1 //===-- UnrollLoop.cpp - Loop unrolling utilities -------------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
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
14 // The process of unrolling can produce extraneous basic blocks linked with
15 // unconditional branches. This will be corrected in the future.
17 //===----------------------------------------------------------------------===//
19 #include "llvm/Transforms/Utils/UnrollLoop.h"
20 #include "llvm/ADT/SmallPtrSet.h"
21 #include "llvm/ADT/Statistic.h"
22 #include "llvm/Analysis/AssumptionCache.h"
23 #include "llvm/Analysis/InstructionSimplify.h"
24 #include "llvm/Analysis/LoopIterator.h"
25 #include "llvm/Analysis/LoopPass.h"
26 #include "llvm/Analysis/ScalarEvolution.h"
27 #include "llvm/IR/BasicBlock.h"
28 #include "llvm/IR/DataLayout.h"
29 #include "llvm/IR/DiagnosticInfo.h"
30 #include "llvm/IR/Dominators.h"
31 #include "llvm/IR/LLVMContext.h"
32 #include "llvm/Support/Debug.h"
33 #include "llvm/Support/raw_ostream.h"
34 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
35 #include "llvm/Transforms/Utils/Cloning.h"
36 #include "llvm/Transforms/Utils/Local.h"
37 #include "llvm/Transforms/Utils/LoopUtils.h"
38 #include "llvm/Transforms/Utils/SimplifyIndVar.h"
41 #define DEBUG_TYPE "loop-unroll"
43 // TODO: Should these be here or in LoopUnroll?
44 STATISTIC(NumCompletelyUnrolled, "Number of loops completely unrolled");
45 STATISTIC(NumUnrolled, "Number of loops unrolled (completely or otherwise)");
47 /// RemapInstruction - Convert the instruction operands from referencing the
48 /// current values into those specified by VMap.
49 static inline void RemapInstruction(Instruction *I,
50 ValueToValueMapTy &VMap) {
51 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
52 Value *Op = I->getOperand(op);
53 ValueToValueMapTy::iterator It = VMap.find(Op);
55 I->setOperand(op, It->second);
58 if (PHINode *PN = dyn_cast<PHINode>(I)) {
59 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
60 ValueToValueMapTy::iterator It = VMap.find(PN->getIncomingBlock(i));
62 PN->setIncomingBlock(i, cast<BasicBlock>(It->second));
67 /// FoldBlockIntoPredecessor - Folds a basic block into its predecessor if it
68 /// only has one predecessor, and that predecessor only has one successor.
69 /// The LoopInfo Analysis that is passed will be kept consistent. If folding is
70 /// successful references to the containing loop must be removed from
71 /// ScalarEvolution by calling ScalarEvolution::forgetLoop because SE may have
72 /// references to the eliminated BB. The argument ForgottenLoops contains a set
73 /// of loops that have already been forgotten to prevent redundant, expensive
74 /// calls to ScalarEvolution::forgetLoop. Returns the new combined block.
76 FoldBlockIntoPredecessor(BasicBlock *BB, LoopInfo* LI, ScalarEvolution *SE,
77 SmallPtrSetImpl<Loop *> &ForgottenLoops) {
78 // Merge basic blocks into their predecessor if there is only one distinct
79 // pred, and if there is only one distinct successor of the predecessor, and
80 // if there are no PHI nodes.
81 BasicBlock *OnlyPred = BB->getSinglePredecessor();
82 if (!OnlyPred) return nullptr;
84 if (OnlyPred->getTerminator()->getNumSuccessors() != 1)
87 DEBUG(dbgs() << "Merging: " << *BB << "into: " << *OnlyPred);
89 // Resolve any PHI nodes at the start of the block. They are all
90 // guaranteed to have exactly one entry if they exist, unless there are
91 // multiple duplicate (but guaranteed to be equal) entries for the
92 // incoming edges. This occurs when there are multiple edges from
93 // OnlyPred to OnlySucc.
94 FoldSingleEntryPHINodes(BB);
96 // Delete the unconditional branch from the predecessor...
97 OnlyPred->getInstList().pop_back();
99 // Make all PHI nodes that referred to BB now refer to Pred as their
101 BB->replaceAllUsesWith(OnlyPred);
103 // Move all definitions in the successor to the predecessor...
104 OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
106 // OldName will be valid until erased.
107 StringRef OldName = BB->getName();
109 // Erase basic block from the function...
111 // ScalarEvolution holds references to loop exit blocks.
113 if (Loop *L = LI->getLoopFor(BB)) {
114 if (ForgottenLoops.insert(L).second)
120 // Inherit predecessor's name if it exists...
121 if (!OldName.empty() && !OnlyPred->hasName())
122 OnlyPred->setName(OldName);
124 BB->eraseFromParent();
129 /// Unroll the given loop by Count. The loop must be in LCSSA form. Returns true
130 /// if unrolling was successful, or false if the loop was unmodified. Unrolling
131 /// can only fail when the loop's latch block is not terminated by a conditional
132 /// branch instruction. However, if the trip count (and multiple) are not known,
133 /// loop unrolling will mostly produce more code that is no faster.
135 /// TripCount is generally defined as the number of times the loop header
136 /// executes. UnrollLoop relaxes the definition to permit early exits: here
137 /// TripCount is the iteration on which control exits LatchBlock if no early
138 /// exits were taken. Note that UnrollLoop assumes that the loop counter test
139 /// terminates LatchBlock in order to remove unnecesssary instances of the
140 /// test. In other words, control may exit the loop prior to TripCount
141 /// iterations via an early branch, but control may not exit the loop from the
142 /// LatchBlock's terminator prior to TripCount iterations.
144 /// Similarly, TripMultiple divides the number of times that the LatchBlock may
145 /// execute without exiting the loop.
147 /// If AllowRuntime is true then UnrollLoop will consider unrolling loops that
148 /// have a runtime (i.e. not compile time constant) trip count. Unrolling these
149 /// loops require a unroll "prologue" that runs "RuntimeTripCount % Count"
150 /// iterations before branching into the unrolled loop. UnrollLoop will not
151 /// runtime-unroll the loop if computing RuntimeTripCount will be expensive and
152 /// AllowExpensiveTripCount is false.
154 /// The LoopInfo Analysis that is passed will be kept consistent.
156 /// If a LoopPassManager is passed in, and the loop is fully removed, it will be
157 /// removed from the LoopPassManager as well. LPM can also be NULL.
159 /// This utility preserves LoopInfo. It will also preserve ScalarEvolution and
160 /// DominatorTree if they are non-null.
161 bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount,
162 bool AllowRuntime, bool AllowExpensiveTripCount,
163 unsigned TripMultiple, LoopInfo *LI, ScalarEvolution *SE,
164 DominatorTree *DT, AssumptionCache *AC,
165 bool PreserveLCSSA, LPPassManager *LPM) {
166 BasicBlock *Preheader = L->getLoopPreheader();
168 DEBUG(dbgs() << " Can't unroll; loop preheader-insertion failed.\n");
172 BasicBlock *LatchBlock = L->getLoopLatch();
174 DEBUG(dbgs() << " Can't unroll; loop exit-block-insertion failed.\n");
178 // Loops with indirectbr cannot be cloned.
179 if (!L->isSafeToClone()) {
180 DEBUG(dbgs() << " Can't unroll; Loop body cannot be cloned.\n");
184 BasicBlock *Header = L->getHeader();
185 BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator());
187 if (!BI || BI->isUnconditional()) {
188 // The loop-rotate pass can be helpful to avoid this in many cases.
190 " Can't unroll; loop not terminated by a conditional branch.\n");
194 if (Header->hasAddressTaken()) {
195 // The loop-rotate pass can be helpful to avoid this in many cases.
197 " Won't unroll loop: address of header block is taken.\n");
202 DEBUG(dbgs() << " Trip Count = " << TripCount << "\n");
203 if (TripMultiple != 1)
204 DEBUG(dbgs() << " Trip Multiple = " << TripMultiple << "\n");
206 // Effectively "DCE" unrolled iterations that are beyond the tripcount
207 // and will never be executed.
208 if (TripCount != 0 && Count > TripCount)
211 // Don't enter the unroll code if there is nothing to do. This way we don't
212 // need to support "partial unrolling by 1".
213 if (TripCount == 0 && Count < 2)
217 assert(TripMultiple > 0);
218 assert(TripCount == 0 || TripCount % TripMultiple == 0);
220 // Are we eliminating the loop control altogether?
221 bool CompletelyUnroll = Count == TripCount;
222 SmallVector<BasicBlock *, 4> ExitBlocks;
223 L->getExitBlocks(ExitBlocks);
224 Loop *ParentL = L->getParentLoop();
225 bool AllExitsAreInsideParentLoop = !ParentL ||
226 std::all_of(ExitBlocks.begin(), ExitBlocks.end(),
227 [&](BasicBlock *BB) { return ParentL->contains(BB); });
229 // We assume a run-time trip count if the compiler cannot
230 // figure out the loop trip count and the unroll-runtime
231 // flag is specified.
232 bool RuntimeTripCount = (TripCount == 0 && Count > 0 && AllowRuntime);
234 if (RuntimeTripCount &&
235 !UnrollRuntimeLoopProlog(L, Count, AllowExpensiveTripCount, LI, SE, DT,
239 // Notify ScalarEvolution that the loop will be substantially changed,
240 // if not outright eliminated.
244 // If we know the trip count, we know the multiple...
245 unsigned BreakoutTrip = 0;
246 if (TripCount != 0) {
247 BreakoutTrip = TripCount % Count;
250 // Figure out what multiple to use.
251 BreakoutTrip = TripMultiple =
252 (unsigned)GreatestCommonDivisor64(Count, TripMultiple);
255 // Report the unrolling decision.
256 DebugLoc LoopLoc = L->getStartLoc();
257 Function *F = Header->getParent();
258 LLVMContext &Ctx = F->getContext();
260 if (CompletelyUnroll) {
261 DEBUG(dbgs() << "COMPLETELY UNROLLING loop %" << Header->getName()
262 << " with trip count " << TripCount << "!\n");
263 emitOptimizationRemark(Ctx, DEBUG_TYPE, *F, LoopLoc,
264 Twine("completely unrolled loop with ") +
265 Twine(TripCount) + " iterations");
267 auto EmitDiag = [&](const Twine &T) {
268 emitOptimizationRemark(Ctx, DEBUG_TYPE, *F, LoopLoc,
269 "unrolled loop by a factor of " + Twine(Count) +
273 DEBUG(dbgs() << "UNROLLING loop %" << Header->getName()
275 if (TripMultiple == 0 || BreakoutTrip != TripMultiple) {
276 DEBUG(dbgs() << " with a breakout at trip " << BreakoutTrip);
277 EmitDiag(" with a breakout at trip " + Twine(BreakoutTrip));
278 } else if (TripMultiple != 1) {
279 DEBUG(dbgs() << " with " << TripMultiple << " trips per branch");
280 EmitDiag(" with " + Twine(TripMultiple) + " trips per branch");
281 } else if (RuntimeTripCount) {
282 DEBUG(dbgs() << " with run-time trip count");
283 EmitDiag(" with run-time trip count");
285 DEBUG(dbgs() << "!\n");
288 bool ContinueOnTrue = L->contains(BI->getSuccessor(0));
289 BasicBlock *LoopExit = BI->getSuccessor(ContinueOnTrue);
291 // For the first iteration of the loop, we should use the precloned values for
292 // PHI nodes. Insert associations now.
293 ValueToValueMapTy LastValueMap;
294 std::vector<PHINode*> OrigPHINode;
295 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
296 OrigPHINode.push_back(cast<PHINode>(I));
299 std::vector<BasicBlock*> Headers;
300 std::vector<BasicBlock*> Latches;
301 Headers.push_back(Header);
302 Latches.push_back(LatchBlock);
304 // The current on-the-fly SSA update requires blocks to be processed in
305 // reverse postorder so that LastValueMap contains the correct value at each
307 LoopBlocksDFS DFS(L);
310 // Stash the DFS iterators before adding blocks to the loop.
311 LoopBlocksDFS::RPOIterator BlockBegin = DFS.beginRPO();
312 LoopBlocksDFS::RPOIterator BlockEnd = DFS.endRPO();
314 for (unsigned It = 1; It != Count; ++It) {
315 std::vector<BasicBlock*> NewBlocks;
316 SmallDenseMap<const Loop *, Loop *, 4> NewLoops;
319 for (LoopBlocksDFS::RPOIterator BB = BlockBegin; BB != BlockEnd; ++BB) {
320 ValueToValueMapTy VMap;
321 BasicBlock *New = CloneBasicBlock(*BB, VMap, "." + Twine(It));
322 Header->getParent()->getBasicBlockList().push_back(New);
324 // Tell LI about New.
326 assert(LI->getLoopFor(*BB) == L && "Header should not be in a sub-loop");
327 L->addBasicBlockToLoop(New, *LI);
329 // Figure out which loop New is in.
330 const Loop *OldLoop = LI->getLoopFor(*BB);
331 assert(OldLoop && "Should (at least) be in the loop being unrolled!");
333 Loop *&NewLoop = NewLoops[OldLoop];
335 // Found a new sub-loop.
336 assert(*BB == OldLoop->getHeader() &&
337 "Header should be first in RPO");
339 Loop *NewLoopParent = NewLoops.lookup(OldLoop->getParentLoop());
340 assert(NewLoopParent &&
341 "Expected parent loop before sub-loop in RPO");
343 NewLoopParent->addChildLoop(NewLoop);
345 // Forget the old loop, since its inputs may have changed.
347 SE->forgetLoop(OldLoop);
349 NewLoop->addBasicBlockToLoop(New, *LI);
353 // Loop over all of the PHI nodes in the block, changing them to use
354 // the incoming values from the previous block.
355 for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) {
356 PHINode *NewPHI = cast<PHINode>(VMap[OrigPHINode[i]]);
357 Value *InVal = NewPHI->getIncomingValueForBlock(LatchBlock);
358 if (Instruction *InValI = dyn_cast<Instruction>(InVal))
359 if (It > 1 && L->contains(InValI))
360 InVal = LastValueMap[InValI];
361 VMap[OrigPHINode[i]] = InVal;
362 New->getInstList().erase(NewPHI);
365 // Update our running map of newest clones
366 LastValueMap[*BB] = New;
367 for (ValueToValueMapTy::iterator VI = VMap.begin(), VE = VMap.end();
369 LastValueMap[VI->first] = VI->second;
371 // Add phi entries for newly created values to all exit blocks.
372 for (succ_iterator SI = succ_begin(*BB), SE = succ_end(*BB);
374 if (L->contains(*SI))
376 for (BasicBlock::iterator BBI = (*SI)->begin();
377 PHINode *phi = dyn_cast<PHINode>(BBI); ++BBI) {
378 Value *Incoming = phi->getIncomingValueForBlock(*BB);
379 ValueToValueMapTy::iterator It = LastValueMap.find(Incoming);
380 if (It != LastValueMap.end())
381 Incoming = It->second;
382 phi->addIncoming(Incoming, New);
385 // Keep track of new headers and latches as we create them, so that
386 // we can insert the proper branches later.
388 Headers.push_back(New);
389 if (*BB == LatchBlock)
390 Latches.push_back(New);
392 NewBlocks.push_back(New);
395 // Remap all instructions in the most recent iteration
396 for (unsigned i = 0; i < NewBlocks.size(); ++i)
397 for (BasicBlock::iterator I = NewBlocks[i]->begin(),
398 E = NewBlocks[i]->end(); I != E; ++I)
399 ::RemapInstruction(&*I, LastValueMap);
402 // Loop over the PHI nodes in the original block, setting incoming values.
403 for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) {
404 PHINode *PN = OrigPHINode[i];
405 if (CompletelyUnroll) {
406 PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader));
407 Header->getInstList().erase(PN);
409 else if (Count > 1) {
410 Value *InVal = PN->removeIncomingValue(LatchBlock, false);
411 // If this value was defined in the loop, take the value defined by the
412 // last iteration of the loop.
413 if (Instruction *InValI = dyn_cast<Instruction>(InVal)) {
414 if (L->contains(InValI))
415 InVal = LastValueMap[InVal];
417 assert(Latches.back() == LastValueMap[LatchBlock] && "bad last latch");
418 PN->addIncoming(InVal, Latches.back());
422 // Now that all the basic blocks for the unrolled iterations are in place,
423 // set up the branches to connect them.
424 for (unsigned i = 0, e = Latches.size(); i != e; ++i) {
425 // The original branch was replicated in each unrolled iteration.
426 BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator());
428 // The branch destination.
429 unsigned j = (i + 1) % e;
430 BasicBlock *Dest = Headers[j];
431 bool NeedConditional = true;
433 if (RuntimeTripCount && j != 0) {
434 NeedConditional = false;
437 // For a complete unroll, make the last iteration end with a branch
438 // to the exit block.
439 if (CompletelyUnroll) {
442 NeedConditional = false;
445 // If we know the trip count or a multiple of it, we can safely use an
446 // unconditional branch for some iterations.
447 if (j != BreakoutTrip && (TripMultiple == 0 || j % TripMultiple != 0)) {
448 NeedConditional = false;
451 if (NeedConditional) {
452 // Update the conditional branch's successor for the following
454 Term->setSuccessor(!ContinueOnTrue, Dest);
456 // Remove phi operands at this loop exit
457 if (Dest != LoopExit) {
458 BasicBlock *BB = Latches[i];
459 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB);
461 if (*SI == Headers[i])
463 for (BasicBlock::iterator BBI = (*SI)->begin();
464 PHINode *Phi = dyn_cast<PHINode>(BBI); ++BBI) {
465 Phi->removeIncomingValue(BB, false);
469 // Replace the conditional branch with an unconditional one.
470 BranchInst::Create(Dest, Term);
471 Term->eraseFromParent();
475 // Merge adjacent basic blocks, if possible.
476 SmallPtrSet<Loop *, 4> ForgottenLoops;
477 for (unsigned i = 0, e = Latches.size(); i != e; ++i) {
478 BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator());
479 if (Term->isUnconditional()) {
480 BasicBlock *Dest = Term->getSuccessor(0);
481 if (BasicBlock *Fold = FoldBlockIntoPredecessor(Dest, LI, SE,
483 std::replace(Latches.begin(), Latches.end(), Dest, Fold);
487 // FIXME: We could register any cloned assumptions instead of clearing the
488 // whole function's cache.
491 // FIXME: Reconstruct dom info, because it is not preserved properly.
492 // Incrementally updating domtree after loop unrolling would be easy.
494 DT->recalculate(*L->getHeader()->getParent());
496 // Simplify any new induction variables in the partially unrolled loop.
497 if (SE && !CompletelyUnroll) {
498 SmallVector<WeakVH, 16> DeadInsts;
499 simplifyLoopIVs(L, SE, DT, LI, DeadInsts);
501 // Aggressively clean up dead instructions that simplifyLoopIVs already
502 // identified. Any remaining should be cleaned up below.
503 while (!DeadInsts.empty())
504 if (Instruction *Inst =
505 dyn_cast_or_null<Instruction>(&*DeadInsts.pop_back_val()))
506 RecursivelyDeleteTriviallyDeadInstructions(Inst);
509 // At this point, the code is well formed. We now do a quick sweep over the
510 // inserted code, doing constant propagation and dead code elimination as we
512 const DataLayout &DL = Header->getModule()->getDataLayout();
513 const std::vector<BasicBlock*> &NewLoopBlocks = L->getBlocks();
514 for (std::vector<BasicBlock*>::const_iterator BB = NewLoopBlocks.begin(),
515 BBE = NewLoopBlocks.end(); BB != BBE; ++BB)
516 for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ) {
517 Instruction *Inst = &*I++;
519 if (isInstructionTriviallyDead(Inst))
520 (*BB)->getInstList().erase(Inst);
521 else if (Value *V = SimplifyInstruction(Inst, DL))
522 if (LI->replacementPreservesLCSSAForm(Inst, V)) {
523 Inst->replaceAllUsesWith(V);
524 (*BB)->getInstList().erase(Inst);
528 NumCompletelyUnrolled += CompletelyUnroll;
531 Loop *OuterL = L->getParentLoop();
532 // Remove the loop from the LoopPassManager if it's completely removed.
533 if (CompletelyUnroll && LPM != nullptr)
534 LPM->deleteLoopFromQueue(L);
536 // If we have a pass and a DominatorTree we should re-simplify impacted loops
537 // to ensure subsequent analyses can rely on this form. We want to simplify
538 // at least one layer outside of the loop that was unrolled so that any
539 // changes to the parent loop exposed by the unrolling are considered.
541 if (!OuterL && !CompletelyUnroll)
544 bool Simplified = simplifyLoop(OuterL, DT, LI, SE, AC, PreserveLCSSA);
546 // LCSSA must be performed on the outermost affected loop. The unrolled
547 // loop's last loop latch is guaranteed to be in the outermost loop after
548 // deleteLoopFromQueue updates LoopInfo.
549 Loop *LatchLoop = LI->getLoopFor(Latches.back());
550 if (!OuterL->contains(LatchLoop))
551 while (OuterL->getParentLoop() != LatchLoop)
552 OuterL = OuterL->getParentLoop();
554 if (CompletelyUnroll && (!AllExitsAreInsideParentLoop || Simplified))
555 formLCSSARecursively(*OuterL, *DT, LI, SE);
557 assert(OuterL->isLCSSAForm(*DT) &&
558 "Loops should be in LCSSA form after loop-unroll.");
565 /// Given an llvm.loop loop id metadata node, returns the loop hint metadata
566 /// node with the given name (for example, "llvm.loop.unroll.count"). If no
567 /// such metadata node exists, then nullptr is returned.
568 MDNode *llvm::GetUnrollMetadata(MDNode *LoopID, StringRef Name) {
569 // First operand should refer to the loop id itself.
570 assert(LoopID->getNumOperands() > 0 && "requires at least one operand");
571 assert(LoopID->getOperand(0) == LoopID && "invalid loop id");
573 for (unsigned i = 1, e = LoopID->getNumOperands(); i < e; ++i) {
574 MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(i));
578 MDString *S = dyn_cast<MDString>(MD->getOperand(0));
582 if (Name.equals(S->getString()))