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1 //===- LoopInfo.cpp - Natural Loop Calculator -----------------------------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file defines the LoopInfo class that is used to identify natural loops
11 // and determine the loop depth of various nodes of the CFG.  Note that the
12 // loops identified may actually be several natural loops that share the same
13 // header node... not just a single natural loop.
14 //
15 //===----------------------------------------------------------------------===//
16 
17 #include "llvm/Analysis/LoopInfo.h"
18 #include "llvm/Constants.h"
19 #include "llvm/Instructions.h"
20 #include "llvm/Analysis/Dominators.h"
21 #include "llvm/Analysis/LoopIterator.h"
22 #include "llvm/Assembly/Writer.h"
23 #include "llvm/Support/CFG.h"
24 #include "llvm/Support/CommandLine.h"
25 #include "llvm/Support/Debug.h"
26 #include "llvm/ADT/DepthFirstIterator.h"
27 #include "llvm/ADT/SmallPtrSet.h"
28 #include <algorithm>
29 using namespace llvm;
30 
31 // Always verify loopinfo if expensive checking is enabled.
32 #ifdef XDEBUG
33 static bool VerifyLoopInfo = true;
34 #else
35 static bool VerifyLoopInfo = false;
36 #endif
37 static cl::opt<bool,true>
38 VerifyLoopInfoX("verify-loop-info", cl::location(VerifyLoopInfo),
39                 cl::desc("Verify loop info (time consuming)"));
40 
41 char LoopInfo::ID = 0;
42 INITIALIZE_PASS_BEGIN(LoopInfo, "loops", "Natural Loop Information", true, true)
INITIALIZE_PASS_DEPENDENCY(DominatorTree)43 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
44 INITIALIZE_PASS_END(LoopInfo, "loops", "Natural Loop Information", true, true)
45 
46 //===----------------------------------------------------------------------===//
47 // Loop implementation
48 //
49 
50 /// isLoopInvariant - Return true if the specified value is loop invariant
51 ///
52 bool Loop::isLoopInvariant(Value *V) const {
53   if (Instruction *I = dyn_cast<Instruction>(V))
54     return !contains(I);
55   return true;  // All non-instructions are loop invariant
56 }
57 
58 /// hasLoopInvariantOperands - Return true if all the operands of the
59 /// specified instruction are loop invariant.
hasLoopInvariantOperands(Instruction * I) const60 bool Loop::hasLoopInvariantOperands(Instruction *I) const {
61   for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
62     if (!isLoopInvariant(I->getOperand(i)))
63       return false;
64 
65   return true;
66 }
67 
68 /// makeLoopInvariant - If the given value is an instruciton inside of the
69 /// loop and it can be hoisted, do so to make it trivially loop-invariant.
70 /// Return true if the value after any hoisting is loop invariant. This
71 /// function can be used as a slightly more aggressive replacement for
72 /// isLoopInvariant.
73 ///
74 /// If InsertPt is specified, it is the point to hoist instructions to.
75 /// If null, the terminator of the loop preheader is used.
76 ///
makeLoopInvariant(Value * V,bool & Changed,Instruction * InsertPt) const77 bool Loop::makeLoopInvariant(Value *V, bool &Changed,
78                              Instruction *InsertPt) const {
79   if (Instruction *I = dyn_cast<Instruction>(V))
80     return makeLoopInvariant(I, Changed, InsertPt);
81   return true;  // All non-instructions are loop-invariant.
82 }
83 
84 /// makeLoopInvariant - If the given instruction is inside of the
85 /// loop and it can be hoisted, do so to make it trivially loop-invariant.
86 /// Return true if the instruction after any hoisting is loop invariant. This
87 /// function can be used as a slightly more aggressive replacement for
88 /// isLoopInvariant.
89 ///
90 /// If InsertPt is specified, it is the point to hoist instructions to.
91 /// If null, the terminator of the loop preheader is used.
92 ///
makeLoopInvariant(Instruction * I,bool & Changed,Instruction * InsertPt) const93 bool Loop::makeLoopInvariant(Instruction *I, bool &Changed,
94                              Instruction *InsertPt) const {
95   // Test if the value is already loop-invariant.
96   if (isLoopInvariant(I))
97     return true;
98   if (!I->isSafeToSpeculativelyExecute())
99     return false;
100   if (I->mayReadFromMemory())
101     return false;
102   // The landingpad instruction is immobile.
103   if (isa<LandingPadInst>(I))
104     return false;
105   // Determine the insertion point, unless one was given.
106   if (!InsertPt) {
107     BasicBlock *Preheader = getLoopPreheader();
108     // Without a preheader, hoisting is not feasible.
109     if (!Preheader)
110       return false;
111     InsertPt = Preheader->getTerminator();
112   }
113   // Don't hoist instructions with loop-variant operands.
114   for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
115     if (!makeLoopInvariant(I->getOperand(i), Changed, InsertPt))
116       return false;
117 
118   // Hoist.
119   I->moveBefore(InsertPt);
120   Changed = true;
121   return true;
122 }
123 
124 /// getCanonicalInductionVariable - Check to see if the loop has a canonical
125 /// induction variable: an integer recurrence that starts at 0 and increments
126 /// by one each time through the loop.  If so, return the phi node that
127 /// corresponds to it.
128 ///
129 /// The IndVarSimplify pass transforms loops to have a canonical induction
130 /// variable.
131 ///
getCanonicalInductionVariable() const132 PHINode *Loop::getCanonicalInductionVariable() const {
133   BasicBlock *H = getHeader();
134 
135   BasicBlock *Incoming = 0, *Backedge = 0;
136   pred_iterator PI = pred_begin(H);
137   assert(PI != pred_end(H) &&
138          "Loop must have at least one backedge!");
139   Backedge = *PI++;
140   if (PI == pred_end(H)) return 0;  // dead loop
141   Incoming = *PI++;
142   if (PI != pred_end(H)) return 0;  // multiple backedges?
143 
144   if (contains(Incoming)) {
145     if (contains(Backedge))
146       return 0;
147     std::swap(Incoming, Backedge);
148   } else if (!contains(Backedge))
149     return 0;
150 
151   // Loop over all of the PHI nodes, looking for a canonical indvar.
152   for (BasicBlock::iterator I = H->begin(); isa<PHINode>(I); ++I) {
153     PHINode *PN = cast<PHINode>(I);
154     if (ConstantInt *CI =
155         dyn_cast<ConstantInt>(PN->getIncomingValueForBlock(Incoming)))
156       if (CI->isNullValue())
157         if (Instruction *Inc =
158             dyn_cast<Instruction>(PN->getIncomingValueForBlock(Backedge)))
159           if (Inc->getOpcode() == Instruction::Add &&
160                 Inc->getOperand(0) == PN)
161             if (ConstantInt *CI = dyn_cast<ConstantInt>(Inc->getOperand(1)))
162               if (CI->equalsInt(1))
163                 return PN;
164   }
165   return 0;
166 }
167 
168 /// getTripCount - Return a loop-invariant LLVM value indicating the number of
169 /// times the loop will be executed.  Note that this means that the backedge
170 /// of the loop executes N-1 times.  If the trip-count cannot be determined,
171 /// this returns null.
172 ///
173 /// The IndVarSimplify pass transforms loops to have a form that this
174 /// function easily understands.
175 ///
getTripCount() const176 Value *Loop::getTripCount() const {
177   // Canonical loops will end with a 'cmp ne I, V', where I is the incremented
178   // canonical induction variable and V is the trip count of the loop.
179   PHINode *IV = getCanonicalInductionVariable();
180   if (IV == 0 || IV->getNumIncomingValues() != 2) return 0;
181 
182   bool P0InLoop = contains(IV->getIncomingBlock(0));
183   Value *Inc = IV->getIncomingValue(!P0InLoop);
184   BasicBlock *BackedgeBlock = IV->getIncomingBlock(!P0InLoop);
185 
186   if (BranchInst *BI = dyn_cast<BranchInst>(BackedgeBlock->getTerminator()))
187     if (BI->isConditional()) {
188       if (ICmpInst *ICI = dyn_cast<ICmpInst>(BI->getCondition())) {
189         if (ICI->getOperand(0) == Inc) {
190           if (BI->getSuccessor(0) == getHeader()) {
191             if (ICI->getPredicate() == ICmpInst::ICMP_NE)
192               return ICI->getOperand(1);
193           } else if (ICI->getPredicate() == ICmpInst::ICMP_EQ) {
194             return ICI->getOperand(1);
195           }
196         }
197       }
198     }
199 
200   return 0;
201 }
202 
203 /// getSmallConstantTripCount - Returns the trip count of this loop as a
204 /// normal unsigned value, if possible. Returns 0 if the trip count is unknown
205 /// or not constant. Will also return 0 if the trip count is very large
206 /// (>= 2^32)
getSmallConstantTripCount() const207 unsigned Loop::getSmallConstantTripCount() const {
208   Value* TripCount = this->getTripCount();
209   if (TripCount) {
210     if (ConstantInt *TripCountC = dyn_cast<ConstantInt>(TripCount)) {
211       // Guard against huge trip counts.
212       if (TripCountC->getValue().getActiveBits() <= 32) {
213         return (unsigned)TripCountC->getZExtValue();
214       }
215     }
216   }
217   return 0;
218 }
219 
220 /// getSmallConstantTripMultiple - Returns the largest constant divisor of the
221 /// trip count of this loop as a normal unsigned value, if possible. This
222 /// means that the actual trip count is always a multiple of the returned
223 /// value (don't forget the trip count could very well be zero as well!).
224 ///
225 /// Returns 1 if the trip count is unknown or not guaranteed to be the
226 /// multiple of a constant (which is also the case if the trip count is simply
227 /// constant, use getSmallConstantTripCount for that case), Will also return 1
228 /// if the trip count is very large (>= 2^32).
getSmallConstantTripMultiple() const229 unsigned Loop::getSmallConstantTripMultiple() const {
230   Value* TripCount = this->getTripCount();
231   // This will hold the ConstantInt result, if any
232   ConstantInt *Result = NULL;
233   if (TripCount) {
234     // See if the trip count is constant itself
235     Result = dyn_cast<ConstantInt>(TripCount);
236     // if not, see if it is a multiplication
237     if (!Result)
238       if (BinaryOperator *BO = dyn_cast<BinaryOperator>(TripCount)) {
239         switch (BO->getOpcode()) {
240         case BinaryOperator::Mul:
241           Result = dyn_cast<ConstantInt>(BO->getOperand(1));
242           break;
243         case BinaryOperator::Shl:
244           if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1)))
245             if (CI->getValue().getActiveBits() <= 5)
246               return 1u << CI->getZExtValue();
247           break;
248         default:
249           break;
250         }
251       }
252   }
253   // Guard against huge trip counts.
254   if (Result && Result->getValue().getActiveBits() <= 32) {
255     return (unsigned)Result->getZExtValue();
256   } else {
257     return 1;
258   }
259 }
260 
261 /// isLCSSAForm - Return true if the Loop is in LCSSA form
isLCSSAForm(DominatorTree & DT) const262 bool Loop::isLCSSAForm(DominatorTree &DT) const {
263   // Sort the blocks vector so that we can use binary search to do quick
264   // lookups.
265   SmallPtrSet<BasicBlock*, 16> LoopBBs(block_begin(), block_end());
266 
267   for (block_iterator BI = block_begin(), E = block_end(); BI != E; ++BI) {
268     BasicBlock *BB = *BI;
269     for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E;++I)
270       for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
271            ++UI) {
272         User *U = *UI;
273         BasicBlock *UserBB = cast<Instruction>(U)->getParent();
274         if (PHINode *P = dyn_cast<PHINode>(U))
275           UserBB = P->getIncomingBlock(UI);
276 
277         // Check the current block, as a fast-path, before checking whether
278         // the use is anywhere in the loop.  Most values are used in the same
279         // block they are defined in.  Also, blocks not reachable from the
280         // entry are special; uses in them don't need to go through PHIs.
281         if (UserBB != BB &&
282             !LoopBBs.count(UserBB) &&
283             DT.isReachableFromEntry(UserBB))
284           return false;
285       }
286   }
287 
288   return true;
289 }
290 
291 /// isLoopSimplifyForm - Return true if the Loop is in the form that
292 /// the LoopSimplify form transforms loops to, which is sometimes called
293 /// normal form.
isLoopSimplifyForm() const294 bool Loop::isLoopSimplifyForm() const {
295   // Normal-form loops have a preheader, a single backedge, and all of their
296   // exits have all their predecessors inside the loop.
297   return getLoopPreheader() && getLoopLatch() && hasDedicatedExits();
298 }
299 
300 /// hasDedicatedExits - Return true if no exit block for the loop
301 /// has a predecessor that is outside the loop.
hasDedicatedExits() const302 bool Loop::hasDedicatedExits() const {
303   // Sort the blocks vector so that we can use binary search to do quick
304   // lookups.
305   SmallPtrSet<BasicBlock *, 16> LoopBBs(block_begin(), block_end());
306   // Each predecessor of each exit block of a normal loop is contained
307   // within the loop.
308   SmallVector<BasicBlock *, 4> ExitBlocks;
309   getExitBlocks(ExitBlocks);
310   for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
311     for (pred_iterator PI = pred_begin(ExitBlocks[i]),
312          PE = pred_end(ExitBlocks[i]); PI != PE; ++PI)
313       if (!LoopBBs.count(*PI))
314         return false;
315   // All the requirements are met.
316   return true;
317 }
318 
319 /// getUniqueExitBlocks - Return all unique successor blocks of this loop.
320 /// These are the blocks _outside of the current loop_ which are branched to.
321 /// This assumes that loop exits are in canonical form.
322 ///
323 void
getUniqueExitBlocks(SmallVectorImpl<BasicBlock * > & ExitBlocks) const324 Loop::getUniqueExitBlocks(SmallVectorImpl<BasicBlock *> &ExitBlocks) const {
325   assert(hasDedicatedExits() &&
326          "getUniqueExitBlocks assumes the loop has canonical form exits!");
327 
328   // Sort the blocks vector so that we can use binary search to do quick
329   // lookups.
330   SmallVector<BasicBlock *, 128> LoopBBs(block_begin(), block_end());
331   std::sort(LoopBBs.begin(), LoopBBs.end());
332 
333   SmallVector<BasicBlock *, 32> switchExitBlocks;
334 
335   for (block_iterator BI = block_begin(), BE = block_end(); BI != BE; ++BI) {
336 
337     BasicBlock *current = *BI;
338     switchExitBlocks.clear();
339 
340     for (succ_iterator I = succ_begin(*BI), E = succ_end(*BI); I != E; ++I) {
341       // If block is inside the loop then it is not a exit block.
342       if (std::binary_search(LoopBBs.begin(), LoopBBs.end(), *I))
343         continue;
344 
345       pred_iterator PI = pred_begin(*I);
346       BasicBlock *firstPred = *PI;
347 
348       // If current basic block is this exit block's first predecessor
349       // then only insert exit block in to the output ExitBlocks vector.
350       // This ensures that same exit block is not inserted twice into
351       // ExitBlocks vector.
352       if (current != firstPred)
353         continue;
354 
355       // If a terminator has more then two successors, for example SwitchInst,
356       // then it is possible that there are multiple edges from current block
357       // to one exit block.
358       if (std::distance(succ_begin(current), succ_end(current)) <= 2) {
359         ExitBlocks.push_back(*I);
360         continue;
361       }
362 
363       // In case of multiple edges from current block to exit block, collect
364       // only one edge in ExitBlocks. Use switchExitBlocks to keep track of
365       // duplicate edges.
366       if (std::find(switchExitBlocks.begin(), switchExitBlocks.end(), *I)
367           == switchExitBlocks.end()) {
368         switchExitBlocks.push_back(*I);
369         ExitBlocks.push_back(*I);
370       }
371     }
372   }
373 }
374 
375 /// getUniqueExitBlock - If getUniqueExitBlocks would return exactly one
376 /// block, return that block. Otherwise return null.
getUniqueExitBlock() const377 BasicBlock *Loop::getUniqueExitBlock() const {
378   SmallVector<BasicBlock *, 8> UniqueExitBlocks;
379   getUniqueExitBlocks(UniqueExitBlocks);
380   if (UniqueExitBlocks.size() == 1)
381     return UniqueExitBlocks[0];
382   return 0;
383 }
384 
dump() const385 void Loop::dump() const {
386   print(dbgs());
387 }
388 
389 //===----------------------------------------------------------------------===//
390 // UnloopUpdater implementation
391 //
392 
393 namespace {
394 /// Find the new parent loop for all blocks within the "unloop" whose last
395 /// backedges has just been removed.
396 class UnloopUpdater {
397   Loop *Unloop;
398   LoopInfo *LI;
399 
400   LoopBlocksDFS DFS;
401 
402   // Map unloop's immediate subloops to their nearest reachable parents. Nested
403   // loops within these subloops will not change parents. However, an immediate
404   // subloop's new parent will be the nearest loop reachable from either its own
405   // exits *or* any of its nested loop's exits.
406   DenseMap<Loop*, Loop*> SubloopParents;
407 
408   // Flag the presence of an irreducible backedge whose destination is a block
409   // directly contained by the original unloop.
410   bool FoundIB;
411 
412 public:
UnloopUpdater(Loop * UL,LoopInfo * LInfo)413   UnloopUpdater(Loop *UL, LoopInfo *LInfo) :
414     Unloop(UL), LI(LInfo), DFS(UL), FoundIB(false) {}
415 
416   void updateBlockParents();
417 
418   void removeBlocksFromAncestors();
419 
420   void updateSubloopParents();
421 
422 protected:
423   Loop *getNearestLoop(BasicBlock *BB, Loop *BBLoop);
424 };
425 } // end anonymous namespace
426 
427 /// updateBlockParents - Update the parent loop for all blocks that are directly
428 /// contained within the original "unloop".
updateBlockParents()429 void UnloopUpdater::updateBlockParents() {
430   if (Unloop->getNumBlocks()) {
431     // Perform a post order CFG traversal of all blocks within this loop,
432     // propagating the nearest loop from sucessors to predecessors.
433     LoopBlocksTraversal Traversal(DFS, LI);
434     for (LoopBlocksTraversal::POTIterator POI = Traversal.begin(),
435            POE = Traversal.end(); POI != POE; ++POI) {
436 
437       Loop *L = LI->getLoopFor(*POI);
438       Loop *NL = getNearestLoop(*POI, L);
439 
440       if (NL != L) {
441         // For reducible loops, NL is now an ancestor of Unloop.
442         assert((NL != Unloop && (!NL || NL->contains(Unloop))) &&
443                "uninitialized successor");
444         LI->changeLoopFor(*POI, NL);
445       }
446       else {
447         // Or the current block is part of a subloop, in which case its parent
448         // is unchanged.
449         assert((FoundIB || Unloop->contains(L)) && "uninitialized successor");
450       }
451     }
452   }
453   // Each irreducible loop within the unloop induces a round of iteration using
454   // the DFS result cached by Traversal.
455   bool Changed = FoundIB;
456   for (unsigned NIters = 0; Changed; ++NIters) {
457     assert(NIters < Unloop->getNumBlocks() && "runaway iterative algorithm");
458 
459     // Iterate over the postorder list of blocks, propagating the nearest loop
460     // from successors to predecessors as before.
461     Changed = false;
462     for (LoopBlocksDFS::POIterator POI = DFS.beginPostorder(),
463            POE = DFS.endPostorder(); POI != POE; ++POI) {
464 
465       Loop *L = LI->getLoopFor(*POI);
466       Loop *NL = getNearestLoop(*POI, L);
467       if (NL != L) {
468         assert(NL != Unloop && (!NL || NL->contains(Unloop)) &&
469                "uninitialized successor");
470         LI->changeLoopFor(*POI, NL);
471         Changed = true;
472       }
473     }
474   }
475 }
476 
477 /// removeBlocksFromAncestors - Remove unloop's blocks from all ancestors below
478 /// their new parents.
removeBlocksFromAncestors()479 void UnloopUpdater::removeBlocksFromAncestors() {
480   // Remove unloop's blocks from all ancestors below their new parents.
481   for (Loop::block_iterator BI = Unloop->block_begin(),
482          BE = Unloop->block_end(); BI != BE; ++BI) {
483     Loop *NewParent = LI->getLoopFor(*BI);
484     // If this block is an immediate subloop, remove all blocks (including
485     // nested subloops) from ancestors below the new parent loop.
486     // Otherwise, if this block is in a nested subloop, skip it.
487     if (SubloopParents.count(NewParent))
488       NewParent = SubloopParents[NewParent];
489     else if (Unloop->contains(NewParent))
490       continue;
491 
492     // Remove blocks from former Ancestors except Unloop itself which will be
493     // deleted.
494     for (Loop *OldParent = Unloop->getParentLoop(); OldParent != NewParent;
495          OldParent = OldParent->getParentLoop()) {
496       assert(OldParent && "new loop is not an ancestor of the original");
497       OldParent->removeBlockFromLoop(*BI);
498     }
499   }
500 }
501 
502 /// updateSubloopParents - Update the parent loop for all subloops directly
503 /// nested within unloop.
updateSubloopParents()504 void UnloopUpdater::updateSubloopParents() {
505   while (!Unloop->empty()) {
506     Loop *Subloop = *llvm::prior(Unloop->end());
507     Unloop->removeChildLoop(llvm::prior(Unloop->end()));
508 
509     assert(SubloopParents.count(Subloop) && "DFS failed to visit subloop");
510     if (SubloopParents[Subloop])
511       SubloopParents[Subloop]->addChildLoop(Subloop);
512     else
513       LI->addTopLevelLoop(Subloop);
514   }
515 }
516 
517 /// getNearestLoop - Return the nearest parent loop among this block's
518 /// successors. If a successor is a subloop header, consider its parent to be
519 /// the nearest parent of the subloop's exits.
520 ///
521 /// For subloop blocks, simply update SubloopParents and return NULL.
getNearestLoop(BasicBlock * BB,Loop * BBLoop)522 Loop *UnloopUpdater::getNearestLoop(BasicBlock *BB, Loop *BBLoop) {
523 
524   // Initially for blocks directly contained by Unloop, NearLoop == Unloop and
525   // is considered uninitialized.
526   Loop *NearLoop = BBLoop;
527 
528   Loop *Subloop = 0;
529   if (NearLoop != Unloop && Unloop->contains(NearLoop)) {
530     Subloop = NearLoop;
531     // Find the subloop ancestor that is directly contained within Unloop.
532     while (Subloop->getParentLoop() != Unloop) {
533       Subloop = Subloop->getParentLoop();
534       assert(Subloop && "subloop is not an ancestor of the original loop");
535     }
536     // Get the current nearest parent of the Subloop exits, initially Unloop.
537     if (!SubloopParents.count(Subloop))
538       SubloopParents[Subloop] = Unloop;
539     NearLoop = SubloopParents[Subloop];
540   }
541 
542   succ_iterator I = succ_begin(BB), E = succ_end(BB);
543   if (I == E) {
544     assert(!Subloop && "subloop blocks must have a successor");
545     NearLoop = 0; // unloop blocks may now exit the function.
546   }
547   for (; I != E; ++I) {
548     if (*I == BB)
549       continue; // self loops are uninteresting
550 
551     Loop *L = LI->getLoopFor(*I);
552     if (L == Unloop) {
553       // This successor has not been processed. This path must lead to an
554       // irreducible backedge.
555       assert((FoundIB || !DFS.hasPostorder(*I)) && "should have seen IB");
556       FoundIB = true;
557     }
558     if (L != Unloop && Unloop->contains(L)) {
559       // Successor is in a subloop.
560       if (Subloop)
561         continue; // Branching within subloops. Ignore it.
562 
563       // BB branches from the original into a subloop header.
564       assert(L->getParentLoop() == Unloop && "cannot skip into nested loops");
565 
566       // Get the current nearest parent of the Subloop's exits.
567       L = SubloopParents[L];
568       // L could be Unloop if the only exit was an irreducible backedge.
569     }
570     if (L == Unloop) {
571       continue;
572     }
573     // Handle critical edges from Unloop into a sibling loop.
574     if (L && !L->contains(Unloop)) {
575       L = L->getParentLoop();
576     }
577     // Remember the nearest parent loop among successors or subloop exits.
578     if (NearLoop == Unloop || !NearLoop || NearLoop->contains(L))
579       NearLoop = L;
580   }
581   if (Subloop) {
582     SubloopParents[Subloop] = NearLoop;
583     return BBLoop;
584   }
585   return NearLoop;
586 }
587 
588 //===----------------------------------------------------------------------===//
589 // LoopInfo implementation
590 //
runOnFunction(Function &)591 bool LoopInfo::runOnFunction(Function &) {
592   releaseMemory();
593   LI.Calculate(getAnalysis<DominatorTree>().getBase());    // Update
594   return false;
595 }
596 
597 /// updateUnloop - The last backedge has been removed from a loop--now the
598 /// "unloop". Find a new parent for the blocks contained within unloop and
599 /// update the loop tree. We don't necessarily have valid dominators at this
600 /// point, but LoopInfo is still valid except for the removal of this loop.
601 ///
602 /// Note that Unloop may now be an empty loop. Calling Loop::getHeader without
603 /// checking first is illegal.
updateUnloop(Loop * Unloop)604 void LoopInfo::updateUnloop(Loop *Unloop) {
605 
606   // First handle the special case of no parent loop to simplify the algorithm.
607   if (!Unloop->getParentLoop()) {
608     // Since BBLoop had no parent, Unloop blocks are no longer in a loop.
609     for (Loop::block_iterator I = Unloop->block_begin(),
610          E = Unloop->block_end(); I != E; ++I) {
611 
612       // Don't reparent blocks in subloops.
613       if (getLoopFor(*I) != Unloop)
614         continue;
615 
616       // Blocks no longer have a parent but are still referenced by Unloop until
617       // the Unloop object is deleted.
618       LI.changeLoopFor(*I, 0);
619     }
620 
621     // Remove the loop from the top-level LoopInfo object.
622     for (LoopInfo::iterator I = LI.begin();; ++I) {
623       assert(I != LI.end() && "Couldn't find loop");
624       if (*I == Unloop) {
625         LI.removeLoop(I);
626         break;
627       }
628     }
629 
630     // Move all of the subloops to the top-level.
631     while (!Unloop->empty())
632       LI.addTopLevelLoop(Unloop->removeChildLoop(llvm::prior(Unloop->end())));
633 
634     return;
635   }
636 
637   // Update the parent loop for all blocks within the loop. Blocks within
638   // subloops will not change parents.
639   UnloopUpdater Updater(Unloop, this);
640   Updater.updateBlockParents();
641 
642   // Remove blocks from former ancestor loops.
643   Updater.removeBlocksFromAncestors();
644 
645   // Add direct subloops as children in their new parent loop.
646   Updater.updateSubloopParents();
647 
648   // Remove unloop from its parent loop.
649   Loop *ParentLoop = Unloop->getParentLoop();
650   for (Loop::iterator I = ParentLoop->begin();; ++I) {
651     assert(I != ParentLoop->end() && "Couldn't find loop");
652     if (*I == Unloop) {
653       ParentLoop->removeChildLoop(I);
654       break;
655     }
656   }
657 }
658 
verifyAnalysis() const659 void LoopInfo::verifyAnalysis() const {
660   // LoopInfo is a FunctionPass, but verifying every loop in the function
661   // each time verifyAnalysis is called is very expensive. The
662   // -verify-loop-info option can enable this. In order to perform some
663   // checking by default, LoopPass has been taught to call verifyLoop
664   // manually during loop pass sequences.
665 
666   if (!VerifyLoopInfo) return;
667 
668   DenseSet<const Loop*> Loops;
669   for (iterator I = begin(), E = end(); I != E; ++I) {
670     assert(!(*I)->getParentLoop() && "Top-level loop has a parent!");
671     (*I)->verifyLoopNest(&Loops);
672   }
673 
674   // Verify that blocks are mapped to valid loops.
675   //
676   // FIXME: With an up-to-date DFS (see LoopIterator.h) and DominatorTree, we
677   // could also verify that the blocks are still in the correct loops.
678   for (DenseMap<BasicBlock*, Loop*>::const_iterator I = LI.BBMap.begin(),
679          E = LI.BBMap.end(); I != E; ++I) {
680     assert(Loops.count(I->second) && "orphaned loop");
681     assert(I->second->contains(I->first) && "orphaned block");
682   }
683 }
684 
getAnalysisUsage(AnalysisUsage & AU) const685 void LoopInfo::getAnalysisUsage(AnalysisUsage &AU) const {
686   AU.setPreservesAll();
687   AU.addRequired<DominatorTree>();
688 }
689 
print(raw_ostream & OS,const Module *) const690 void LoopInfo::print(raw_ostream &OS, const Module*) const {
691   LI.print(OS);
692 }
693 
694 //===----------------------------------------------------------------------===//
695 // LoopBlocksDFS implementation
696 //
697 
698 /// Traverse the loop blocks and store the DFS result.
699 /// Useful for clients that just want the final DFS result and don't need to
700 /// visit blocks during the initial traversal.
perform(LoopInfo * LI)701 void LoopBlocksDFS::perform(LoopInfo *LI) {
702   LoopBlocksTraversal Traversal(*this, LI);
703   for (LoopBlocksTraversal::POTIterator POI = Traversal.begin(),
704          POE = Traversal.end(); POI != POE; ++POI) ;
705 }
706