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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/ADT/DepthFirstIterator.h"
19 #include "llvm/ADT/SmallPtrSet.h"
20 #include "llvm/Analysis/LoopInfoImpl.h"
21 #include "llvm/Analysis/LoopIterator.h"
22 #include "llvm/Analysis/ValueTracking.h"
23 #include "llvm/IR/CFG.h"
24 #include "llvm/IR/Constants.h"
25 #include "llvm/IR/DebugLoc.h"
26 #include "llvm/IR/Dominators.h"
27 #include "llvm/IR/Instructions.h"
28 #include "llvm/IR/LLVMContext.h"
29 #include "llvm/IR/Metadata.h"
30 #include "llvm/IR/PassManager.h"
31 #include "llvm/Support/CommandLine.h"
32 #include "llvm/Support/Debug.h"
33 #include "llvm/Support/raw_ostream.h"
34 #include <algorithm>
35 using namespace llvm;
36 
37 // Explicitly instantiate methods in LoopInfoImpl.h for IR-level Loops.
38 template class llvm::LoopBase<BasicBlock, Loop>;
39 template class llvm::LoopInfoBase<BasicBlock, Loop>;
40 
41 // Always verify loopinfo if expensive checking is enabled.
42 #ifdef EXPENSIVE_CHECKS
43 static bool VerifyLoopInfo = true;
44 #else
45 static bool VerifyLoopInfo = false;
46 #endif
47 static cl::opt<bool,true>
48 VerifyLoopInfoX("verify-loop-info", cl::location(VerifyLoopInfo),
49                 cl::desc("Verify loop info (time consuming)"));
50 
51 //===----------------------------------------------------------------------===//
52 // Loop implementation
53 //
54 
isLoopInvariant(const Value * V) const55 bool Loop::isLoopInvariant(const Value *V) const {
56   if (const Instruction *I = dyn_cast<Instruction>(V))
57     return !contains(I);
58   return true;  // All non-instructions are loop invariant
59 }
60 
hasLoopInvariantOperands(const Instruction * I) const61 bool Loop::hasLoopInvariantOperands(const Instruction *I) const {
62   return all_of(I->operands(), [this](Value *V) { return isLoopInvariant(V); });
63 }
64 
makeLoopInvariant(Value * V,bool & Changed,Instruction * InsertPt) const65 bool Loop::makeLoopInvariant(Value *V, bool &Changed,
66                              Instruction *InsertPt) const {
67   if (Instruction *I = dyn_cast<Instruction>(V))
68     return makeLoopInvariant(I, Changed, InsertPt);
69   return true;  // All non-instructions are loop-invariant.
70 }
71 
makeLoopInvariant(Instruction * I,bool & Changed,Instruction * InsertPt) const72 bool Loop::makeLoopInvariant(Instruction *I, bool &Changed,
73                              Instruction *InsertPt) const {
74   // Test if the value is already loop-invariant.
75   if (isLoopInvariant(I))
76     return true;
77   if (!isSafeToSpeculativelyExecute(I))
78     return false;
79   if (I->mayReadFromMemory())
80     return false;
81   // EH block instructions are immobile.
82   if (I->isEHPad())
83     return false;
84   // Determine the insertion point, unless one was given.
85   if (!InsertPt) {
86     BasicBlock *Preheader = getLoopPreheader();
87     // Without a preheader, hoisting is not feasible.
88     if (!Preheader)
89       return false;
90     InsertPt = Preheader->getTerminator();
91   }
92   // Don't hoist instructions with loop-variant operands.
93   for (Value *Operand : I->operands())
94     if (!makeLoopInvariant(Operand, Changed, InsertPt))
95       return false;
96 
97   // Hoist.
98   I->moveBefore(InsertPt);
99 
100   // There is possibility of hoisting this instruction above some arbitrary
101   // condition. Any metadata defined on it can be control dependent on this
102   // condition. Conservatively strip it here so that we don't give any wrong
103   // information to the optimizer.
104   I->dropUnknownNonDebugMetadata();
105 
106   Changed = true;
107   return true;
108 }
109 
getCanonicalInductionVariable() const110 PHINode *Loop::getCanonicalInductionVariable() const {
111   BasicBlock *H = getHeader();
112 
113   BasicBlock *Incoming = nullptr, *Backedge = nullptr;
114   pred_iterator PI = pred_begin(H);
115   assert(PI != pred_end(H) &&
116          "Loop must have at least one backedge!");
117   Backedge = *PI++;
118   if (PI == pred_end(H)) return nullptr;  // dead loop
119   Incoming = *PI++;
120   if (PI != pred_end(H)) return nullptr;  // multiple backedges?
121 
122   if (contains(Incoming)) {
123     if (contains(Backedge))
124       return nullptr;
125     std::swap(Incoming, Backedge);
126   } else if (!contains(Backedge))
127     return nullptr;
128 
129   // Loop over all of the PHI nodes, looking for a canonical indvar.
130   for (BasicBlock::iterator I = H->begin(); isa<PHINode>(I); ++I) {
131     PHINode *PN = cast<PHINode>(I);
132     if (ConstantInt *CI =
133         dyn_cast<ConstantInt>(PN->getIncomingValueForBlock(Incoming)))
134       if (CI->isNullValue())
135         if (Instruction *Inc =
136             dyn_cast<Instruction>(PN->getIncomingValueForBlock(Backedge)))
137           if (Inc->getOpcode() == Instruction::Add &&
138                 Inc->getOperand(0) == PN)
139             if (ConstantInt *CI = dyn_cast<ConstantInt>(Inc->getOperand(1)))
140               if (CI->equalsInt(1))
141                 return PN;
142   }
143   return nullptr;
144 }
145 
isLCSSAForm(DominatorTree & DT) const146 bool Loop::isLCSSAForm(DominatorTree &DT) const {
147   for (BasicBlock *BB : this->blocks()) {
148     for (Instruction &I : *BB) {
149       // Tokens can't be used in PHI nodes and live-out tokens prevent loop
150       // optimizations, so for the purposes of considered LCSSA form, we
151       // can ignore them.
152       if (I.getType()->isTokenTy())
153         continue;
154 
155       for (Use &U : I.uses()) {
156         Instruction *UI = cast<Instruction>(U.getUser());
157         BasicBlock *UserBB = UI->getParent();
158         if (PHINode *P = dyn_cast<PHINode>(UI))
159           UserBB = P->getIncomingBlock(U);
160 
161         // Check the current block, as a fast-path, before checking whether
162         // the use is anywhere in the loop.  Most values are used in the same
163         // block they are defined in.  Also, blocks not reachable from the
164         // entry are special; uses in them don't need to go through PHIs.
165         if (UserBB != BB &&
166             !contains(UserBB) &&
167             DT.isReachableFromEntry(UserBB))
168           return false;
169       }
170     }
171   }
172 
173   return true;
174 }
175 
isRecursivelyLCSSAForm(DominatorTree & DT) const176 bool Loop::isRecursivelyLCSSAForm(DominatorTree &DT) const {
177   if (!isLCSSAForm(DT))
178     return false;
179 
180   return std::all_of(begin(), end(), [&](const Loop *L) {
181     return L->isRecursivelyLCSSAForm(DT);
182   });
183 }
184 
isLoopSimplifyForm() const185 bool Loop::isLoopSimplifyForm() const {
186   // Normal-form loops have a preheader, a single backedge, and all of their
187   // exits have all their predecessors inside the loop.
188   return getLoopPreheader() && getLoopLatch() && hasDedicatedExits();
189 }
190 
191 // Routines that reform the loop CFG and split edges often fail on indirectbr.
isSafeToClone() const192 bool Loop::isSafeToClone() const {
193   // Return false if any loop blocks contain indirectbrs, or there are any calls
194   // to noduplicate functions.
195   for (BasicBlock *BB : this->blocks()) {
196     if (isa<IndirectBrInst>(BB->getTerminator()))
197       return false;
198 
199     for (Instruction &I : *BB)
200       if (auto CS = CallSite(&I))
201         if (CS.cannotDuplicate())
202           return false;
203   }
204   return true;
205 }
206 
getLoopID() const207 MDNode *Loop::getLoopID() const {
208   MDNode *LoopID = nullptr;
209   if (isLoopSimplifyForm()) {
210     LoopID = getLoopLatch()->getTerminator()->getMetadata(LLVMContext::MD_loop);
211   } else {
212     // Go through each predecessor of the loop header and check the
213     // terminator for the metadata.
214     BasicBlock *H = getHeader();
215     for (BasicBlock *BB : this->blocks()) {
216       TerminatorInst *TI = BB->getTerminator();
217       MDNode *MD = nullptr;
218 
219       // Check if this terminator branches to the loop header.
220       for (BasicBlock *Successor : TI->successors()) {
221         if (Successor == H) {
222           MD = TI->getMetadata(LLVMContext::MD_loop);
223           break;
224         }
225       }
226       if (!MD)
227         return nullptr;
228 
229       if (!LoopID)
230         LoopID = MD;
231       else if (MD != LoopID)
232         return nullptr;
233     }
234   }
235   if (!LoopID || LoopID->getNumOperands() == 0 ||
236       LoopID->getOperand(0) != LoopID)
237     return nullptr;
238   return LoopID;
239 }
240 
setLoopID(MDNode * LoopID) const241 void Loop::setLoopID(MDNode *LoopID) const {
242   assert(LoopID && "Loop ID should not be null");
243   assert(LoopID->getNumOperands() > 0 && "Loop ID needs at least one operand");
244   assert(LoopID->getOperand(0) == LoopID && "Loop ID should refer to itself");
245 
246   if (isLoopSimplifyForm()) {
247     getLoopLatch()->getTerminator()->setMetadata(LLVMContext::MD_loop, LoopID);
248     return;
249   }
250 
251   BasicBlock *H = getHeader();
252   for (BasicBlock *BB : this->blocks()) {
253     TerminatorInst *TI = BB->getTerminator();
254     for (BasicBlock *Successor : TI->successors()) {
255       if (Successor == H)
256         TI->setMetadata(LLVMContext::MD_loop, LoopID);
257     }
258   }
259 }
260 
isAnnotatedParallel() const261 bool Loop::isAnnotatedParallel() const {
262   MDNode *DesiredLoopIdMetadata = getLoopID();
263 
264   if (!DesiredLoopIdMetadata)
265       return false;
266 
267   // The loop branch contains the parallel loop metadata. In order to ensure
268   // that any parallel-loop-unaware optimization pass hasn't added loop-carried
269   // dependencies (thus converted the loop back to a sequential loop), check
270   // that all the memory instructions in the loop contain parallelism metadata
271   // that point to the same unique "loop id metadata" the loop branch does.
272   for (BasicBlock *BB : this->blocks()) {
273     for (Instruction &I : *BB) {
274       if (!I.mayReadOrWriteMemory())
275         continue;
276 
277       // The memory instruction can refer to the loop identifier metadata
278       // directly or indirectly through another list metadata (in case of
279       // nested parallel loops). The loop identifier metadata refers to
280       // itself so we can check both cases with the same routine.
281       MDNode *LoopIdMD =
282           I.getMetadata(LLVMContext::MD_mem_parallel_loop_access);
283 
284       if (!LoopIdMD)
285         return false;
286 
287       bool LoopIdMDFound = false;
288       for (const MDOperand &MDOp : LoopIdMD->operands()) {
289         if (MDOp == DesiredLoopIdMetadata) {
290           LoopIdMDFound = true;
291           break;
292         }
293       }
294 
295       if (!LoopIdMDFound)
296         return false;
297     }
298   }
299   return true;
300 }
301 
getStartLoc() const302 DebugLoc Loop::getStartLoc() const {
303   // If we have a debug location in the loop ID, then use it.
304   if (MDNode *LoopID = getLoopID())
305     for (unsigned i = 1, ie = LoopID->getNumOperands(); i < ie; ++i)
306       if (DILocation *L = dyn_cast<DILocation>(LoopID->getOperand(i)))
307         return DebugLoc(L);
308 
309   // Try the pre-header first.
310   if (BasicBlock *PHeadBB = getLoopPreheader())
311     if (DebugLoc DL = PHeadBB->getTerminator()->getDebugLoc())
312       return DL;
313 
314   // If we have no pre-header or there are no instructions with debug
315   // info in it, try the header.
316   if (BasicBlock *HeadBB = getHeader())
317     return HeadBB->getTerminator()->getDebugLoc();
318 
319   return DebugLoc();
320 }
321 
hasDedicatedExits() const322 bool Loop::hasDedicatedExits() const {
323   // Each predecessor of each exit block of a normal loop is contained
324   // within the loop.
325   SmallVector<BasicBlock *, 4> ExitBlocks;
326   getExitBlocks(ExitBlocks);
327   for (BasicBlock *BB : ExitBlocks)
328     for (BasicBlock *Predecessor : predecessors(BB))
329       if (!contains(Predecessor))
330         return false;
331   // All the requirements are met.
332   return true;
333 }
334 
335 void
getUniqueExitBlocks(SmallVectorImpl<BasicBlock * > & ExitBlocks) const336 Loop::getUniqueExitBlocks(SmallVectorImpl<BasicBlock *> &ExitBlocks) const {
337   assert(hasDedicatedExits() &&
338          "getUniqueExitBlocks assumes the loop has canonical form exits!");
339 
340   SmallVector<BasicBlock *, 32> SwitchExitBlocks;
341   for (BasicBlock *BB : this->blocks()) {
342     SwitchExitBlocks.clear();
343     for (BasicBlock *Successor : successors(BB)) {
344       // If block is inside the loop then it is not an exit block.
345       if (contains(Successor))
346         continue;
347 
348       pred_iterator PI = pred_begin(Successor);
349       BasicBlock *FirstPred = *PI;
350 
351       // If current basic block is this exit block's first predecessor
352       // then only insert exit block in to the output ExitBlocks vector.
353       // This ensures that same exit block is not inserted twice into
354       // ExitBlocks vector.
355       if (BB != FirstPred)
356         continue;
357 
358       // If a terminator has more then two successors, for example SwitchInst,
359       // then it is possible that there are multiple edges from current block
360       // to one exit block.
361       if (std::distance(succ_begin(BB), succ_end(BB)) <= 2) {
362         ExitBlocks.push_back(Successor);
363         continue;
364       }
365 
366       // In case of multiple edges from current block to exit block, collect
367       // only one edge in ExitBlocks. Use switchExitBlocks to keep track of
368       // duplicate edges.
369       if (std::find(SwitchExitBlocks.begin(), SwitchExitBlocks.end(), Successor)
370           == SwitchExitBlocks.end()) {
371         SwitchExitBlocks.push_back(Successor);
372         ExitBlocks.push_back(Successor);
373       }
374     }
375   }
376 }
377 
getUniqueExitBlock() const378 BasicBlock *Loop::getUniqueExitBlock() const {
379   SmallVector<BasicBlock *, 8> UniqueExitBlocks;
380   getUniqueExitBlocks(UniqueExitBlocks);
381   if (UniqueExitBlocks.size() == 1)
382     return UniqueExitBlocks[0];
383   return nullptr;
384 }
385 
386 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
dump() const387 LLVM_DUMP_METHOD void Loop::dump() const {
388   print(dbgs());
389 }
390 #endif
391 
392 //===----------------------------------------------------------------------===//
393 // UnloopUpdater implementation
394 //
395 
396 namespace {
397 /// Find the new parent loop for all blocks within the "unloop" whose last
398 /// backedges has just been removed.
399 class UnloopUpdater {
400   Loop &Unloop;
401   LoopInfo *LI;
402 
403   LoopBlocksDFS DFS;
404 
405   // Map unloop's immediate subloops to their nearest reachable parents. Nested
406   // loops within these subloops will not change parents. However, an immediate
407   // subloop's new parent will be the nearest loop reachable from either its own
408   // exits *or* any of its nested loop's exits.
409   DenseMap<Loop*, Loop*> SubloopParents;
410 
411   // Flag the presence of an irreducible backedge whose destination is a block
412   // directly contained by the original unloop.
413   bool FoundIB;
414 
415 public:
UnloopUpdater(Loop * UL,LoopInfo * LInfo)416   UnloopUpdater(Loop *UL, LoopInfo *LInfo) :
417     Unloop(*UL), LI(LInfo), DFS(UL), FoundIB(false) {}
418 
419   void updateBlockParents();
420 
421   void removeBlocksFromAncestors();
422 
423   void updateSubloopParents();
424 
425 protected:
426   Loop *getNearestLoop(BasicBlock *BB, Loop *BBLoop);
427 };
428 } // end anonymous namespace
429 
430 /// Update the parent loop for all blocks that are directly contained within the
431 /// original "unloop".
updateBlockParents()432 void UnloopUpdater::updateBlockParents() {
433   if (Unloop.getNumBlocks()) {
434     // Perform a post order CFG traversal of all blocks within this loop,
435     // propagating the nearest loop from sucessors to predecessors.
436     LoopBlocksTraversal Traversal(DFS, LI);
437     for (BasicBlock *POI : Traversal) {
438 
439       Loop *L = LI->getLoopFor(POI);
440       Loop *NL = getNearestLoop(POI, L);
441 
442       if (NL != L) {
443         // For reducible loops, NL is now an ancestor of Unloop.
444         assert((NL != &Unloop && (!NL || NL->contains(&Unloop))) &&
445                "uninitialized successor");
446         LI->changeLoopFor(POI, NL);
447       }
448       else {
449         // Or the current block is part of a subloop, in which case its parent
450         // is unchanged.
451         assert((FoundIB || Unloop.contains(L)) && "uninitialized successor");
452       }
453     }
454   }
455   // Each irreducible loop within the unloop induces a round of iteration using
456   // the DFS result cached by Traversal.
457   bool Changed = FoundIB;
458   for (unsigned NIters = 0; Changed; ++NIters) {
459     assert(NIters < Unloop.getNumBlocks() && "runaway iterative algorithm");
460 
461     // Iterate over the postorder list of blocks, propagating the nearest loop
462     // from successors to predecessors as before.
463     Changed = false;
464     for (LoopBlocksDFS::POIterator POI = DFS.beginPostorder(),
465            POE = DFS.endPostorder(); POI != POE; ++POI) {
466 
467       Loop *L = LI->getLoopFor(*POI);
468       Loop *NL = getNearestLoop(*POI, L);
469       if (NL != L) {
470         assert(NL != &Unloop && (!NL || NL->contains(&Unloop)) &&
471                "uninitialized successor");
472         LI->changeLoopFor(*POI, NL);
473         Changed = true;
474       }
475     }
476   }
477 }
478 
479 /// Remove unloop's blocks from all ancestors below their new parents.
removeBlocksFromAncestors()480 void UnloopUpdater::removeBlocksFromAncestors() {
481   // Remove all unloop's blocks (including those in nested subloops) from
482   // ancestors below the new parent loop.
483   for (Loop::block_iterator BI = Unloop.block_begin(),
484          BE = Unloop.block_end(); BI != BE; ++BI) {
485     Loop *OuterParent = LI->getLoopFor(*BI);
486     if (Unloop.contains(OuterParent)) {
487       while (OuterParent->getParentLoop() != &Unloop)
488         OuterParent = OuterParent->getParentLoop();
489       OuterParent = SubloopParents[OuterParent];
490     }
491     // Remove blocks from former Ancestors except Unloop itself which will be
492     // deleted.
493     for (Loop *OldParent = Unloop.getParentLoop(); OldParent != OuterParent;
494          OldParent = OldParent->getParentLoop()) {
495       assert(OldParent && "new loop is not an ancestor of the original");
496       OldParent->removeBlockFromLoop(*BI);
497     }
498   }
499 }
500 
501 /// Update the parent loop for all subloops directly nested within unloop.
updateSubloopParents()502 void UnloopUpdater::updateSubloopParents() {
503   while (!Unloop.empty()) {
504     Loop *Subloop = *std::prev(Unloop.end());
505     Unloop.removeChildLoop(std::prev(Unloop.end()));
506 
507     assert(SubloopParents.count(Subloop) && "DFS failed to visit subloop");
508     if (Loop *Parent = SubloopParents[Subloop])
509       Parent->addChildLoop(Subloop);
510     else
511       LI->addTopLevelLoop(Subloop);
512   }
513 }
514 
515 /// Return the nearest parent loop among this block's successors. If a successor
516 /// is a subloop header, consider its parent to be the nearest parent of the
517 /// subloop's exits.
518 ///
519 /// For subloop blocks, simply update SubloopParents and return NULL.
getNearestLoop(BasicBlock * BB,Loop * BBLoop)520 Loop *UnloopUpdater::getNearestLoop(BasicBlock *BB, Loop *BBLoop) {
521 
522   // Initially for blocks directly contained by Unloop, NearLoop == Unloop and
523   // is considered uninitialized.
524   Loop *NearLoop = BBLoop;
525 
526   Loop *Subloop = nullptr;
527   if (NearLoop != &Unloop && Unloop.contains(NearLoop)) {
528     Subloop = NearLoop;
529     // Find the subloop ancestor that is directly contained within Unloop.
530     while (Subloop->getParentLoop() != &Unloop) {
531       Subloop = Subloop->getParentLoop();
532       assert(Subloop && "subloop is not an ancestor of the original loop");
533     }
534     // Get the current nearest parent of the Subloop exits, initially Unloop.
535     NearLoop =
536       SubloopParents.insert(std::make_pair(Subloop, &Unloop)).first->second;
537   }
538 
539   succ_iterator I = succ_begin(BB), E = succ_end(BB);
540   if (I == E) {
541     assert(!Subloop && "subloop blocks must have a successor");
542     NearLoop = nullptr; // unloop blocks may now exit the function.
543   }
544   for (; I != E; ++I) {
545     if (*I == BB)
546       continue; // self loops are uninteresting
547 
548     Loop *L = LI->getLoopFor(*I);
549     if (L == &Unloop) {
550       // This successor has not been processed. This path must lead to an
551       // irreducible backedge.
552       assert((FoundIB || !DFS.hasPostorder(*I)) && "should have seen IB");
553       FoundIB = true;
554     }
555     if (L != &Unloop && Unloop.contains(L)) {
556       // Successor is in a subloop.
557       if (Subloop)
558         continue; // Branching within subloops. Ignore it.
559 
560       // BB branches from the original into a subloop header.
561       assert(L->getParentLoop() == &Unloop && "cannot skip into nested loops");
562 
563       // Get the current nearest parent of the Subloop's exits.
564       L = SubloopParents[L];
565       // L could be Unloop if the only exit was an irreducible backedge.
566     }
567     if (L == &Unloop) {
568       continue;
569     }
570     // Handle critical edges from Unloop into a sibling loop.
571     if (L && !L->contains(&Unloop)) {
572       L = L->getParentLoop();
573     }
574     // Remember the nearest parent loop among successors or subloop exits.
575     if (NearLoop == &Unloop || !NearLoop || NearLoop->contains(L))
576       NearLoop = L;
577   }
578   if (Subloop) {
579     SubloopParents[Subloop] = NearLoop;
580     return BBLoop;
581   }
582   return NearLoop;
583 }
584 
LoopInfo(const DominatorTreeBase<BasicBlock> & DomTree)585 LoopInfo::LoopInfo(const DominatorTreeBase<BasicBlock> &DomTree) {
586   analyze(DomTree);
587 }
588 
markAsRemoved(Loop * Unloop)589 void LoopInfo::markAsRemoved(Loop *Unloop) {
590   assert(!Unloop->isInvalid() && "Loop has already been removed");
591   Unloop->invalidate();
592   RemovedLoops.push_back(Unloop);
593 
594   // First handle the special case of no parent loop to simplify the algorithm.
595   if (!Unloop->getParentLoop()) {
596     // Since BBLoop had no parent, Unloop blocks are no longer in a loop.
597     for (Loop::block_iterator I = Unloop->block_begin(),
598                               E = Unloop->block_end();
599          I != E; ++I) {
600 
601       // Don't reparent blocks in subloops.
602       if (getLoopFor(*I) != Unloop)
603         continue;
604 
605       // Blocks no longer have a parent but are still referenced by Unloop until
606       // the Unloop object is deleted.
607       changeLoopFor(*I, nullptr);
608     }
609 
610     // Remove the loop from the top-level LoopInfo object.
611     for (iterator I = begin();; ++I) {
612       assert(I != end() && "Couldn't find loop");
613       if (*I == Unloop) {
614         removeLoop(I);
615         break;
616       }
617     }
618 
619     // Move all of the subloops to the top-level.
620     while (!Unloop->empty())
621       addTopLevelLoop(Unloop->removeChildLoop(std::prev(Unloop->end())));
622 
623     return;
624   }
625 
626   // Update the parent loop for all blocks within the loop. Blocks within
627   // subloops will not change parents.
628   UnloopUpdater Updater(Unloop, this);
629   Updater.updateBlockParents();
630 
631   // Remove blocks from former ancestor loops.
632   Updater.removeBlocksFromAncestors();
633 
634   // Add direct subloops as children in their new parent loop.
635   Updater.updateSubloopParents();
636 
637   // Remove unloop from its parent loop.
638   Loop *ParentLoop = Unloop->getParentLoop();
639   for (Loop::iterator I = ParentLoop->begin();; ++I) {
640     assert(I != ParentLoop->end() && "Couldn't find loop");
641     if (*I == Unloop) {
642       ParentLoop->removeChildLoop(I);
643       break;
644     }
645   }
646 }
647 
648 char LoopAnalysis::PassID;
649 
run(Function & F,AnalysisManager<Function> & AM)650 LoopInfo LoopAnalysis::run(Function &F, AnalysisManager<Function> &AM) {
651   // FIXME: Currently we create a LoopInfo from scratch for every function.
652   // This may prove to be too wasteful due to deallocating and re-allocating
653   // memory each time for the underlying map and vector datastructures. At some
654   // point it may prove worthwhile to use a freelist and recycle LoopInfo
655   // objects. I don't want to add that kind of complexity until the scope of
656   // the problem is better understood.
657   LoopInfo LI;
658   LI.analyze(AM.getResult<DominatorTreeAnalysis>(F));
659   return LI;
660 }
661 
run(Function & F,AnalysisManager<Function> & AM)662 PreservedAnalyses LoopPrinterPass::run(Function &F,
663                                        AnalysisManager<Function> &AM) {
664   AM.getResult<LoopAnalysis>(F).print(OS);
665   return PreservedAnalyses::all();
666 }
667 
PrintLoopPass()668 PrintLoopPass::PrintLoopPass() : OS(dbgs()) {}
PrintLoopPass(raw_ostream & OS,const std::string & Banner)669 PrintLoopPass::PrintLoopPass(raw_ostream &OS, const std::string &Banner)
670     : OS(OS), Banner(Banner) {}
671 
run(Loop & L,AnalysisManager<Loop> &)672 PreservedAnalyses PrintLoopPass::run(Loop &L, AnalysisManager<Loop> &) {
673   OS << Banner;
674   for (auto *Block : L.blocks())
675     if (Block)
676       Block->print(OS);
677     else
678       OS << "Printing <null> block";
679   return PreservedAnalyses::all();
680 }
681 
682 //===----------------------------------------------------------------------===//
683 // LoopInfo implementation
684 //
685 
686 char LoopInfoWrapperPass::ID = 0;
687 INITIALIZE_PASS_BEGIN(LoopInfoWrapperPass, "loops", "Natural Loop Information",
688                       true, true)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)689 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
690 INITIALIZE_PASS_END(LoopInfoWrapperPass, "loops", "Natural Loop Information",
691                     true, true)
692 
693 bool LoopInfoWrapperPass::runOnFunction(Function &) {
694   releaseMemory();
695   LI.analyze(getAnalysis<DominatorTreeWrapperPass>().getDomTree());
696   return false;
697 }
698 
verifyAnalysis() const699 void LoopInfoWrapperPass::verifyAnalysis() const {
700   // LoopInfoWrapperPass is a FunctionPass, but verifying every loop in the
701   // function each time verifyAnalysis is called is very expensive. The
702   // -verify-loop-info option can enable this. In order to perform some
703   // checking by default, LoopPass has been taught to call verifyLoop manually
704   // during loop pass sequences.
705   if (VerifyLoopInfo)
706     LI.verify();
707 }
708 
getAnalysisUsage(AnalysisUsage & AU) const709 void LoopInfoWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
710   AU.setPreservesAll();
711   AU.addRequired<DominatorTreeWrapperPass>();
712 }
713 
print(raw_ostream & OS,const Module *) const714 void LoopInfoWrapperPass::print(raw_ostream &OS, const Module *) const {
715   LI.print(OS);
716 }
717 
718 //===----------------------------------------------------------------------===//
719 // LoopBlocksDFS implementation
720 //
721 
722 /// Traverse the loop blocks and store the DFS result.
723 /// Useful for clients that just want the final DFS result and don't need to
724 /// visit blocks during the initial traversal.
perform(LoopInfo * LI)725 void LoopBlocksDFS::perform(LoopInfo *LI) {
726   LoopBlocksTraversal Traversal(*this, LI);
727   for (LoopBlocksTraversal::POTIterator POI = Traversal.begin(),
728          POE = Traversal.end(); POI != POE; ++POI) ;
729 }
730