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1 //===- LoopSimplify.cpp - Loop Canonicalization Pass ----------------------===//
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 pass performs several transformations to transform natural loops into a
11 // simpler form, which makes subsequent analyses and transformations simpler and
12 // more effective.
13 //
14 // Loop pre-header insertion guarantees that there is a single, non-critical
15 // entry edge from outside of the loop to the loop header.  This simplifies a
16 // number of analyses and transformations, such as LICM.
17 //
18 // Loop exit-block insertion guarantees that all exit blocks from the loop
19 // (blocks which are outside of the loop that have predecessors inside of the
20 // loop) only have predecessors from inside of the loop (and are thus dominated
21 // by the loop header).  This simplifies transformations such as store-sinking
22 // that are built into LICM.
23 //
24 // This pass also guarantees that loops will have exactly one backedge.
25 //
26 // Indirectbr instructions introduce several complications. If the loop
27 // contains or is entered by an indirectbr instruction, it may not be possible
28 // to transform the loop and make these guarantees. Client code should check
29 // that these conditions are true before relying on them.
30 //
31 // Note that the simplifycfg pass will clean up blocks which are split out but
32 // end up being unnecessary, so usage of this pass should not pessimize
33 // generated code.
34 //
35 // This pass obviously modifies the CFG, but updates loop information and
36 // dominator information.
37 //
38 //===----------------------------------------------------------------------===//
39 
40 #include "llvm/Transforms/Scalar.h"
41 #include "llvm/ADT/DepthFirstIterator.h"
42 #include "llvm/ADT/SetOperations.h"
43 #include "llvm/ADT/SetVector.h"
44 #include "llvm/ADT/SmallVector.h"
45 #include "llvm/ADT/Statistic.h"
46 #include "llvm/Analysis/AliasAnalysis.h"
47 #include "llvm/Analysis/BasicAliasAnalysis.h"
48 #include "llvm/Analysis/AssumptionCache.h"
49 #include "llvm/Analysis/DependenceAnalysis.h"
50 #include "llvm/Analysis/GlobalsModRef.h"
51 #include "llvm/Analysis/InstructionSimplify.h"
52 #include "llvm/Analysis/LoopInfo.h"
53 #include "llvm/Analysis/ScalarEvolution.h"
54 #include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
55 #include "llvm/IR/CFG.h"
56 #include "llvm/IR/Constants.h"
57 #include "llvm/IR/DataLayout.h"
58 #include "llvm/IR/Dominators.h"
59 #include "llvm/IR/Function.h"
60 #include "llvm/IR/Instructions.h"
61 #include "llvm/IR/IntrinsicInst.h"
62 #include "llvm/IR/LLVMContext.h"
63 #include "llvm/IR/Module.h"
64 #include "llvm/IR/Type.h"
65 #include "llvm/Support/Debug.h"
66 #include "llvm/Support/raw_ostream.h"
67 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
68 #include "llvm/Transforms/Utils/Local.h"
69 #include "llvm/Transforms/Utils/LoopUtils.h"
70 using namespace llvm;
71 
72 #define DEBUG_TYPE "loop-simplify"
73 
74 STATISTIC(NumInserted, "Number of pre-header or exit blocks inserted");
75 STATISTIC(NumNested  , "Number of nested loops split out");
76 
77 // If the block isn't already, move the new block to right after some 'outside
78 // block' block.  This prevents the preheader from being placed inside the loop
79 // body, e.g. when the loop hasn't been rotated.
placeSplitBlockCarefully(BasicBlock * NewBB,SmallVectorImpl<BasicBlock * > & SplitPreds,Loop * L)80 static void placeSplitBlockCarefully(BasicBlock *NewBB,
81                                      SmallVectorImpl<BasicBlock *> &SplitPreds,
82                                      Loop *L) {
83   // Check to see if NewBB is already well placed.
84   Function::iterator BBI = --NewBB->getIterator();
85   for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
86     if (&*BBI == SplitPreds[i])
87       return;
88   }
89 
90   // If it isn't already after an outside block, move it after one.  This is
91   // always good as it makes the uncond branch from the outside block into a
92   // fall-through.
93 
94   // Figure out *which* outside block to put this after.  Prefer an outside
95   // block that neighbors a BB actually in the loop.
96   BasicBlock *FoundBB = nullptr;
97   for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
98     Function::iterator BBI = SplitPreds[i]->getIterator();
99     if (++BBI != NewBB->getParent()->end() && L->contains(&*BBI)) {
100       FoundBB = SplitPreds[i];
101       break;
102     }
103   }
104 
105   // If our heuristic for a *good* bb to place this after doesn't find
106   // anything, just pick something.  It's likely better than leaving it within
107   // the loop.
108   if (!FoundBB)
109     FoundBB = SplitPreds[0];
110   NewBB->moveAfter(FoundBB);
111 }
112 
113 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
114 /// preheader, this method is called to insert one.  This method has two phases:
115 /// preheader insertion and analysis updating.
116 ///
InsertPreheaderForLoop(Loop * L,DominatorTree * DT,LoopInfo * LI,bool PreserveLCSSA)117 BasicBlock *llvm::InsertPreheaderForLoop(Loop *L, DominatorTree *DT,
118                                          LoopInfo *LI, bool PreserveLCSSA) {
119   BasicBlock *Header = L->getHeader();
120 
121   // Compute the set of predecessors of the loop that are not in the loop.
122   SmallVector<BasicBlock*, 8> OutsideBlocks;
123   for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
124        PI != PE; ++PI) {
125     BasicBlock *P = *PI;
126     if (!L->contains(P)) {         // Coming in from outside the loop?
127       // If the loop is branched to from an indirect branch, we won't
128       // be able to fully transform the loop, because it prohibits
129       // edge splitting.
130       if (isa<IndirectBrInst>(P->getTerminator())) return nullptr;
131 
132       // Keep track of it.
133       OutsideBlocks.push_back(P);
134     }
135   }
136 
137   // Split out the loop pre-header.
138   BasicBlock *PreheaderBB;
139   PreheaderBB = SplitBlockPredecessors(Header, OutsideBlocks, ".preheader", DT,
140                                        LI, PreserveLCSSA);
141   if (!PreheaderBB)
142     return nullptr;
143 
144   DEBUG(dbgs() << "LoopSimplify: Creating pre-header "
145                << PreheaderBB->getName() << "\n");
146 
147   // Make sure that NewBB is put someplace intelligent, which doesn't mess up
148   // code layout too horribly.
149   placeSplitBlockCarefully(PreheaderBB, OutsideBlocks, L);
150 
151   return PreheaderBB;
152 }
153 
154 /// \brief Ensure that the loop preheader dominates all exit blocks.
155 ///
156 /// This method is used to split exit blocks that have predecessors outside of
157 /// the loop.
rewriteLoopExitBlock(Loop * L,BasicBlock * Exit,DominatorTree * DT,LoopInfo * LI,bool PreserveLCSSA)158 static BasicBlock *rewriteLoopExitBlock(Loop *L, BasicBlock *Exit,
159                                         DominatorTree *DT, LoopInfo *LI,
160                                         bool PreserveLCSSA) {
161   SmallVector<BasicBlock*, 8> LoopBlocks;
162   for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I) {
163     BasicBlock *P = *I;
164     if (L->contains(P)) {
165       // Don't do this if the loop is exited via an indirect branch.
166       if (isa<IndirectBrInst>(P->getTerminator())) return nullptr;
167 
168       LoopBlocks.push_back(P);
169     }
170   }
171 
172   assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
173   BasicBlock *NewExitBB = nullptr;
174 
175   NewExitBB = SplitBlockPredecessors(Exit, LoopBlocks, ".loopexit", DT, LI,
176                                      PreserveLCSSA);
177   if (!NewExitBB)
178     return nullptr;
179 
180   DEBUG(dbgs() << "LoopSimplify: Creating dedicated exit block "
181                << NewExitBB->getName() << "\n");
182   return NewExitBB;
183 }
184 
185 /// Add the specified block, and all of its predecessors, to the specified set,
186 /// if it's not already in there.  Stop predecessor traversal when we reach
187 /// StopBlock.
addBlockAndPredsToSet(BasicBlock * InputBB,BasicBlock * StopBlock,std::set<BasicBlock * > & Blocks)188 static void addBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
189                                   std::set<BasicBlock*> &Blocks) {
190   SmallVector<BasicBlock *, 8> Worklist;
191   Worklist.push_back(InputBB);
192   do {
193     BasicBlock *BB = Worklist.pop_back_val();
194     if (Blocks.insert(BB).second && BB != StopBlock)
195       // If BB is not already processed and it is not a stop block then
196       // insert its predecessor in the work list
197       for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
198         BasicBlock *WBB = *I;
199         Worklist.push_back(WBB);
200       }
201   } while (!Worklist.empty());
202 }
203 
204 /// \brief The first part of loop-nestification is to find a PHI node that tells
205 /// us how to partition the loops.
findPHIToPartitionLoops(Loop * L,DominatorTree * DT,AssumptionCache * AC)206 static PHINode *findPHIToPartitionLoops(Loop *L, DominatorTree *DT,
207                                         AssumptionCache *AC) {
208   const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
209   for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
210     PHINode *PN = cast<PHINode>(I);
211     ++I;
212     if (Value *V = SimplifyInstruction(PN, DL, nullptr, DT, AC)) {
213       // This is a degenerate PHI already, don't modify it!
214       PN->replaceAllUsesWith(V);
215       PN->eraseFromParent();
216       continue;
217     }
218 
219     // Scan this PHI node looking for a use of the PHI node by itself.
220     for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
221       if (PN->getIncomingValue(i) == PN &&
222           L->contains(PN->getIncomingBlock(i)))
223         // We found something tasty to remove.
224         return PN;
225   }
226   return nullptr;
227 }
228 
229 /// \brief If this loop has multiple backedges, try to pull one of them out into
230 /// a nested loop.
231 ///
232 /// This is important for code that looks like
233 /// this:
234 ///
235 ///  Loop:
236 ///     ...
237 ///     br cond, Loop, Next
238 ///     ...
239 ///     br cond2, Loop, Out
240 ///
241 /// To identify this common case, we look at the PHI nodes in the header of the
242 /// loop.  PHI nodes with unchanging values on one backedge correspond to values
243 /// that change in the "outer" loop, but not in the "inner" loop.
244 ///
245 /// If we are able to separate out a loop, return the new outer loop that was
246 /// created.
247 ///
separateNestedLoop(Loop * L,BasicBlock * Preheader,DominatorTree * DT,LoopInfo * LI,ScalarEvolution * SE,bool PreserveLCSSA,AssumptionCache * AC)248 static Loop *separateNestedLoop(Loop *L, BasicBlock *Preheader,
249                                 DominatorTree *DT, LoopInfo *LI,
250                                 ScalarEvolution *SE, bool PreserveLCSSA,
251                                 AssumptionCache *AC) {
252   // Don't try to separate loops without a preheader.
253   if (!Preheader)
254     return nullptr;
255 
256   // The header is not a landing pad; preheader insertion should ensure this.
257   BasicBlock *Header = L->getHeader();
258   assert(!Header->isEHPad() && "Can't insert backedge to EH pad");
259 
260   PHINode *PN = findPHIToPartitionLoops(L, DT, AC);
261   if (!PN) return nullptr;  // No known way to partition.
262 
263   // Pull out all predecessors that have varying values in the loop.  This
264   // handles the case when a PHI node has multiple instances of itself as
265   // arguments.
266   SmallVector<BasicBlock*, 8> OuterLoopPreds;
267   for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
268     if (PN->getIncomingValue(i) != PN ||
269         !L->contains(PN->getIncomingBlock(i))) {
270       // We can't split indirectbr edges.
271       if (isa<IndirectBrInst>(PN->getIncomingBlock(i)->getTerminator()))
272         return nullptr;
273       OuterLoopPreds.push_back(PN->getIncomingBlock(i));
274     }
275   }
276   DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n");
277 
278   // If ScalarEvolution is around and knows anything about values in
279   // this loop, tell it to forget them, because we're about to
280   // substantially change it.
281   if (SE)
282     SE->forgetLoop(L);
283 
284   BasicBlock *NewBB = SplitBlockPredecessors(Header, OuterLoopPreds, ".outer",
285                                              DT, LI, PreserveLCSSA);
286 
287   // Make sure that NewBB is put someplace intelligent, which doesn't mess up
288   // code layout too horribly.
289   placeSplitBlockCarefully(NewBB, OuterLoopPreds, L);
290 
291   // Create the new outer loop.
292   Loop *NewOuter = new Loop();
293 
294   // Change the parent loop to use the outer loop as its child now.
295   if (Loop *Parent = L->getParentLoop())
296     Parent->replaceChildLoopWith(L, NewOuter);
297   else
298     LI->changeTopLevelLoop(L, NewOuter);
299 
300   // L is now a subloop of our outer loop.
301   NewOuter->addChildLoop(L);
302 
303   for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
304        I != E; ++I)
305     NewOuter->addBlockEntry(*I);
306 
307   // Now reset the header in L, which had been moved by
308   // SplitBlockPredecessors for the outer loop.
309   L->moveToHeader(Header);
310 
311   // Determine which blocks should stay in L and which should be moved out to
312   // the Outer loop now.
313   std::set<BasicBlock*> BlocksInL;
314   for (pred_iterator PI=pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) {
315     BasicBlock *P = *PI;
316     if (DT->dominates(Header, P))
317       addBlockAndPredsToSet(P, Header, BlocksInL);
318   }
319 
320   // Scan all of the loop children of L, moving them to OuterLoop if they are
321   // not part of the inner loop.
322   const std::vector<Loop*> &SubLoops = L->getSubLoops();
323   for (size_t I = 0; I != SubLoops.size(); )
324     if (BlocksInL.count(SubLoops[I]->getHeader()))
325       ++I;   // Loop remains in L
326     else
327       NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));
328 
329   // Now that we know which blocks are in L and which need to be moved to
330   // OuterLoop, move any blocks that need it.
331   for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
332     BasicBlock *BB = L->getBlocks()[i];
333     if (!BlocksInL.count(BB)) {
334       // Move this block to the parent, updating the exit blocks sets
335       L->removeBlockFromLoop(BB);
336       if ((*LI)[BB] == L)
337         LI->changeLoopFor(BB, NewOuter);
338       --i;
339     }
340   }
341 
342   return NewOuter;
343 }
344 
345 /// \brief This method is called when the specified loop has more than one
346 /// backedge in it.
347 ///
348 /// If this occurs, revector all of these backedges to target a new basic block
349 /// and have that block branch to the loop header.  This ensures that loops
350 /// have exactly one backedge.
insertUniqueBackedgeBlock(Loop * L,BasicBlock * Preheader,DominatorTree * DT,LoopInfo * LI)351 static BasicBlock *insertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader,
352                                              DominatorTree *DT, LoopInfo *LI) {
353   assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
354 
355   // Get information about the loop
356   BasicBlock *Header = L->getHeader();
357   Function *F = Header->getParent();
358 
359   // Unique backedge insertion currently depends on having a preheader.
360   if (!Preheader)
361     return nullptr;
362 
363   // The header is not an EH pad; preheader insertion should ensure this.
364   assert(!Header->isEHPad() && "Can't insert backedge to EH pad");
365 
366   // Figure out which basic blocks contain back-edges to the loop header.
367   std::vector<BasicBlock*> BackedgeBlocks;
368   for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I){
369     BasicBlock *P = *I;
370 
371     // Indirectbr edges cannot be split, so we must fail if we find one.
372     if (isa<IndirectBrInst>(P->getTerminator()))
373       return nullptr;
374 
375     if (P != Preheader) BackedgeBlocks.push_back(P);
376   }
377 
378   // Create and insert the new backedge block...
379   BasicBlock *BEBlock = BasicBlock::Create(Header->getContext(),
380                                            Header->getName() + ".backedge", F);
381   BranchInst *BETerminator = BranchInst::Create(Header, BEBlock);
382   BETerminator->setDebugLoc(Header->getFirstNonPHI()->getDebugLoc());
383 
384   DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block "
385                << BEBlock->getName() << "\n");
386 
387   // Move the new backedge block to right after the last backedge block.
388   Function::iterator InsertPos = ++BackedgeBlocks.back()->getIterator();
389   F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
390 
391   // Now that the block has been inserted into the function, create PHI nodes in
392   // the backedge block which correspond to any PHI nodes in the header block.
393   for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
394     PHINode *PN = cast<PHINode>(I);
395     PHINode *NewPN = PHINode::Create(PN->getType(), BackedgeBlocks.size(),
396                                      PN->getName()+".be", BETerminator);
397 
398     // Loop over the PHI node, moving all entries except the one for the
399     // preheader over to the new PHI node.
400     unsigned PreheaderIdx = ~0U;
401     bool HasUniqueIncomingValue = true;
402     Value *UniqueValue = nullptr;
403     for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
404       BasicBlock *IBB = PN->getIncomingBlock(i);
405       Value *IV = PN->getIncomingValue(i);
406       if (IBB == Preheader) {
407         PreheaderIdx = i;
408       } else {
409         NewPN->addIncoming(IV, IBB);
410         if (HasUniqueIncomingValue) {
411           if (!UniqueValue)
412             UniqueValue = IV;
413           else if (UniqueValue != IV)
414             HasUniqueIncomingValue = false;
415         }
416       }
417     }
418 
419     // Delete all of the incoming values from the old PN except the preheader's
420     assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
421     if (PreheaderIdx != 0) {
422       PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
423       PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
424     }
425     // Nuke all entries except the zero'th.
426     for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
427       PN->removeIncomingValue(e-i, false);
428 
429     // Finally, add the newly constructed PHI node as the entry for the BEBlock.
430     PN->addIncoming(NewPN, BEBlock);
431 
432     // As an optimization, if all incoming values in the new PhiNode (which is a
433     // subset of the incoming values of the old PHI node) have the same value,
434     // eliminate the PHI Node.
435     if (HasUniqueIncomingValue) {
436       NewPN->replaceAllUsesWith(UniqueValue);
437       BEBlock->getInstList().erase(NewPN);
438     }
439   }
440 
441   // Now that all of the PHI nodes have been inserted and adjusted, modify the
442   // backedge blocks to just to the BEBlock instead of the header.
443   for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
444     TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
445     for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
446       if (TI->getSuccessor(Op) == Header)
447         TI->setSuccessor(Op, BEBlock);
448   }
449 
450   //===--- Update all analyses which we must preserve now -----------------===//
451 
452   // Update Loop Information - we know that this block is now in the current
453   // loop and all parent loops.
454   L->addBasicBlockToLoop(BEBlock, *LI);
455 
456   // Update dominator information
457   DT->splitBlock(BEBlock);
458 
459   return BEBlock;
460 }
461 
462 /// \brief Simplify one loop and queue further loops for simplification.
simplifyOneLoop(Loop * L,SmallVectorImpl<Loop * > & Worklist,DominatorTree * DT,LoopInfo * LI,ScalarEvolution * SE,AssumptionCache * AC,bool PreserveLCSSA)463 static bool simplifyOneLoop(Loop *L, SmallVectorImpl<Loop *> &Worklist,
464                             DominatorTree *DT, LoopInfo *LI,
465                             ScalarEvolution *SE, AssumptionCache *AC,
466                             bool PreserveLCSSA) {
467   bool Changed = false;
468 ReprocessLoop:
469 
470   // Check to see that no blocks (other than the header) in this loop have
471   // predecessors that are not in the loop.  This is not valid for natural
472   // loops, but can occur if the blocks are unreachable.  Since they are
473   // unreachable we can just shamelessly delete those CFG edges!
474   for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
475        BB != E; ++BB) {
476     if (*BB == L->getHeader()) continue;
477 
478     SmallPtrSet<BasicBlock*, 4> BadPreds;
479     for (pred_iterator PI = pred_begin(*BB),
480          PE = pred_end(*BB); PI != PE; ++PI) {
481       BasicBlock *P = *PI;
482       if (!L->contains(P))
483         BadPreds.insert(P);
484     }
485 
486     // Delete each unique out-of-loop (and thus dead) predecessor.
487     for (BasicBlock *P : BadPreds) {
488 
489       DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor "
490                    << P->getName() << "\n");
491 
492       // Inform each successor of each dead pred.
493       for (succ_iterator SI = succ_begin(P), SE = succ_end(P); SI != SE; ++SI)
494         (*SI)->removePredecessor(P);
495       // Zap the dead pred's terminator and replace it with unreachable.
496       TerminatorInst *TI = P->getTerminator();
497        TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
498       P->getTerminator()->eraseFromParent();
499       new UnreachableInst(P->getContext(), P);
500       Changed = true;
501     }
502   }
503 
504   // If there are exiting blocks with branches on undef, resolve the undef in
505   // the direction which will exit the loop. This will help simplify loop
506   // trip count computations.
507   SmallVector<BasicBlock*, 8> ExitingBlocks;
508   L->getExitingBlocks(ExitingBlocks);
509   for (SmallVectorImpl<BasicBlock *>::iterator I = ExitingBlocks.begin(),
510        E = ExitingBlocks.end(); I != E; ++I)
511     if (BranchInst *BI = dyn_cast<BranchInst>((*I)->getTerminator()))
512       if (BI->isConditional()) {
513         if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) {
514 
515           DEBUG(dbgs() << "LoopSimplify: Resolving \"br i1 undef\" to exit in "
516                        << (*I)->getName() << "\n");
517 
518           BI->setCondition(ConstantInt::get(Cond->getType(),
519                                             !L->contains(BI->getSuccessor(0))));
520 
521           // This may make the loop analyzable, force SCEV recomputation.
522           if (SE)
523             SE->forgetLoop(L);
524 
525           Changed = true;
526         }
527       }
528 
529   // Does the loop already have a preheader?  If so, don't insert one.
530   BasicBlock *Preheader = L->getLoopPreheader();
531   if (!Preheader) {
532     Preheader = InsertPreheaderForLoop(L, DT, LI, PreserveLCSSA);
533     if (Preheader) {
534       ++NumInserted;
535       Changed = true;
536     }
537   }
538 
539   // Next, check to make sure that all exit nodes of the loop only have
540   // predecessors that are inside of the loop.  This check guarantees that the
541   // loop preheader/header will dominate the exit blocks.  If the exit block has
542   // predecessors from outside of the loop, split the edge now.
543   SmallVector<BasicBlock*, 8> ExitBlocks;
544   L->getExitBlocks(ExitBlocks);
545 
546   SmallSetVector<BasicBlock *, 8> ExitBlockSet(ExitBlocks.begin(),
547                                                ExitBlocks.end());
548   for (SmallSetVector<BasicBlock *, 8>::iterator I = ExitBlockSet.begin(),
549          E = ExitBlockSet.end(); I != E; ++I) {
550     BasicBlock *ExitBlock = *I;
551     for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock);
552          PI != PE; ++PI)
553       // Must be exactly this loop: no subloops, parent loops, or non-loop preds
554       // allowed.
555       if (!L->contains(*PI)) {
556         if (rewriteLoopExitBlock(L, ExitBlock, DT, LI, PreserveLCSSA)) {
557           ++NumInserted;
558           Changed = true;
559         }
560         break;
561       }
562   }
563 
564   // If the header has more than two predecessors at this point (from the
565   // preheader and from multiple backedges), we must adjust the loop.
566   BasicBlock *LoopLatch = L->getLoopLatch();
567   if (!LoopLatch) {
568     // If this is really a nested loop, rip it out into a child loop.  Don't do
569     // this for loops with a giant number of backedges, just factor them into a
570     // common backedge instead.
571     if (L->getNumBackEdges() < 8) {
572       if (Loop *OuterL =
573               separateNestedLoop(L, Preheader, DT, LI, SE, PreserveLCSSA, AC)) {
574         ++NumNested;
575         // Enqueue the outer loop as it should be processed next in our
576         // depth-first nest walk.
577         Worklist.push_back(OuterL);
578 
579         // This is a big restructuring change, reprocess the whole loop.
580         Changed = true;
581         // GCC doesn't tail recursion eliminate this.
582         // FIXME: It isn't clear we can't rely on LLVM to TRE this.
583         goto ReprocessLoop;
584       }
585     }
586 
587     // If we either couldn't, or didn't want to, identify nesting of the loops,
588     // insert a new block that all backedges target, then make it jump to the
589     // loop header.
590     LoopLatch = insertUniqueBackedgeBlock(L, Preheader, DT, LI);
591     if (LoopLatch) {
592       ++NumInserted;
593       Changed = true;
594     }
595   }
596 
597   const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
598 
599   // Scan over the PHI nodes in the loop header.  Since they now have only two
600   // incoming values (the loop is canonicalized), we may have simplified the PHI
601   // down to 'X = phi [X, Y]', which should be replaced with 'Y'.
602   PHINode *PN;
603   for (BasicBlock::iterator I = L->getHeader()->begin();
604        (PN = dyn_cast<PHINode>(I++)); )
605     if (Value *V = SimplifyInstruction(PN, DL, nullptr, DT, AC)) {
606       if (SE) SE->forgetValue(PN);
607       PN->replaceAllUsesWith(V);
608       PN->eraseFromParent();
609     }
610 
611   // If this loop has multiple exits and the exits all go to the same
612   // block, attempt to merge the exits. This helps several passes, such
613   // as LoopRotation, which do not support loops with multiple exits.
614   // SimplifyCFG also does this (and this code uses the same utility
615   // function), however this code is loop-aware, where SimplifyCFG is
616   // not. That gives it the advantage of being able to hoist
617   // loop-invariant instructions out of the way to open up more
618   // opportunities, and the disadvantage of having the responsibility
619   // to preserve dominator information.
620   bool UniqueExit = true;
621   if (!ExitBlocks.empty())
622     for (unsigned i = 1, e = ExitBlocks.size(); i != e; ++i)
623       if (ExitBlocks[i] != ExitBlocks[0]) {
624         UniqueExit = false;
625         break;
626       }
627   if (UniqueExit) {
628     for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
629       BasicBlock *ExitingBlock = ExitingBlocks[i];
630       if (!ExitingBlock->getSinglePredecessor()) continue;
631       BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
632       if (!BI || !BI->isConditional()) continue;
633       CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
634       if (!CI || CI->getParent() != ExitingBlock) continue;
635 
636       // Attempt to hoist out all instructions except for the
637       // comparison and the branch.
638       bool AllInvariant = true;
639       bool AnyInvariant = false;
640       for (BasicBlock::iterator I = ExitingBlock->begin(); &*I != BI; ) {
641         Instruction *Inst = &*I++;
642         // Skip debug info intrinsics.
643         if (isa<DbgInfoIntrinsic>(Inst))
644           continue;
645         if (Inst == CI)
646           continue;
647         if (!L->makeLoopInvariant(Inst, AnyInvariant,
648                                   Preheader ? Preheader->getTerminator()
649                                             : nullptr)) {
650           AllInvariant = false;
651           break;
652         }
653       }
654       if (AnyInvariant) {
655         Changed = true;
656         // The loop disposition of all SCEV expressions that depend on any
657         // hoisted values have also changed.
658         if (SE)
659           SE->forgetLoopDispositions(L);
660       }
661       if (!AllInvariant) continue;
662 
663       // The block has now been cleared of all instructions except for
664       // a comparison and a conditional branch. SimplifyCFG may be able
665       // to fold it now.
666       if (!FoldBranchToCommonDest(BI))
667         continue;
668 
669       // Success. The block is now dead, so remove it from the loop,
670       // update the dominator tree and delete it.
671       DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block "
672                    << ExitingBlock->getName() << "\n");
673 
674       // Notify ScalarEvolution before deleting this block. Currently assume the
675       // parent loop doesn't change (spliting edges doesn't count). If blocks,
676       // CFG edges, or other values in the parent loop change, then we need call
677       // to forgetLoop() for the parent instead.
678       if (SE)
679         SE->forgetLoop(L);
680 
681       assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock));
682       Changed = true;
683       LI->removeBlock(ExitingBlock);
684 
685       DomTreeNode *Node = DT->getNode(ExitingBlock);
686       const std::vector<DomTreeNodeBase<BasicBlock> *> &Children =
687         Node->getChildren();
688       while (!Children.empty()) {
689         DomTreeNode *Child = Children.front();
690         DT->changeImmediateDominator(Child, Node->getIDom());
691       }
692       DT->eraseNode(ExitingBlock);
693 
694       BI->getSuccessor(0)->removePredecessor(ExitingBlock);
695       BI->getSuccessor(1)->removePredecessor(ExitingBlock);
696       ExitingBlock->eraseFromParent();
697     }
698   }
699 
700   return Changed;
701 }
702 
simplifyLoop(Loop * L,DominatorTree * DT,LoopInfo * LI,ScalarEvolution * SE,AssumptionCache * AC,bool PreserveLCSSA)703 bool llvm::simplifyLoop(Loop *L, DominatorTree *DT, LoopInfo *LI,
704                         ScalarEvolution *SE, AssumptionCache *AC,
705                         bool PreserveLCSSA) {
706   bool Changed = false;
707 
708   // Worklist maintains our depth-first queue of loops in this nest to process.
709   SmallVector<Loop *, 4> Worklist;
710   Worklist.push_back(L);
711 
712   // Walk the worklist from front to back, pushing newly found sub loops onto
713   // the back. This will let us process loops from back to front in depth-first
714   // order. We can use this simple process because loops form a tree.
715   for (unsigned Idx = 0; Idx != Worklist.size(); ++Idx) {
716     Loop *L2 = Worklist[Idx];
717     Worklist.append(L2->begin(), L2->end());
718   }
719 
720   while (!Worklist.empty())
721     Changed |= simplifyOneLoop(Worklist.pop_back_val(), Worklist, DT, LI, SE,
722                                AC, PreserveLCSSA);
723 
724   return Changed;
725 }
726 
727 namespace {
728   struct LoopSimplify : public FunctionPass {
729     static char ID; // Pass identification, replacement for typeid
LoopSimplify__anond4c09a7c0111::LoopSimplify730     LoopSimplify() : FunctionPass(ID) {
731       initializeLoopSimplifyPass(*PassRegistry::getPassRegistry());
732     }
733 
734     DominatorTree *DT;
735     LoopInfo *LI;
736     ScalarEvolution *SE;
737     AssumptionCache *AC;
738 
739     bool runOnFunction(Function &F) override;
740 
getAnalysisUsage__anond4c09a7c0111::LoopSimplify741     void getAnalysisUsage(AnalysisUsage &AU) const override {
742       AU.addRequired<AssumptionCacheTracker>();
743 
744       // We need loop information to identify the loops...
745       AU.addRequired<DominatorTreeWrapperPass>();
746       AU.addPreserved<DominatorTreeWrapperPass>();
747 
748       AU.addRequired<LoopInfoWrapperPass>();
749       AU.addPreserved<LoopInfoWrapperPass>();
750 
751       AU.addPreserved<BasicAAWrapperPass>();
752       AU.addPreserved<AAResultsWrapperPass>();
753       AU.addPreserved<GlobalsAAWrapperPass>();
754       AU.addPreserved<ScalarEvolutionWrapperPass>();
755       AU.addPreserved<SCEVAAWrapperPass>();
756       AU.addPreserved<DependenceAnalysis>();
757       AU.addPreservedID(BreakCriticalEdgesID);  // No critical edges added.
758     }
759 
760     /// verifyAnalysis() - Verify LoopSimplifyForm's guarantees.
761     void verifyAnalysis() const override;
762   };
763 }
764 
765 char LoopSimplify::ID = 0;
766 INITIALIZE_PASS_BEGIN(LoopSimplify, "loop-simplify",
767                 "Canonicalize natural loops", false, false)
768 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
769 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
770 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
771 INITIALIZE_PASS_DEPENDENCY(BasicAAWrapperPass)
772 INITIALIZE_PASS_DEPENDENCY(GlobalsAAWrapperPass)
773 INITIALIZE_PASS_DEPENDENCY(SCEVAAWrapperPass)
774 INITIALIZE_PASS_END(LoopSimplify, "loop-simplify",
775                 "Canonicalize natural loops", false, false)
776 
777 // Publicly exposed interface to pass...
778 char &llvm::LoopSimplifyID = LoopSimplify::ID;
createLoopSimplifyPass()779 Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
780 
781 /// runOnFunction - Run down all loops in the CFG (recursively, but we could do
782 /// it in any convenient order) inserting preheaders...
783 ///
runOnFunction(Function & F)784 bool LoopSimplify::runOnFunction(Function &F) {
785   bool Changed = false;
786   LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
787   DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
788   auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
789   SE = SEWP ? &SEWP->getSE() : nullptr;
790   AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
791   bool PreserveLCSSA = mustPreserveAnalysisID(LCSSAID);
792 
793   // Simplify each loop nest in the function.
794   for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
795     Changed |= simplifyLoop(*I, DT, LI, SE, AC, PreserveLCSSA);
796 
797   return Changed;
798 }
799 
800 // FIXME: Restore this code when we re-enable verification in verifyAnalysis
801 // below.
802 #if 0
803 static void verifyLoop(Loop *L) {
804   // Verify subloops.
805   for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
806     verifyLoop(*I);
807 
808   // It used to be possible to just assert L->isLoopSimplifyForm(), however
809   // with the introduction of indirectbr, there are now cases where it's
810   // not possible to transform a loop as necessary. We can at least check
811   // that there is an indirectbr near any time there's trouble.
812 
813   // Indirectbr can interfere with preheader and unique backedge insertion.
814   if (!L->getLoopPreheader() || !L->getLoopLatch()) {
815     bool HasIndBrPred = false;
816     for (pred_iterator PI = pred_begin(L->getHeader()),
817          PE = pred_end(L->getHeader()); PI != PE; ++PI)
818       if (isa<IndirectBrInst>((*PI)->getTerminator())) {
819         HasIndBrPred = true;
820         break;
821       }
822     assert(HasIndBrPred &&
823            "LoopSimplify has no excuse for missing loop header info!");
824     (void)HasIndBrPred;
825   }
826 
827   // Indirectbr can interfere with exit block canonicalization.
828   if (!L->hasDedicatedExits()) {
829     bool HasIndBrExiting = false;
830     SmallVector<BasicBlock*, 8> ExitingBlocks;
831     L->getExitingBlocks(ExitingBlocks);
832     for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
833       if (isa<IndirectBrInst>((ExitingBlocks[i])->getTerminator())) {
834         HasIndBrExiting = true;
835         break;
836       }
837     }
838 
839     assert(HasIndBrExiting &&
840            "LoopSimplify has no excuse for missing exit block info!");
841     (void)HasIndBrExiting;
842   }
843 }
844 #endif
845 
verifyAnalysis() const846 void LoopSimplify::verifyAnalysis() const {
847   // FIXME: This routine is being called mid-way through the loop pass manager
848   // as loop passes destroy this analysis. That's actually fine, but we have no
849   // way of expressing that here. Once all of the passes that destroy this are
850   // hoisted out of the loop pass manager we can add back verification here.
851 #if 0
852   for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
853     verifyLoop(*I);
854 #endif
855 }
856