoids trouble with the pass manager trying to call
     // verifyAnalysis on them.
-    if (skipThisLoop)
+    if (CurrentLoop->isUnloop()) {
       for (unsigned Index = 0; Index < getNumContainedPasses(); ++Index) {
         Pass *P = getContainedPass(Index);
         freePass(P, "<deleted>", ON_LOOP_MSG);
       }
+      delete CurrentLoop;
+    }
 
     // Pop the loop from queue after running all passes.
     LQ.pop_back();