LLVM  4.0.0
LoopUnroll.cpp
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1 //===-- UnrollLoop.cpp - Loop unrolling utilities -------------------------===//
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 implements some loop unrolling utilities. It does not define any
11 // actual pass or policy, but provides a single function to perform loop
12 // unrolling.
13 //
14 // The process of unrolling can produce extraneous basic blocks linked with
15 // unconditional branches. This will be corrected in the future.
16 //
17 //===----------------------------------------------------------------------===//
18 
20 #include "llvm/ADT/SmallPtrSet.h"
21 #include "llvm/ADT/Statistic.h"
25 #include "llvm/Analysis/LoopPass.h"
28 #include "llvm/IR/BasicBlock.h"
29 #include "llvm/IR/DataLayout.h"
30 #include "llvm/IR/Dominators.h"
31 #include "llvm/IR/IntrinsicInst.h"
32 #include "llvm/IR/LLVMContext.h"
33 #include "llvm/Support/Debug.h"
41 using namespace llvm;
42 
43 #define DEBUG_TYPE "loop-unroll"
44 
45 // TODO: Should these be here or in LoopUnroll?
46 STATISTIC(NumCompletelyUnrolled, "Number of loops completely unrolled");
47 STATISTIC(NumUnrolled, "Number of loops unrolled (completely or otherwise)");
48 
49 static cl::opt<bool>
50 UnrollRuntimeEpilog("unroll-runtime-epilog", cl::init(false), cl::Hidden,
51  cl::desc("Allow runtime unrolled loops to be unrolled "
52  "with epilog instead of prolog."));
53 
54 /// Convert the instruction operands from referencing the current values into
55 /// those specified by VMap.
56 static inline void remapInstruction(Instruction *I,
57  ValueToValueMapTy &VMap) {
58  for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
59  Value *Op = I->getOperand(op);
60  ValueToValueMapTy::iterator It = VMap.find(Op);
61  if (It != VMap.end())
62  I->setOperand(op, It->second);
63  }
64 
65  if (PHINode *PN = dyn_cast<PHINode>(I)) {
66  for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
67  ValueToValueMapTy::iterator It = VMap.find(PN->getIncomingBlock(i));
68  if (It != VMap.end())
69  PN->setIncomingBlock(i, cast<BasicBlock>(It->second));
70  }
71  }
72 }
73 
74 /// Folds a basic block into its predecessor if it only has one predecessor, and
75 /// that predecessor only has one successor.
76 /// The LoopInfo Analysis that is passed will be kept consistent. If folding is
77 /// successful references to the containing loop must be removed from
78 /// ScalarEvolution by calling ScalarEvolution::forgetLoop because SE may have
79 /// references to the eliminated BB. The argument ForgottenLoops contains a set
80 /// of loops that have already been forgotten to prevent redundant, expensive
81 /// calls to ScalarEvolution::forgetLoop. Returns the new combined block.
82 static BasicBlock *
84  SmallPtrSetImpl<Loop *> &ForgottenLoops,
85  DominatorTree *DT) {
86  // Merge basic blocks into their predecessor if there is only one distinct
87  // pred, and if there is only one distinct successor of the predecessor, and
88  // if there are no PHI nodes.
89  BasicBlock *OnlyPred = BB->getSinglePredecessor();
90  if (!OnlyPred) return nullptr;
91 
92  if (OnlyPred->getTerminator()->getNumSuccessors() != 1)
93  return nullptr;
94 
95  DEBUG(dbgs() << "Merging: " << *BB << "into: " << *OnlyPred);
96 
97  // Resolve any PHI nodes at the start of the block. They are all
98  // guaranteed to have exactly one entry if they exist, unless there are
99  // multiple duplicate (but guaranteed to be equal) entries for the
100  // incoming edges. This occurs when there are multiple edges from
101  // OnlyPred to OnlySucc.
103 
104  // Delete the unconditional branch from the predecessor...
105  OnlyPred->getInstList().pop_back();
106 
107  // Make all PHI nodes that referred to BB now refer to Pred as their
108  // source...
109  BB->replaceAllUsesWith(OnlyPred);
110 
111  // Move all definitions in the successor to the predecessor...
112  OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
113 
114  // OldName will be valid until erased.
115  StringRef OldName = BB->getName();
116 
117  // Erase the old block and update dominator info.
118  if (DT)
119  if (DomTreeNode *DTN = DT->getNode(BB)) {
120  DomTreeNode *PredDTN = DT->getNode(OnlyPred);
121  SmallVector<DomTreeNode *, 8> Children(DTN->begin(), DTN->end());
122  for (auto *DI : Children)
123  DT->changeImmediateDominator(DI, PredDTN);
124 
125  DT->eraseNode(BB);
126  }
127 
128  // ScalarEvolution holds references to loop exit blocks.
129  if (SE) {
130  if (Loop *L = LI->getLoopFor(BB)) {
131  if (ForgottenLoops.insert(L).second)
132  SE->forgetLoop(L);
133  }
134  }
135  LI->removeBlock(BB);
136 
137  // Inherit predecessor's name if it exists...
138  if (!OldName.empty() && !OnlyPred->hasName())
139  OnlyPred->setName(OldName);
140 
141  BB->eraseFromParent();
142 
143  return OnlyPred;
144 }
145 
146 /// Check if unrolling created a situation where we need to insert phi nodes to
147 /// preserve LCSSA form.
148 /// \param Blocks is a vector of basic blocks representing unrolled loop.
149 /// \param L is the outer loop.
150 /// It's possible that some of the blocks are in L, and some are not. In this
151 /// case, if there is a use is outside L, and definition is inside L, we need to
152 /// insert a phi-node, otherwise LCSSA will be broken.
153 /// The function is just a helper function for llvm::UnrollLoop that returns
154 /// true if this situation occurs, indicating that LCSSA needs to be fixed.
155 static bool needToInsertPhisForLCSSA(Loop *L, std::vector<BasicBlock *> Blocks,
156  LoopInfo *LI) {
157  for (BasicBlock *BB : Blocks) {
158  if (LI->getLoopFor(BB) == L)
159  continue;
160  for (Instruction &I : *BB) {
161  for (Use &U : I.operands()) {
162  if (auto Def = dyn_cast<Instruction>(U)) {
163  Loop *DefLoop = LI->getLoopFor(Def->getParent());
164  if (!DefLoop)
165  continue;
166  if (DefLoop->contains(L))
167  return true;
168  }
169  }
170  }
171  }
172  return false;
173 }
174 
175 /// Adds ClonedBB to LoopInfo, creates a new loop for ClonedBB if necessary
176 /// and adds a mapping from the original loop to the new loop to NewLoops.
177 /// Returns nullptr if no new loop was created and a pointer to the
178 /// original loop OriginalBB was part of otherwise.
180  BasicBlock *ClonedBB, LoopInfo *LI,
181  NewLoopsMap &NewLoops) {
182  // Figure out which loop New is in.
183  const Loop *OldLoop = LI->getLoopFor(OriginalBB);
184  assert(OldLoop && "Should (at least) be in the loop being unrolled!");
185 
186  Loop *&NewLoop = NewLoops[OldLoop];
187  if (!NewLoop) {
188  // Found a new sub-loop.
189  assert(OriginalBB == OldLoop->getHeader() &&
190  "Header should be first in RPO");
191 
192  NewLoop = new Loop();
193  Loop *NewLoopParent = NewLoops.lookup(OldLoop->getParentLoop());
194 
195  if (NewLoopParent)
196  NewLoopParent->addChildLoop(NewLoop);
197  else
198  LI->addTopLevelLoop(NewLoop);
199 
200  NewLoop->addBasicBlockToLoop(ClonedBB, *LI);
201  return OldLoop;
202  } else {
203  NewLoop->addBasicBlockToLoop(ClonedBB, *LI);
204  return nullptr;
205  }
206 }
207 
208 /// Unroll the given loop by Count. The loop must be in LCSSA form. Returns true
209 /// if unrolling was successful, or false if the loop was unmodified. Unrolling
210 /// can only fail when the loop's latch block is not terminated by a conditional
211 /// branch instruction. However, if the trip count (and multiple) are not known,
212 /// loop unrolling will mostly produce more code that is no faster.
213 ///
214 /// TripCount is the upper bound of the iteration on which control exits
215 /// LatchBlock. Control may exit the loop prior to TripCount iterations either
216 /// via an early branch in other loop block or via LatchBlock terminator. This
217 /// is relaxed from the general definition of trip count which is the number of
218 /// times the loop header executes. Note that UnrollLoop assumes that the loop
219 /// counter test is in LatchBlock in order to remove unnecesssary instances of
220 /// the test. If control can exit the loop from the LatchBlock's terminator
221 /// prior to TripCount iterations, flag PreserveCondBr needs to be set.
222 ///
223 /// PreserveCondBr indicates whether the conditional branch of the LatchBlock
224 /// needs to be preserved. It is needed when we use trip count upper bound to
225 /// fully unroll the loop. If PreserveOnlyFirst is also set then only the first
226 /// conditional branch needs to be preserved.
227 ///
228 /// Similarly, TripMultiple divides the number of times that the LatchBlock may
229 /// execute without exiting the loop.
230 ///
231 /// If AllowRuntime is true then UnrollLoop will consider unrolling loops that
232 /// have a runtime (i.e. not compile time constant) trip count. Unrolling these
233 /// loops require a unroll "prologue" that runs "RuntimeTripCount % Count"
234 /// iterations before branching into the unrolled loop. UnrollLoop will not
235 /// runtime-unroll the loop if computing RuntimeTripCount will be expensive and
236 /// AllowExpensiveTripCount is false.
237 ///
238 /// If we want to perform PGO-based loop peeling, PeelCount is set to the
239 /// number of iterations we want to peel off.
240 ///
241 /// The LoopInfo Analysis that is passed will be kept consistent.
242 ///
243 /// This utility preserves LoopInfo. It will also preserve ScalarEvolution and
244 /// DominatorTree if they are non-null.
245 bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount, bool Force,
246  bool AllowRuntime, bool AllowExpensiveTripCount,
247  bool PreserveCondBr, bool PreserveOnlyFirst,
248  unsigned TripMultiple, unsigned PeelCount, LoopInfo *LI,
251  bool PreserveLCSSA) {
252 
253  BasicBlock *Preheader = L->getLoopPreheader();
254  if (!Preheader) {
255  DEBUG(dbgs() << " Can't unroll; loop preheader-insertion failed.\n");
256  return false;
257  }
258 
259  BasicBlock *LatchBlock = L->getLoopLatch();
260  if (!LatchBlock) {
261  DEBUG(dbgs() << " Can't unroll; loop exit-block-insertion failed.\n");
262  return false;
263  }
264 
265  // Loops with indirectbr cannot be cloned.
266  if (!L->isSafeToClone()) {
267  DEBUG(dbgs() << " Can't unroll; Loop body cannot be cloned.\n");
268  return false;
269  }
270 
271  BasicBlock *Header = L->getHeader();
272  BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator());
273 
274  if (!BI || BI->isUnconditional()) {
275  // The loop-rotate pass can be helpful to avoid this in many cases.
276  DEBUG(dbgs() <<
277  " Can't unroll; loop not terminated by a conditional branch.\n");
278  return false;
279  }
280 
281  if (Header->hasAddressTaken()) {
282  // The loop-rotate pass can be helpful to avoid this in many cases.
283  DEBUG(dbgs() <<
284  " Won't unroll loop: address of header block is taken.\n");
285  return false;
286  }
287 
288  if (TripCount != 0)
289  DEBUG(dbgs() << " Trip Count = " << TripCount << "\n");
290  if (TripMultiple != 1)
291  DEBUG(dbgs() << " Trip Multiple = " << TripMultiple << "\n");
292 
293  // Effectively "DCE" unrolled iterations that are beyond the tripcount
294  // and will never be executed.
295  if (TripCount != 0 && Count > TripCount)
296  Count = TripCount;
297 
298  // Don't enter the unroll code if there is nothing to do.
299  if (TripCount == 0 && Count < 2 && PeelCount == 0)
300  return false;
301 
302  assert(Count > 0);
303  assert(TripMultiple > 0);
304  assert(TripCount == 0 || TripCount % TripMultiple == 0);
305 
306  // Are we eliminating the loop control altogether?
307  bool CompletelyUnroll = Count == TripCount;
308  SmallVector<BasicBlock *, 4> ExitBlocks;
309  L->getExitBlocks(ExitBlocks);
310  std::vector<BasicBlock*> OriginalLoopBlocks = L->getBlocks();
311 
312  // Go through all exits of L and see if there are any phi-nodes there. We just
313  // conservatively assume that they're inserted to preserve LCSSA form, which
314  // means that complete unrolling might break this form. We need to either fix
315  // it in-place after the transformation, or entirely rebuild LCSSA. TODO: For
316  // now we just recompute LCSSA for the outer loop, but it should be possible
317  // to fix it in-place.
318  bool NeedToFixLCSSA = PreserveLCSSA && CompletelyUnroll &&
319  any_of(ExitBlocks, [](const BasicBlock *BB) {
320  return isa<PHINode>(BB->begin());
321  });
322 
323  // We assume a run-time trip count if the compiler cannot
324  // figure out the loop trip count and the unroll-runtime
325  // flag is specified.
326  bool RuntimeTripCount = (TripCount == 0 && Count > 0 && AllowRuntime);
327 
328  assert((!RuntimeTripCount || !PeelCount) &&
329  "Did not expect runtime trip-count unrolling "
330  "and peeling for the same loop");
331 
332  if (PeelCount)
333  peelLoop(L, PeelCount, LI, SE, DT, PreserveLCSSA);
334 
335  // Loops containing convergent instructions must have a count that divides
336  // their TripMultiple.
337  DEBUG(
338  {
339  bool HasConvergent = false;
340  for (auto &BB : L->blocks())
341  for (auto &I : *BB)
342  if (auto CS = CallSite(&I))
343  HasConvergent |= CS.isConvergent();
344  assert((!HasConvergent || TripMultiple % Count == 0) &&
345  "Unroll count must divide trip multiple if loop contains a "
346  "convergent operation.");
347  });
348 
349  if (RuntimeTripCount && TripMultiple % Count != 0 &&
350  !UnrollRuntimeLoopRemainder(L, Count, AllowExpensiveTripCount,
351  UnrollRuntimeEpilog, LI, SE, DT,
352  PreserveLCSSA)) {
353  if (Force)
354  RuntimeTripCount = false;
355  else
356  return false;
357  }
358 
359  // Notify ScalarEvolution that the loop will be substantially changed,
360  // if not outright eliminated.
361  if (SE)
362  SE->forgetLoop(L);
363 
364  // If we know the trip count, we know the multiple...
365  unsigned BreakoutTrip = 0;
366  if (TripCount != 0) {
367  BreakoutTrip = TripCount % Count;
368  TripMultiple = 0;
369  } else {
370  // Figure out what multiple to use.
371  BreakoutTrip = TripMultiple =
372  (unsigned)GreatestCommonDivisor64(Count, TripMultiple);
373  }
374 
375  using namespace ore;
376  // Report the unrolling decision.
377  if (CompletelyUnroll) {
378  DEBUG(dbgs() << "COMPLETELY UNROLLING loop %" << Header->getName()
379  << " with trip count " << TripCount << "!\n");
380  ORE->emit(OptimizationRemark(DEBUG_TYPE, "FullyUnrolled", L->getStartLoc(),
381  L->getHeader())
382  << "completely unrolled loop with "
383  << NV("UnrollCount", TripCount) << " iterations");
384  } else if (PeelCount) {
385  DEBUG(dbgs() << "PEELING loop %" << Header->getName()
386  << " with iteration count " << PeelCount << "!\n");
387  ORE->emit(OptimizationRemark(DEBUG_TYPE, "Peeled", L->getStartLoc(),
388  L->getHeader())
389  << " peeled loop by " << NV("PeelCount", PeelCount)
390  << " iterations");
391  } else {
392  OptimizationRemark Diag(DEBUG_TYPE, "PartialUnrolled", L->getStartLoc(),
393  L->getHeader());
394  Diag << "unrolled loop by a factor of " << NV("UnrollCount", Count);
395 
396  DEBUG(dbgs() << "UNROLLING loop %" << Header->getName()
397  << " by " << Count);
398  if (TripMultiple == 0 || BreakoutTrip != TripMultiple) {
399  DEBUG(dbgs() << " with a breakout at trip " << BreakoutTrip);
400  ORE->emit(Diag << " with a breakout at trip "
401  << NV("BreakoutTrip", BreakoutTrip));
402  } else if (TripMultiple != 1) {
403  DEBUG(dbgs() << " with " << TripMultiple << " trips per branch");
404  ORE->emit(Diag << " with " << NV("TripMultiple", TripMultiple)
405  << " trips per branch");
406  } else if (RuntimeTripCount) {
407  DEBUG(dbgs() << " with run-time trip count");
408  ORE->emit(Diag << " with run-time trip count");
409  }
410  DEBUG(dbgs() << "!\n");
411  }
412 
413  bool ContinueOnTrue = L->contains(BI->getSuccessor(0));
414  BasicBlock *LoopExit = BI->getSuccessor(ContinueOnTrue);
415 
416  // For the first iteration of the loop, we should use the precloned values for
417  // PHI nodes. Insert associations now.
418  ValueToValueMapTy LastValueMap;
419  std::vector<PHINode*> OrigPHINode;
420  for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
421  OrigPHINode.push_back(cast<PHINode>(I));
422  }
423 
424  std::vector<BasicBlock*> Headers;
425  std::vector<BasicBlock*> Latches;
426  Headers.push_back(Header);
427  Latches.push_back(LatchBlock);
428 
429  // The current on-the-fly SSA update requires blocks to be processed in
430  // reverse postorder so that LastValueMap contains the correct value at each
431  // exit.
432  LoopBlocksDFS DFS(L);
433  DFS.perform(LI);
434 
435  // Stash the DFS iterators before adding blocks to the loop.
436  LoopBlocksDFS::RPOIterator BlockBegin = DFS.beginRPO();
437  LoopBlocksDFS::RPOIterator BlockEnd = DFS.endRPO();
438 
439  std::vector<BasicBlock*> UnrolledLoopBlocks = L->getBlocks();
440 
441  // Loop Unrolling might create new loops. While we do preserve LoopInfo, we
442  // might break loop-simplified form for these loops (as they, e.g., would
443  // share the same exit blocks). We'll keep track of loops for which we can
444  // break this so that later we can re-simplify them.
445  SmallSetVector<Loop *, 4> LoopsToSimplify;
446  for (Loop *SubLoop : *L)
447  LoopsToSimplify.insert(SubLoop);
448 
449  for (unsigned It = 1; It != Count; ++It) {
450  std::vector<BasicBlock*> NewBlocks;
452  NewLoops[L] = L;
453 
454  for (LoopBlocksDFS::RPOIterator BB = BlockBegin; BB != BlockEnd; ++BB) {
455  ValueToValueMapTy VMap;
456  BasicBlock *New = CloneBasicBlock(*BB, VMap, "." + Twine(It));
457  Header->getParent()->getBasicBlockList().push_back(New);
458 
459  // Tell LI about New.
460  if (*BB == Header) {
461  assert(LI->getLoopFor(*BB) == L && "Header should not be in a sub-loop");
462  L->addBasicBlockToLoop(New, *LI);
463  } else {
464  const Loop *OldLoop = addClonedBlockToLoopInfo(*BB, New, LI, NewLoops);
465  if (OldLoop) {
466  LoopsToSimplify.insert(NewLoops[OldLoop]);
467 
468  // Forget the old loop, since its inputs may have changed.
469  if (SE)
470  SE->forgetLoop(OldLoop);
471  }
472  }
473 
474  if (*BB == Header)
475  // Loop over all of the PHI nodes in the block, changing them to use
476  // the incoming values from the previous block.
477  for (PHINode *OrigPHI : OrigPHINode) {
478  PHINode *NewPHI = cast<PHINode>(VMap[OrigPHI]);
479  Value *InVal = NewPHI->getIncomingValueForBlock(LatchBlock);
480  if (Instruction *InValI = dyn_cast<Instruction>(InVal))
481  if (It > 1 && L->contains(InValI))
482  InVal = LastValueMap[InValI];
483  VMap[OrigPHI] = InVal;
484  New->getInstList().erase(NewPHI);
485  }
486 
487  // Update our running map of newest clones
488  LastValueMap[*BB] = New;
489  for (ValueToValueMapTy::iterator VI = VMap.begin(), VE = VMap.end();
490  VI != VE; ++VI)
491  LastValueMap[VI->first] = VI->second;
492 
493  // Add phi entries for newly created values to all exit blocks.
494  for (BasicBlock *Succ : successors(*BB)) {
495  if (L->contains(Succ))
496  continue;
497  for (BasicBlock::iterator BBI = Succ->begin();
498  PHINode *phi = dyn_cast<PHINode>(BBI); ++BBI) {
499  Value *Incoming = phi->getIncomingValueForBlock(*BB);
500  ValueToValueMapTy::iterator It = LastValueMap.find(Incoming);
501  if (It != LastValueMap.end())
502  Incoming = It->second;
503  phi->addIncoming(Incoming, New);
504  }
505  }
506  // Keep track of new headers and latches as we create them, so that
507  // we can insert the proper branches later.
508  if (*BB == Header)
509  Headers.push_back(New);
510  if (*BB == LatchBlock)
511  Latches.push_back(New);
512 
513  NewBlocks.push_back(New);
514  UnrolledLoopBlocks.push_back(New);
515 
516  // Update DomTree: since we just copy the loop body, and each copy has a
517  // dedicated entry block (copy of the header block), this header's copy
518  // dominates all copied blocks. That means, dominance relations in the
519  // copied body are the same as in the original body.
520  if (DT) {
521  if (*BB == Header)
522  DT->addNewBlock(New, Latches[It - 1]);
523  else {
524  auto BBDomNode = DT->getNode(*BB);
525  auto BBIDom = BBDomNode->getIDom();
526  BasicBlock *OriginalBBIDom = BBIDom->getBlock();
527  DT->addNewBlock(
528  New, cast<BasicBlock>(LastValueMap[cast<Value>(OriginalBBIDom)]));
529  }
530  }
531  }
532 
533  // Remap all instructions in the most recent iteration
534  for (BasicBlock *NewBlock : NewBlocks) {
535  for (Instruction &I : *NewBlock) {
536  ::remapInstruction(&I, LastValueMap);
537  if (auto *II = dyn_cast<IntrinsicInst>(&I))
538  if (II->getIntrinsicID() == Intrinsic::assume)
539  AC->registerAssumption(II);
540  }
541  }
542  }
543 
544  // Loop over the PHI nodes in the original block, setting incoming values.
545  for (PHINode *PN : OrigPHINode) {
546  if (CompletelyUnroll) {
547  PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader));
548  Header->getInstList().erase(PN);
549  }
550  else if (Count > 1) {
551  Value *InVal = PN->removeIncomingValue(LatchBlock, false);
552  // If this value was defined in the loop, take the value defined by the
553  // last iteration of the loop.
554  if (Instruction *InValI = dyn_cast<Instruction>(InVal)) {
555  if (L->contains(InValI))
556  InVal = LastValueMap[InVal];
557  }
558  assert(Latches.back() == LastValueMap[LatchBlock] && "bad last latch");
559  PN->addIncoming(InVal, Latches.back());
560  }
561  }
562 
563  // Now that all the basic blocks for the unrolled iterations are in place,
564  // set up the branches to connect them.
565  for (unsigned i = 0, e = Latches.size(); i != e; ++i) {
566  // The original branch was replicated in each unrolled iteration.
567  BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator());
568 
569  // The branch destination.
570  unsigned j = (i + 1) % e;
571  BasicBlock *Dest = Headers[j];
572  bool NeedConditional = true;
573 
574  if (RuntimeTripCount && j != 0) {
575  NeedConditional = false;
576  }
577 
578  // For a complete unroll, make the last iteration end with a branch
579  // to the exit block.
580  if (CompletelyUnroll) {
581  if (j == 0)
582  Dest = LoopExit;
583  // If using trip count upper bound to completely unroll, we need to keep
584  // the conditional branch except the last one because the loop may exit
585  // after any iteration.
586  assert(NeedConditional &&
587  "NeedCondition cannot be modified by both complete "
588  "unrolling and runtime unrolling");
589  NeedConditional = (PreserveCondBr && j && !(PreserveOnlyFirst && i != 0));
590  } else if (j != BreakoutTrip && (TripMultiple == 0 || j % TripMultiple != 0)) {
591  // If we know the trip count or a multiple of it, we can safely use an
592  // unconditional branch for some iterations.
593  NeedConditional = false;
594  }
595 
596  if (NeedConditional) {
597  // Update the conditional branch's successor for the following
598  // iteration.
599  Term->setSuccessor(!ContinueOnTrue, Dest);
600  } else {
601  // Remove phi operands at this loop exit
602  if (Dest != LoopExit) {
603  BasicBlock *BB = Latches[i];
604  for (BasicBlock *Succ: successors(BB)) {
605  if (Succ == Headers[i])
606  continue;
607  for (BasicBlock::iterator BBI = Succ->begin();
608  PHINode *Phi = dyn_cast<PHINode>(BBI); ++BBI) {
609  Phi->removeIncomingValue(BB, false);
610  }
611  }
612  }
613  // Replace the conditional branch with an unconditional one.
614  BranchInst::Create(Dest, Term);
615  Term->eraseFromParent();
616  }
617  }
618  // Update dominators of blocks we might reach through exits.
619  // Immediate dominator of such block might change, because we add more
620  // routes which can lead to the exit: we can now reach it from the copied
621  // iterations too. Thus, the new idom of the block will be the nearest
622  // common dominator of the previous idom and common dominator of all copies of
623  // the previous idom. This is equivalent to the nearest common dominator of
624  // the previous idom and the first latch, which dominates all copies of the
625  // previous idom.
626  if (DT && Count > 1) {
627  for (auto *BB : OriginalLoopBlocks) {
628  auto *BBDomNode = DT->getNode(BB);
629  SmallVector<BasicBlock *, 16> ChildrenToUpdate;
630  for (auto *ChildDomNode : BBDomNode->getChildren()) {
631  auto *ChildBB = ChildDomNode->getBlock();
632  if (!L->contains(ChildBB))
633  ChildrenToUpdate.push_back(ChildBB);
634  }
635  BasicBlock *NewIDom = DT->findNearestCommonDominator(BB, Latches[0]);
636  for (auto *ChildBB : ChildrenToUpdate)
637  DT->changeImmediateDominator(ChildBB, NewIDom);
638  }
639  }
640 
641  // Merge adjacent basic blocks, if possible.
642  SmallPtrSet<Loop *, 4> ForgottenLoops;
643  for (BasicBlock *Latch : Latches) {
644  BranchInst *Term = cast<BranchInst>(Latch->getTerminator());
645  if (Term->isUnconditional()) {
646  BasicBlock *Dest = Term->getSuccessor(0);
647  if (BasicBlock *Fold =
648  foldBlockIntoPredecessor(Dest, LI, SE, ForgottenLoops, DT)) {
649  // Dest has been folded into Fold. Update our worklists accordingly.
650  std::replace(Latches.begin(), Latches.end(), Dest, Fold);
651  UnrolledLoopBlocks.erase(std::remove(UnrolledLoopBlocks.begin(),
652  UnrolledLoopBlocks.end(), Dest),
653  UnrolledLoopBlocks.end());
654  }
655  }
656  }
657 
658  // FIXME: We only preserve DT info for complete unrolling now. Incrementally
659  // updating domtree after partial loop unrolling should also be easy.
660  if (DT && !CompletelyUnroll)
661  DT->recalculate(*L->getHeader()->getParent());
662  else if (DT)
663  DEBUG(DT->verifyDomTree());
664 
665  // Simplify any new induction variables in the partially unrolled loop.
666  if (SE && !CompletelyUnroll && Count > 1) {
667  SmallVector<WeakVH, 16> DeadInsts;
668  simplifyLoopIVs(L, SE, DT, LI, DeadInsts);
669 
670  // Aggressively clean up dead instructions that simplifyLoopIVs already
671  // identified. Any remaining should be cleaned up below.
672  while (!DeadInsts.empty())
673  if (Instruction *Inst =
674  dyn_cast_or_null<Instruction>(&*DeadInsts.pop_back_val()))
676  }
677 
678  // At this point, the code is well formed. We now do a quick sweep over the
679  // inserted code, doing constant propagation and dead code elimination as we
680  // go.
681  const DataLayout &DL = Header->getModule()->getDataLayout();
682  const std::vector<BasicBlock*> &NewLoopBlocks = L->getBlocks();
683  for (BasicBlock *BB : NewLoopBlocks) {
684  for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
685  Instruction *Inst = &*I++;
686 
687  if (Value *V = SimplifyInstruction(Inst, DL))
688  if (LI->replacementPreservesLCSSAForm(Inst, V))
689  Inst->replaceAllUsesWith(V);
690  if (isInstructionTriviallyDead(Inst))
691  BB->getInstList().erase(Inst);
692  }
693  }
694 
695  // TODO: after peeling or unrolling, previously loop variant conditions are
696  // likely to fold to constants, eagerly propagating those here will require
697  // fewer cleanup passes to be run. Alternatively, a LoopEarlyCSE might be
698  // appropriate.
699 
700  NumCompletelyUnrolled += CompletelyUnroll;
701  ++NumUnrolled;
702 
703  Loop *OuterL = L->getParentLoop();
704  // Update LoopInfo if the loop is completely removed.
705  if (CompletelyUnroll)
706  LI->markAsRemoved(L);
707 
708  // After complete unrolling most of the blocks should be contained in OuterL.
709  // However, some of them might happen to be out of OuterL (e.g. if they
710  // precede a loop exit). In this case we might need to insert PHI nodes in
711  // order to preserve LCSSA form.
712  // We don't need to check this if we already know that we need to fix LCSSA
713  // form.
714  // TODO: For now we just recompute LCSSA for the outer loop in this case, but
715  // it should be possible to fix it in-place.
716  if (PreserveLCSSA && OuterL && CompletelyUnroll && !NeedToFixLCSSA)
717  NeedToFixLCSSA |= ::needToInsertPhisForLCSSA(OuterL, UnrolledLoopBlocks, LI);
718 
719  // If we have a pass and a DominatorTree we should re-simplify impacted loops
720  // to ensure subsequent analyses can rely on this form. We want to simplify
721  // at least one layer outside of the loop that was unrolled so that any
722  // changes to the parent loop exposed by the unrolling are considered.
723  if (DT) {
724  if (!OuterL && !CompletelyUnroll)
725  OuterL = L;
726  if (OuterL) {
727  // OuterL includes all loops for which we can break loop-simplify, so
728  // it's sufficient to simplify only it (it'll recursively simplify inner
729  // loops too).
730  // TODO: That potentially might be compile-time expensive. We should try
731  // to fix the loop-simplified form incrementally.
732  simplifyLoop(OuterL, DT, LI, SE, AC, PreserveLCSSA);
733 
734  // LCSSA must be performed on the outermost affected loop. The unrolled
735  // loop's last loop latch is guaranteed to be in the outermost loop after
736  // LoopInfo's been updated by markAsRemoved.
737  Loop *LatchLoop = LI->getLoopFor(Latches.back());
738  if (!OuterL->contains(LatchLoop))
739  while (OuterL->getParentLoop() != LatchLoop)
740  OuterL = OuterL->getParentLoop();
741 
742  if (NeedToFixLCSSA)
743  formLCSSARecursively(*OuterL, *DT, LI, SE);
744  else
745  assert(OuterL->isLCSSAForm(*DT) &&
746  "Loops should be in LCSSA form after loop-unroll.");
747  } else {
748  // Simplify loops for which we might've broken loop-simplify form.
749  for (Loop *SubLoop : LoopsToSimplify)
750  simplifyLoop(SubLoop, DT, LI, SE, AC, PreserveLCSSA);
751  }
752  }
753 
754  return true;
755 }
756 
757 /// Given an llvm.loop loop id metadata node, returns the loop hint metadata
758 /// node with the given name (for example, "llvm.loop.unroll.count"). If no
759 /// such metadata node exists, then nullptr is returned.
761  // First operand should refer to the loop id itself.
762  assert(LoopID->getNumOperands() > 0 && "requires at least one operand");
763  assert(LoopID->getOperand(0) == LoopID && "invalid loop id");
764 
765  for (unsigned i = 1, e = LoopID->getNumOperands(); i < e; ++i) {
766  MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(i));
767  if (!MD)
768  continue;
769 
770  MDString *S = dyn_cast<MDString>(MD->getOperand(0));
771  if (!S)
772  continue;
773 
774  if (Name.equals(S->getString()))
775  return MD;
776  }
777  return nullptr;
778 }
MachineLoop * L
DomTreeNodeBase< NodeT > * addNewBlock(NodeT *BB, NodeT *DomBB)
Add a new node to the dominator tree information.
SymbolTableList< Instruction >::iterator eraseFromParent()
This method unlinks 'this' from the containing basic block and deletes it.
Definition: Instruction.cpp:76
A parsed version of the target data layout string in and methods for querying it. ...
Definition: DataLayout.h:102
void addIncoming(Value *V, BasicBlock *BB)
Add an incoming value to the end of the PHI list.
DiagnosticInfoOptimizationBase::Argument NV
void removeBlock(BlockT *BB)
This method completely removes BB from all data structures, including all of the Loop objects it is n...
Definition: LoopInfo.h:637
uint64_t GreatestCommonDivisor64(uint64_t A, uint64_t B)
GreatestCommonDivisor64 - Return the greatest common divisor of the two values using Euclid's algorit...
Definition: MathExtras.h:538
bool hasName() const
Definition: Value.h:236
STATISTIC(NumFunctions,"Total number of functions")
iterator erase(iterator where)
Definition: ilist.h:280
static void remapInstruction(Instruction *I, ValueToValueMapTy &VMap)
Convert the instruction operands from referencing the current values into those specified by VMap...
Definition: LoopUnroll.cpp:56
size_t i
ValueT lookup(const KeyT &Val) const
lookup - Return the entry for the specified key, or a default constructed value if no such entry exis...
Definition: DenseMap.h:162
std::error_code remove(const Twine &path, bool IgnoreNonExisting=true)
Remove path.
DebugLoc getStartLoc() const
Return the debug location of the start of this loop.
Definition: LoopInfo.cpp:307
unsigned getNumOperands() const
Definition: User.h:167
unsigned getNumOperands() const
Return number of MDNode operands.
Definition: Metadata.h:1040
static bool needToInsertPhisForLCSSA(Loop *L, std::vector< BasicBlock * > Blocks, LoopInfo *LI)
Check if unrolling created a situation where we need to insert phi nodes to preserve LCSSA form...
Definition: LoopUnroll.cpp:155
The main scalar evolution driver.
A cache of .assume calls within a function.
LoopT * getParentLoop() const
Definition: LoopInfo.h:103
const Function * getParent() const
Return the enclosing method, or null if none.
Definition: BasicBlock.h:100
Metadata node.
Definition: Metadata.h:830
LoopT * getLoopFor(const BlockT *BB) const
Return the inner most loop that BB lives in.
Definition: LoopInfo.h:575
#define op(i)
const std::vector< BlockT * > & getBlocks() const
Get a list of the basic blocks which make up this loop.
Definition: LoopInfo.h:139
BlockT * getHeader() const
Definition: LoopInfo.h:102
bool UnrollLoop(Loop *L, unsigned Count, unsigned TripCount, bool Force, bool AllowRuntime, bool AllowExpensiveTripCount, bool PreserveCondBr, bool PreserveOnlyFirst, unsigned TripMultiple, unsigned PeelCount, LoopInfo *LI, ScalarEvolution *SE, DominatorTree *DT, AssumptionCache *AC, OptimizationRemarkEmitter *ORE, bool PreserveLCSSA)
Unroll the given loop by Count.
Definition: LoopUnroll.cpp:245
StringRef getName() const
Return a constant reference to the value's name.
Definition: Value.cpp:191
BlockT * getLoopLatch() const
If there is a single latch block for this loop, return it.
Definition: LoopInfoImpl.h:157
A templated base class for SmallPtrSet which provides the typesafe interface that is common across al...
Definition: SmallPtrSet.h:345
iterator begin()
Instruction iterator methods.
Definition: BasicBlock.h:228
bool formLCSSARecursively(Loop &L, DominatorTree &DT, LoopInfo *LI, ScalarEvolution *SE)
Put a loop nest into LCSSA form.
Definition: LCSSA.cpp:299
bool isUnconditional() const
Value * removeIncomingValue(unsigned Idx, bool DeletePHIIfEmpty=true)
Remove an incoming value.
Twine - A lightweight data structure for efficiently representing the concatenation of temporary valu...
Definition: Twine.h:81
bool simplifyLoop(Loop *L, DominatorTree *DT, LoopInfo *LI, ScalarEvolution *SE, AssumptionCache *AC, bool PreserveLCSSA)
Simplify each loop in a loop nest recursively.
const Module * getModule() const
Return the module owning the function this basic block belongs to, or nullptr it the function does no...
Definition: BasicBlock.cpp:116
A Use represents the edge between a Value definition and its users.
Definition: Use.h:56
LLVM_NODISCARD LLVM_ATTRIBUTE_ALWAYS_INLINE bool equals(StringRef RHS) const
equals - Check for string equality, this is more efficient than compare() when the relative ordering ...
Definition: StringRef.h:166
bool hasAddressTaken() const
Returns true if there are any uses of this basic block other than direct branches, switches, etc.
Definition: BasicBlock.h:308
void setName(const Twine &Name)
Change the name of the value.
Definition: Value.cpp:257
LLVM_NODISCARD bool empty() const
Definition: SmallVector.h:60
void getExitBlocks(SmallVectorImpl< BlockT * > &ExitBlocks) const
Return all of the successor blocks of this loop.
Definition: LoopInfoImpl.h:65
bool insert(const value_type &X)
Insert a new element into the SetVector.
Definition: SetVector.h:136
void addBasicBlockToLoop(BlockT *NewBB, LoopInfoBase< BlockT, LoopT > &LI)
This method is used by other analyses to update loop information.
Definition: LoopInfoImpl.h:188
void addChildLoop(LoopT *NewChild)
Add the specified loop to be a child of this loop.
Definition: LoopInfo.h:279
BasicBlock * getSuccessor(unsigned i) const
iterator find(const KeyT &Val)
Definition: ValueMap.h:158
Base class for the actual dominator tree node.
void replaceAllUsesWith(Value *V)
Change all uses of this to point to a new Value.
Definition: Value.cpp:401
void perform(LoopInfo *LI)
Traverse the loop blocks and store the DFS result.
Definition: LoopInfo.cpp:754
Concrete subclass of DominatorTreeBase that is used to compute a normal dominator tree...
Definition: Dominators.h:96
static GCRegistry::Add< CoreCLRGC > E("coreclr","CoreCLR-compatible GC")
iterator_range< block_iterator > blocks() const
Definition: LoopInfo.h:143
unsigned getNumSuccessors() const
Return the number of successors that this terminator has.
Definition: InstrTypes.h:74
initializer< Ty > init(const Ty &Val)
Definition: CommandLine.h:395
BlockT * getLoopPreheader() const
If there is a preheader for this loop, return it.
Definition: LoopInfoImpl.h:109
LLVM Basic Block Representation.
Definition: BasicBlock.h:51
void eraseNode(NodeT *BB)
eraseNode - Removes a node from the dominator tree.
Conditional or Unconditional Branch instruction.
std::pair< iterator, bool > insert(PtrType Ptr)
Inserts Ptr if and only if there is no element in the container equal to Ptr.
Definition: SmallPtrSet.h:368
Diagnostic information for applied optimization remarks.
void splice(iterator where, iplist_impl &L2)
Definition: ilist.h:342
bool contains(const LoopT *L) const
Return true if the specified loop is contained within in this loop.
Definition: LoopInfo.h:109
bool any_of(R &&Range, UnaryPredicate P)
Provide wrappers to std::any_of which take ranges instead of having to pass begin/end explicitly...
Definition: STLExtras.h:743
const InstListType & getInstList() const
Return the underlying instruction list container.
Definition: BasicBlock.h:249
bool isSafeToClone() const
Return true if the loop body is safe to clone in practice.
Definition: LoopInfo.cpp:197
std::vector< BasicBlock * >::const_reverse_iterator RPOIterator
Definition: LoopIterator.h:102
for(unsigned i=0, e=MI->getNumOperands();i!=e;++i)
Value * getOperand(unsigned i) const
Definition: User.h:145
NodeT * findNearestCommonDominator(NodeT *A, NodeT *B)
findNearestCommonDominator - Find nearest common dominator basic block for basic block A and B...
void markAsRemoved(Loop *L)
Update LoopInfo after removing the last backedge from a loop.
Definition: LoopInfo.cpp:613
void verifyDomTree() const
Verify the correctness of the domtree by re-computing it.
Definition: Dominators.cpp:284
bool RecursivelyDeleteTriviallyDeadInstructions(Value *V, const TargetLibraryInfo *TLI=nullptr)
If the specified value is a trivially dead instruction, delete it.
Definition: Local.cpp:355
void setSuccessor(unsigned idx, BasicBlock *NewSucc)
static void DFS(BasicBlock *Root, SetVector< BasicBlock * > &Set)
iterator end()
Definition: ValueMap.h:138
StringRef getString() const
Definition: Metadata.cpp:424
const MDOperand & getOperand(unsigned I) const
Definition: Metadata.h:1034
A SetVector that performs no allocations if smaller than a certain size.
Definition: SetVector.h:292
Iterator for intrusive lists based on ilist_node.
const BasicBlockListType & getBasicBlockList() const
Definition: Function.h:512
SmallPtrSet - This class implements a set which is optimized for holding SmallSize or less elements...
Definition: SmallPtrSet.h:425
void emit(DiagnosticInfoOptimizationBase &OptDiag)
The new interface to emit remarks.
iterator end()
Definition: BasicBlock.h:230
This is a 'vector' (really, a variable-sized array), optimized for the case when the array is small...
Definition: SmallVector.h:843
LLVM_NODISCARD T pop_back_val()
Definition: SmallVector.h:382
static BranchInst * Create(BasicBlock *IfTrue, Instruction *InsertBefore=nullptr)
void addTopLevelLoop(LoopT *New)
This adds the specified loop to the collection of top-level loops.
Definition: LoopInfo.h:629
void setOperand(unsigned i, Value *Val)
Definition: User.h:150
raw_ostream & dbgs()
dbgs() - This returns a reference to a raw_ostream for debugging messages.
Definition: Debug.cpp:132
const Loop * addClonedBlockToLoopInfo(BasicBlock *OriginalBB, BasicBlock *ClonedBB, LoopInfo *LI, NewLoopsMap &NewLoops)
Adds ClonedBB to LoopInfo, creates a new loop for ClonedBB if necessary and adds a mapping from the o...
Definition: LoopUnroll.cpp:179
Store the result of a depth first search within basic blocks contained by a single loop...
Definition: LoopIterator.h:98
void push_back(pointer val)
Definition: ilist.h:326
bool simplifyLoopIVs(Loop *L, ScalarEvolution *SE, DominatorTree *DT, LoopInfo *LI, SmallVectorImpl< WeakVH > &Dead)
SimplifyLoopIVs - Simplify users of induction variables within this loop.
Value * getIncomingValueForBlock(const BasicBlock *BB) const
BasicBlock * getSinglePredecessor()
Return the predecessor of this block if it has a single predecessor block.
Definition: BasicBlock.cpp:226
bool peelLoop(Loop *L, unsigned PeelCount, LoopInfo *LI, ScalarEvolution *SE, DominatorTree *DT, bool PreserveLCSSA)
Peel off the first PeelCount iterations of loop L.
void FoldSingleEntryPHINodes(BasicBlock *BB, MemoryDependenceResults *MemDep=nullptr)
We know that BB has one predecessor.
LLVM_NODISCARD LLVM_ATTRIBUTE_ALWAYS_INLINE bool empty() const
empty - Check if the string is empty.
Definition: StringRef.h:130
MDNode * GetUnrollMetadata(MDNode *LoopID, StringRef Name)
Given an llvm.loop loop id metadata node, returns the loop hint metadata node with the given name (fo...
Definition: LoopUnroll.cpp:760
bool UnrollRuntimeLoopRemainder(Loop *L, unsigned Count, bool AllowExpensiveTripCount, bool UseEpilogRemainder, LoopInfo *LI, ScalarEvolution *SE, DominatorTree *DT, bool PreserveLCSSA)
Insert code in the prolog/epilog code when unrolling a loop with a run-time trip-count.
const DataLayout & getDataLayout() const
Get the data layout for the module's target platform.
Definition: Module.cpp:384
void registerAssumption(CallInst *CI)
Add an .assume intrinsic to this function's cache.
void forgetLoop(const Loop *L)
This method should be called by the client when it has changed a loop in a way that may effect Scalar...
static cl::opt< bool > UnrollRuntimeEpilog("unroll-runtime-epilog", cl::init(false), cl::Hidden, cl::desc("Allow runtime unrolled loops to be unrolled ""with epilog instead of prolog."))
Represents a single loop in the control flow graph.
Definition: LoopInfo.h:368
SymbolTableList< BasicBlock >::iterator eraseFromParent()
Unlink 'this' from the containing function and delete it.
Definition: BasicBlock.cpp:97
#define I(x, y, z)
Definition: MD5.cpp:54
TerminatorInst * getTerminator()
Returns the terminator instruction if the block is well formed or null if the block is not well forme...
Definition: BasicBlock.cpp:124
LLVM_NODISCARD std::enable_if<!is_simple_type< Y >::value, typename cast_retty< X, const Y >::ret_type >::type dyn_cast(const Y &Val)
Definition: Casting.h:287
void changeImmediateDominator(DomTreeNodeBase< NodeT > *N, DomTreeNodeBase< NodeT > *NewIDom)
changeImmediateDominator - This method is used to update the dominator tree information when a node's...
RPOIterator beginRPO() const
Reverse iterate over the cached postorder blocks.
Definition: LoopIterator.h:137
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
bool isInstructionTriviallyDead(Instruction *I, const TargetLibraryInfo *TLI=nullptr)
Return true if the result produced by the instruction is not used, and the instruction has no side ef...
Definition: Local.cpp:288
LLVM Value Representation.
Definition: Value.h:71
succ_range successors(BasicBlock *BB)
Definition: IR/CFG.h:143
#define DEBUG_TYPE
Definition: LoopUnroll.cpp:43
#define DEBUG(X)
Definition: Debug.h:100
void recalculate(FT &F)
recalculate - compute a dominator tree for the given function
StringRef - Represent a constant reference to a string, i.e.
Definition: StringRef.h:47
A single uniqued string.
Definition: Metadata.h:586
static BasicBlock * foldBlockIntoPredecessor(BasicBlock *BB, LoopInfo *LI, ScalarEvolution *SE, SmallPtrSetImpl< Loop * > &ForgottenLoops, DominatorTree *DT)
Folds a basic block into its predecessor if it only has one predecessor, and that predecessor only ha...
Definition: LoopUnroll.cpp:83
Value * SimplifyInstruction(Instruction *I, const DataLayout &DL, const TargetLibraryInfo *TLI=nullptr, const DominatorTree *DT=nullptr, AssumptionCache *AC=nullptr)
See if we can compute a simplified version of this instruction.
bool replacementPreservesLCSSAForm(Instruction *From, Value *To)
Returns true if replacing From with To everywhere is guaranteed to preserve LCSSA form...
Definition: LoopInfo.h:695
DomTreeNodeBase< NodeT > * getNode(NodeT *BB) const
getNode - return the (Post)DominatorTree node for the specified basic block.
void pop_back()
Definition: ilist.h:331
BasicBlock * CloneBasicBlock(const BasicBlock *BB, ValueToValueMapTy &VMap, const Twine &NameSuffix="", Function *F=nullptr, ClonedCodeInfo *CodeInfo=nullptr)
CloneBasicBlock - Return a copy of the specified basic block, but without embedding the block into a ...
The optimization diagnostic interface.
iterator begin()
Definition: ValueMap.h:137
RPOIterator endRPO() const
Definition: LoopIterator.h:141