An algorithm for incrementally updating LoopInfo within a

LoopPassManager. The incremental update should be extremely cheap in
most cases and can be used in places where it's not feasible to
regenerate the entire loop forest.

- "Unloop" is a node in the loop tree whose last backedge has been removed.
- Perform reverse dataflow on the block inside Unloop to propagate the
  nearest loop from the block's successors.
- For reducible CFG, each block in unloop is visited exactly
  once. This is because unloop no longer has a backedge and blocks
  within subloops don't change parents.
- Immediate subloops are summarized by the nearest loop reachable from
  their exits or exits within nested subloops.
- At completion the unloop blocks each have a new parent loop, and
  each immediate subloop has a new parent.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@136844 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 242 insertions, 0 deletions

diff --git a/lib/Analysis/LoopInfo.cpp b/lib/Analysis/LoopInfo.cpp
index cfe71d18a4..6ab71b81d8 100644
--- a/lib/Analysis/LoopInfo.cpp
+++ b/lib/Analysis/LoopInfo.cpp
@@ -18,6 +18,7 @@
 #include "llvm/Constants.h"
 #include "llvm/Instructions.h"
 #include "llvm/Analysis/Dominators.h"
+#include "llvm/Analysis/LoopIterator.h"
 #include "llvm/Assembly/Writer.h"
 #include "llvm/Support/CFG.h"
 #include "llvm/Support/CommandLine.h"
@@ -383,6 +384,174 @@ void Loop::dump() const {
 }
 
 //===----------------------------------------------------------------------===//
+// UnloopUpdater implementation
+//
+
+/// Find the new parent loop for all blocks within the "unloop" whose last
+/// backedges has just been removed.
+class UnloopUpdater {
+  Loop *Unloop;
+  LoopInfo *LI;
+
+  LoopBlocksDFS DFS;
+
+  // Map unloop's immediate subloops to their nearest reachable parents. Nested
+  // loops within these subloops will not change parents. However, an immediate
+  // subloop's new parent will be the nearest loop reachable from either its own
+  // exits *or* any of its nested loop's exits.
+  DenseMap<Loop*, Loop*> SubloopParents;
+
+  // Flag the presence of an irreducible backedge whose destination is a block
+  // directly contained by the original unloop.
+  bool FoundIB;
+
+public:
+  UnloopUpdater(Loop *UL, LoopInfo *LInfo) :
+    Unloop(UL), LI(LInfo), DFS(UL), FoundIB(false) {}
+
+  void updateBlockParents();
+
+  void updateSubloopParents();
+
+protected:
+  Loop *getNearestLoop(BasicBlock *BB, Loop *BBLoop);
+};
+
+/// updateBlockParents - Update the parent loop for all blocks that are directly
+/// contained within the original "unloop".
+void UnloopUpdater::updateBlockParents() {
+  {
+    // Perform a post order CFG traversal of all blocks within this loop,
+    // propagating the nearest loop from sucessors to predecessors.
+    LoopBlocksTraversal Traversal(DFS, LI);
+    for (LoopBlocksTraversal::POTIterator POI = Traversal.begin(),
+           POE = Traversal.end(); POI != POE; ++POI) {
+
+      Loop *L = LI->getLoopFor(*POI);
+      Loop *NL = getNearestLoop(*POI, L);
+
+      if (NL != L) {
+        // For reducible loops, NL is now an ancestor of Unloop.
+        assert((NL != Unloop && (!NL || NL->contains(Unloop))) &&
+               "uninitialized successor");
+        LI->changeLoopFor(*POI, NL);
+      }
+      else {
+        // Or the current block is part of a subloop, in which case its parent
+        // is unchanged.
+        assert((FoundIB || Unloop->contains(L)) && "uninitialized successor");
+      }
+    }
+  }
+  // Each irreducible loop within the unloop induces a round of iteration using
+  // the DFS result cached by Traversal.
+  bool Changed = FoundIB;
+  for (unsigned NIters = 0; Changed; ++NIters) {
+    assert(NIters < Unloop->getNumBlocks() && "runaway iterative algorithm");
+
+    // Iterate over the postorder list of blocks, propagating the nearest loop
+    // from successors to predecessors as before.
+    Changed = false;
+    for (LoopBlocksDFS::POIterator POI = DFS.beginPostorder(),
+           POE = DFS.endPostorder(); POI != POE; ++POI) {
+
+      Loop *L = LI->getLoopFor(*POI);
+      Loop *NL = getNearestLoop(*POI, L);
+      if (NL != L) {
+        assert(NL != Unloop && (!NL || NL->contains(Unloop)) &&
+               "uninitialized successor");
+        LI->changeLoopFor(*POI, NL);
+        Changed = true;
+      }
+    }
+  }
+}
+
+/// updateSubloopParents - Update the parent loop for all subloops directly
+/// nested within unloop.
+void UnloopUpdater::updateSubloopParents() {
+  while (!Unloop->empty()) {
+    Loop *Subloop = *(Unloop->end()-1);
+    Unloop->removeChildLoop(Unloop->end()-1);
+
+    assert(SubloopParents.count(Subloop) && "DFS failed to visit subloop");
+    if (SubloopParents[Subloop])
+      SubloopParents[Subloop]->addChildLoop(Subloop);
+  }
+}
+
+/// getNearestLoop - Return the nearest parent loop among this block's
+/// successors. If a successor is a subloop header, consider its parent to be
+/// the nearest parent of the subloop's exits.
+///
+/// For subloop blocks, simply update SubloopParents and return NULL.
+Loop *UnloopUpdater::getNearestLoop(BasicBlock *BB, Loop *BBLoop) {
+
+  // Initialy for blocks directly contained by Unloop, NearLoop == Unloop and is
+  // considered uninitialized.
+  Loop *NearLoop = BBLoop;
+
+  Loop *Subloop = 0;
+  if (NearLoop != Unloop && Unloop->contains(NearLoop)) {
+    Subloop = NearLoop;
+    // Find the subloop ancestor that is directly contained within Unloop.
+    while (Subloop->getParentLoop() != Unloop) {
+      Subloop = Subloop->getParentLoop();
+      assert(Subloop && "subloop is not an ancestor of the original loop");
+    }
+    // Get the current nearest parent of the Subloop exits, initially Unloop.
+    if (!SubloopParents.count(Subloop))
+      SubloopParents[Subloop] = Unloop;
+    NearLoop = SubloopParents[Subloop];
+  }
+
+  succ_iterator I = succ_begin(BB), E = succ_end(BB);
+  if (I == E) {
+    assert(!Subloop && "subloop blocks must have a successor");
+    NearLoop = 0; // unloop blocks may now exit the function.
+  }
+  for (; I != E; ++I) {
+    if (*I == BB)
+      continue; // self loops are uninteresting
+
+    Loop *L = LI->getLoopFor(*I);
+    if (L == Unloop) {
+      // This successor has not been processed. This path must lead to an
+      // irreducible backedge.
+      assert((FoundIB || !DFS.hasPostorder(*I)) && "should have seen IB");
+      FoundIB = true;
+    }
+    if (L != Unloop && Unloop->contains(L)) {
+      // Successor is in a subloop.
+      if (Subloop)
+        continue; // Branching within subloops. Ignore it.
+
+      // BB branches from the original into a subloop header.
+      assert(L->getParentLoop() == Unloop && "cannot skip into nested loops");
+
+      // Get the current nearest parent of the Subloop's exits.
+      L = SubloopParents[L];
+      // L could be Unloop if the only exit was an irreducible backedge.
+    }
+    if (L == Unloop) {
+      continue;
+    }
+    // Handle critical edges from Unloop into a sibling loop.
+    if (L && !L->contains(Unloop)) {
+      L = L->getParentLoop();
+    }
+    // Remember the nearest parent loop among successors or subloop exits.
+    if (NearLoop == Unloop || !NearLoop || NearLoop->contains(L))
+      NearLoop = L;
+  }
+  if (Subloop) {
+    SubloopParents[Subloop] = NearLoop;
+    return BBLoop;
+  }
+  return NearLoop;
+}
+
+//===----------------------------------------------------------------------===//
 // LoopInfo implementation
 //
 bool LoopInfo::runOnFunction(Function &) {
@@ -391,6 +560,79 @@ bool LoopInfo::runOnFunction(Function &) {
   return false;
 }
 
+/// 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 necessarilly have valid dominators at this
+/// point, but LoopInfo is still valid except for the removal of this loop.
+void LoopInfo::updateUnloop(Loop *Unloop) {
+
+  // First handle the special case of no parent loop to simplify the algorithm.
+  if (!Unloop->getParentLoop()) {
+    // Since BBLoop had no parent, Unloop blocks are no longer in a loop.
+    for (Loop::block_iterator I = Unloop->block_begin(),
+         E = Unloop->block_end(); I != E; ++I) {
+
+      // Don't reparent blocks in subloops.
+      if (getLoopFor(*I) != Unloop)
+        continue;
+
+      // Blocks no longer have a parent but are still referenced by Unloop until
+      // the Unloop object is deleted.
+      LI.changeLoopFor(*I, 0);
+    }
+
+    // Remove the loop from the top-level LoopInfo object.
+    for (LoopInfo::iterator I = LI.begin(), E = LI.end();; ++I) {
+      assert(I != E && "Couldn't find loop");
+      if (*I == Unloop) {
+        LI.removeLoop(I);
+        break;
+      }
+    }
+
+    // Move all of the subloops to the top-level.
+    while (!Unloop->empty())
+      LI.addTopLevelLoop(Unloop->removeChildLoop(Unloop->end()-1));
+
+    return;
+  }
+
+  // Update the parent loop for all blocks within the loop. Blocks within
+  // subloops will not change parents.
+  UnloopUpdater Updater(Unloop, this);
+  Updater.updateBlockParents();
+
+  // Remove unloop's blocks from all ancestors below their new parents.
+  for (Loop::block_iterator BI = Unloop->block_begin(),
+         BE = Unloop->block_end(); BI != BE; ++BI) {
+    Loop *NewParent = getLoopFor(*BI);
+    // If this block is in a subloop, skip it.
+    if (Unloop->contains(NewParent))
+      continue;
+
+    // Remove blocks from former Ancestors except Unloop itself which will be
+    // deleted.
+    for (Loop *OldParent = Unloop->getParentLoop(); OldParent != NewParent;
+         OldParent = OldParent->getParentLoop()) {
+      assert(OldParent && "new loop is not an ancestor of the original");
+      OldParent->removeBlockFromLoop(*BI);
+    }
+  }
+
+  // Add direct subloops as children in their new parent loop.
+  Updater.updateSubloopParents();
+
+  // Remove unloop from its parent loop.
+  Loop *ParentLoop = Unloop->getParentLoop();
+  for (Loop::iterator I = ParentLoop->begin(), E = ParentLoop->end();; ++I) {
+    assert(I != E && "Couldn't find loop");
+    if (*I == Unloop) {
+      ParentLoop->removeChildLoop(I);
+      break;
+    }
+  }
+}
+
 void LoopInfo::verifyAnalysis() const {
   // LoopInfo is a FunctionPass, but verifying every loop in the function
   // each time verifyAnalysis is called is very expensive. The