1 //===- UnrollLoopPeel.cpp - Loop peeling 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 for peeling loops
11 // with dynamically inferred (from PGO) trip counts. See LoopUnroll.cpp for
12 // unrolling loops with compile-time constant trip counts.
14 //===----------------------------------------------------------------------===//
16 #include "llvm/ADT/DenseMap.h"
17 #include "llvm/ADT/Optional.h"
18 #include "llvm/ADT/SmallVector.h"
19 #include "llvm/ADT/Statistic.h"
20 #include "llvm/Analysis/LoopInfo.h"
21 #include "llvm/Analysis/LoopIterator.h"
22 #include "llvm/Analysis/ScalarEvolution.h"
23 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
24 #include "llvm/Analysis/TargetTransformInfo.h"
25 #include "llvm/IR/BasicBlock.h"
26 #include "llvm/IR/Dominators.h"
27 #include "llvm/IR/Function.h"
28 #include "llvm/IR/InstrTypes.h"
29 #include "llvm/IR/Instruction.h"
30 #include "llvm/IR/Instructions.h"
31 #include "llvm/IR/LLVMContext.h"
32 #include "llvm/IR/MDBuilder.h"
33 #include "llvm/IR/Metadata.h"
34 #include "llvm/IR/PatternMatch.h"
35 #include "llvm/Support/Casting.h"
36 #include "llvm/Support/CommandLine.h"
37 #include "llvm/Support/Debug.h"
38 #include "llvm/Support/raw_ostream.h"
39 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
40 #include "llvm/Transforms/Utils/Cloning.h"
41 #include "llvm/Transforms/Utils/LoopSimplify.h"
42 #include "llvm/Transforms/Utils/LoopUtils.h"
43 #include "llvm/Transforms/Utils/UnrollLoop.h"
44 #include "llvm/Transforms/Utils/ValueMapper.h"
51 using namespace llvm::PatternMatch;
53 #define DEBUG_TYPE "loop-unroll"
55 STATISTIC(NumPeeled, "Number of loops peeled");
57 static cl::opt<unsigned> UnrollPeelMaxCount(
58 "unroll-peel-max-count", cl::init(7), cl::Hidden,
59 cl::desc("Max average trip count which will cause loop peeling."));
61 static cl::opt<unsigned> UnrollForcePeelCount(
62 "unroll-force-peel-count", cl::init(0), cl::Hidden,
63 cl::desc("Force a peel count regardless of profiling information."));
65 // Designates that a Phi is estimated to become invariant after an "infinite"
66 // number of loop iterations (i.e. only may become an invariant if the loop is
68 static const unsigned InfiniteIterationsToInvariance =
69 std::numeric_limits<unsigned>::max();
71 // Check whether we are capable of peeling this loop.
72 bool llvm::canPeel(Loop *L) {
73 // Make sure the loop is in simplified form
74 if (!L->isLoopSimplifyForm())
77 // Only peel loops that contain a single exit
78 if (!L->getExitingBlock() || !L->getUniqueExitBlock())
81 // Don't try to peel loops where the latch is not the exiting block.
82 // This can be an indication of two different things:
83 // 1) The loop is not rotated.
84 // 2) The loop contains irreducible control flow that involves the latch.
85 if (L->getLoopLatch() != L->getExitingBlock())
91 // This function calculates the number of iterations after which the given Phi
92 // becomes an invariant. The pre-calculated values are memorized in the map. The
93 // function (shortcut is I) is calculated according to the following definition:
94 // Given %x = phi <Inputs from above the loop>, ..., [%y, %back.edge].
95 // If %y is a loop invariant, then I(%x) = 1.
96 // If %y is a Phi from the loop header, I(%x) = I(%y) + 1.
97 // Otherwise, I(%x) is infinite.
98 // TODO: Actually if %y is an expression that depends only on Phi %z and some
99 // loop invariants, we can estimate I(%x) = I(%z) + 1. The example
101 // %x = phi(0, %a), <-- becomes invariant starting from 3rd iteration.
104 static unsigned calculateIterationsToInvariance(
105 PHINode *Phi, Loop *L, BasicBlock *BackEdge,
106 SmallDenseMap<PHINode *, unsigned> &IterationsToInvariance) {
107 assert(Phi->getParent() == L->getHeader() &&
108 "Non-loop Phi should not be checked for turning into invariant.");
109 assert(BackEdge == L->getLoopLatch() && "Wrong latch?");
110 // If we already know the answer, take it from the map.
111 auto I = IterationsToInvariance.find(Phi);
112 if (I != IterationsToInvariance.end())
115 // Otherwise we need to analyze the input from the back edge.
116 Value *Input = Phi->getIncomingValueForBlock(BackEdge);
117 // Place infinity to map to avoid infinite recursion for cycled Phis. Such
118 // cycles can never stop on an invariant.
119 IterationsToInvariance[Phi] = InfiniteIterationsToInvariance;
120 unsigned ToInvariance = InfiniteIterationsToInvariance;
122 if (L->isLoopInvariant(Input))
124 else if (PHINode *IncPhi = dyn_cast<PHINode>(Input)) {
125 // Only consider Phis in header block.
126 if (IncPhi->getParent() != L->getHeader())
127 return InfiniteIterationsToInvariance;
128 // If the input becomes an invariant after X iterations, then our Phi
129 // becomes an invariant after X + 1 iterations.
130 unsigned InputToInvariance = calculateIterationsToInvariance(
131 IncPhi, L, BackEdge, IterationsToInvariance);
132 if (InputToInvariance != InfiniteIterationsToInvariance)
133 ToInvariance = InputToInvariance + 1u;
136 // If we found that this Phi lies in an invariant chain, update the map.
137 if (ToInvariance != InfiniteIterationsToInvariance)
138 IterationsToInvariance[Phi] = ToInvariance;
142 // Return the number of iterations to peel off that make conditions in the
143 // body true/false. For example, if we peel 2 iterations off the loop below,
144 // the condition i < 2 can be evaluated at compile time.
145 // for (i = 0; i < n; i++)
151 static unsigned countToEliminateCompares(Loop &L, unsigned MaxPeelCount,
152 ScalarEvolution &SE) {
153 assert(L.isLoopSimplifyForm() && "Loop needs to be in loop simplify form");
154 unsigned DesiredPeelCount = 0;
156 for (auto *BB : L.blocks()) {
157 auto *BI = dyn_cast<BranchInst>(BB->getTerminator());
158 if (!BI || BI->isUnconditional())
161 // Ignore loop exit condition.
162 if (L.getLoopLatch() == BB)
165 Value *Condition = BI->getCondition();
166 Value *LeftVal, *RightVal;
167 CmpInst::Predicate Pred;
168 if (!match(Condition, m_ICmp(Pred, m_Value(LeftVal), m_Value(RightVal))))
171 const SCEV *LeftSCEV = SE.getSCEV(LeftVal);
172 const SCEV *RightSCEV = SE.getSCEV(RightVal);
174 // Do not consider predicates that are known to be true or false
175 // independently of the loop iteration.
176 if (SE.isKnownPredicate(Pred, LeftSCEV, RightSCEV) ||
177 SE.isKnownPredicate(ICmpInst::getInversePredicate(Pred), LeftSCEV,
181 // Check if we have a condition with one AddRec and one non AddRec
182 // expression. Normalize LeftSCEV to be the AddRec.
183 if (!isa<SCEVAddRecExpr>(LeftSCEV)) {
184 if (isa<SCEVAddRecExpr>(RightSCEV)) {
185 std::swap(LeftSCEV, RightSCEV);
186 Pred = ICmpInst::getSwappedPredicate(Pred);
191 const SCEVAddRecExpr *LeftAR = cast<SCEVAddRecExpr>(LeftSCEV);
193 // Avoid huge SCEV computations in the loop below, make sure we only
194 // consider AddRecs of the loop we are trying to peel and avoid
195 // non-monotonic predicates, as we will not be able to simplify the loop
197 // FIXME: For the non-monotonic predicates ICMP_EQ and ICMP_NE we can
198 // simplify the loop, if we peel 1 additional iteration, if there
201 if (!LeftAR->isAffine() || LeftAR->getLoop() != &L ||
202 !SE.isMonotonicPredicate(LeftAR, Pred, Increasing))
206 // Check if extending the current DesiredPeelCount lets us evaluate Pred
207 // or !Pred in the loop body statically.
208 unsigned NewPeelCount = DesiredPeelCount;
210 const SCEV *IterVal = LeftAR->evaluateAtIteration(
211 SE.getConstant(LeftSCEV->getType(), NewPeelCount), SE);
213 // If the original condition is not known, get the negated predicate
214 // (which holds on the else branch) and check if it is known. This allows
215 // us to peel of iterations that make the original condition false.
216 if (!SE.isKnownPredicate(Pred, IterVal, RightSCEV))
217 Pred = ICmpInst::getInversePredicate(Pred);
219 const SCEV *Step = LeftAR->getStepRecurrence(SE);
220 while (NewPeelCount < MaxPeelCount &&
221 SE.isKnownPredicate(Pred, IterVal, RightSCEV)) {
222 IterVal = SE.getAddExpr(IterVal, Step);
226 // Only peel the loop if the monotonic predicate !Pred becomes known in the
227 // first iteration of the loop body after peeling.
228 if (NewPeelCount > DesiredPeelCount &&
229 SE.isKnownPredicate(ICmpInst::getInversePredicate(Pred), IterVal,
231 DesiredPeelCount = NewPeelCount;
234 return DesiredPeelCount;
237 // Return the number of iterations we want to peel off.
238 void llvm::computePeelCount(Loop *L, unsigned LoopSize,
239 TargetTransformInfo::UnrollingPreferences &UP,
240 unsigned &TripCount, ScalarEvolution &SE) {
241 assert(LoopSize > 0 && "Zero loop size is not allowed!");
242 // Save the UP.PeelCount value set by the target in
243 // TTI.getUnrollingPreferences or by the flag -unroll-peel-count.
244 unsigned TargetPeelCount = UP.PeelCount;
249 // Only try to peel innermost loops.
253 // If the user provided a peel count, use that.
254 bool UserPeelCount = UnrollForcePeelCount.getNumOccurrences() > 0;
256 LLVM_DEBUG(dbgs() << "Force-peeling first " << UnrollForcePeelCount
257 << " iterations.\n");
258 UP.PeelCount = UnrollForcePeelCount;
262 // Skip peeling if it's disabled.
263 if (!UP.AllowPeeling)
266 // Here we try to get rid of Phis which become invariants after 1, 2, ..., N
267 // iterations of the loop. For this we compute the number for iterations after
268 // which every Phi is guaranteed to become an invariant, and try to peel the
269 // maximum number of iterations among these values, thus turning all those
270 // Phis into invariants.
271 // First, check that we can peel at least one iteration.
272 if (2 * LoopSize <= UP.Threshold && UnrollPeelMaxCount > 0) {
273 // Store the pre-calculated values here.
274 SmallDenseMap<PHINode *, unsigned> IterationsToInvariance;
275 // Now go through all Phis to calculate their the number of iterations they
276 // need to become invariants.
277 // Start the max computation with the UP.PeelCount value set by the target
278 // in TTI.getUnrollingPreferences or by the flag -unroll-peel-count.
279 unsigned DesiredPeelCount = TargetPeelCount;
280 BasicBlock *BackEdge = L->getLoopLatch();
281 assert(BackEdge && "Loop is not in simplified form?");
282 for (auto BI = L->getHeader()->begin(); isa<PHINode>(&*BI); ++BI) {
283 PHINode *Phi = cast<PHINode>(&*BI);
284 unsigned ToInvariance = calculateIterationsToInvariance(
285 Phi, L, BackEdge, IterationsToInvariance);
286 if (ToInvariance != InfiniteIterationsToInvariance)
287 DesiredPeelCount = std::max(DesiredPeelCount, ToInvariance);
290 // Pay respect to limitations implied by loop size and the max peel count.
291 unsigned MaxPeelCount = UnrollPeelMaxCount;
292 MaxPeelCount = std::min(MaxPeelCount, UP.Threshold / LoopSize - 1);
294 DesiredPeelCount = std::max(DesiredPeelCount,
295 countToEliminateCompares(*L, MaxPeelCount, SE));
297 if (DesiredPeelCount > 0) {
298 DesiredPeelCount = std::min(DesiredPeelCount, MaxPeelCount);
299 // Consider max peel count limitation.
300 assert(DesiredPeelCount > 0 && "Wrong loop size estimation?");
301 LLVM_DEBUG(dbgs() << "Peel " << DesiredPeelCount
302 << " iteration(s) to turn"
303 << " some Phis into invariants.\n");
304 UP.PeelCount = DesiredPeelCount;
309 // Bail if we know the statically calculated trip count.
310 // In this case we rather prefer partial unrolling.
314 // If we don't know the trip count, but have reason to believe the average
315 // trip count is low, peeling should be beneficial, since we will usually
316 // hit the peeled section.
317 // We only do this in the presence of profile information, since otherwise
318 // our estimates of the trip count are not reliable enough.
319 if (L->getHeader()->getParent()->hasProfileData()) {
320 Optional<unsigned> PeelCount = getLoopEstimatedTripCount(L);
324 LLVM_DEBUG(dbgs() << "Profile-based estimated trip count is " << *PeelCount
328 if ((*PeelCount <= UnrollPeelMaxCount) &&
329 (LoopSize * (*PeelCount + 1) <= UP.Threshold)) {
330 LLVM_DEBUG(dbgs() << "Peeling first " << *PeelCount
331 << " iterations.\n");
332 UP.PeelCount = *PeelCount;
335 LLVM_DEBUG(dbgs() << "Requested peel count: " << *PeelCount << "\n");
336 LLVM_DEBUG(dbgs() << "Max peel count: " << UnrollPeelMaxCount << "\n");
337 LLVM_DEBUG(dbgs() << "Peel cost: " << LoopSize * (*PeelCount + 1)
339 LLVM_DEBUG(dbgs() << "Max peel cost: " << UP.Threshold << "\n");
344 /// Update the branch weights of the latch of a peeled-off loop
346 /// This sets the branch weights for the latch of the recently peeled off loop
347 /// iteration correctly.
348 /// Our goal is to make sure that:
349 /// a) The total weight of all the copies of the loop body is preserved.
350 /// b) The total weight of the loop exit is preserved.
351 /// c) The body weight is reasonably distributed between the peeled iterations.
353 /// \param Header The copy of the header block that belongs to next iteration.
354 /// \param LatchBR The copy of the latch branch that belongs to this iteration.
355 /// \param IterNumber The serial number of the iteration that was just
357 /// \param AvgIters The average number of iterations we expect the loop to have.
358 /// \param[in,out] PeeledHeaderWeight The total number of dynamic loop
359 /// iterations that are unaccounted for. As an input, it represents the number
360 /// of times we expect to enter the header of the iteration currently being
361 /// peeled off. The output is the number of times we expect to enter the
362 /// header of the next iteration.
363 static void updateBranchWeights(BasicBlock *Header, BranchInst *LatchBR,
364 unsigned IterNumber, unsigned AvgIters,
365 uint64_t &PeeledHeaderWeight) {
366 // FIXME: Pick a more realistic distribution.
367 // Currently the proportion of weight we assign to the fall-through
368 // side of the branch drops linearly with the iteration number, and we use
369 // a 0.9 fudge factor to make the drop-off less sharp...
370 if (PeeledHeaderWeight) {
371 uint64_t FallThruWeight =
372 PeeledHeaderWeight * ((float)(AvgIters - IterNumber) / AvgIters * 0.9);
373 uint64_t ExitWeight = PeeledHeaderWeight - FallThruWeight;
374 PeeledHeaderWeight -= ExitWeight;
376 unsigned HeaderIdx = (LatchBR->getSuccessor(0) == Header ? 0 : 1);
377 MDBuilder MDB(LatchBR->getContext());
379 HeaderIdx ? MDB.createBranchWeights(ExitWeight, FallThruWeight)
380 : MDB.createBranchWeights(FallThruWeight, ExitWeight);
381 LatchBR->setMetadata(LLVMContext::MD_prof, WeightNode);
385 /// Clones the body of the loop L, putting it between \p InsertTop and \p
387 /// \param IterNumber The serial number of the iteration currently being
389 /// \param Exit The exit block of the original loop.
390 /// \param[out] NewBlocks A list of the blocks in the newly created clone
391 /// \param[out] VMap The value map between the loop and the new clone.
392 /// \param LoopBlocks A helper for DFS-traversal of the loop.
393 /// \param LVMap A value-map that maps instructions from the original loop to
394 /// instructions in the last peeled-off iteration.
395 static void cloneLoopBlocks(Loop *L, unsigned IterNumber, BasicBlock *InsertTop,
396 BasicBlock *InsertBot, BasicBlock *Exit,
397 SmallVectorImpl<BasicBlock *> &NewBlocks,
398 LoopBlocksDFS &LoopBlocks, ValueToValueMapTy &VMap,
399 ValueToValueMapTy &LVMap, DominatorTree *DT,
401 BasicBlock *Header = L->getHeader();
402 BasicBlock *Latch = L->getLoopLatch();
403 BasicBlock *PreHeader = L->getLoopPreheader();
405 Function *F = Header->getParent();
406 LoopBlocksDFS::RPOIterator BlockBegin = LoopBlocks.beginRPO();
407 LoopBlocksDFS::RPOIterator BlockEnd = LoopBlocks.endRPO();
408 Loop *ParentLoop = L->getParentLoop();
410 // For each block in the original loop, create a new copy,
411 // and update the value map with the newly created values.
412 for (LoopBlocksDFS::RPOIterator BB = BlockBegin; BB != BlockEnd; ++BB) {
413 BasicBlock *NewBB = CloneBasicBlock(*BB, VMap, ".peel", F);
414 NewBlocks.push_back(NewBB);
417 ParentLoop->addBasicBlockToLoop(NewBB, *LI);
421 // If dominator tree is available, insert nodes to represent cloned blocks.
424 DT->addNewBlock(NewBB, InsertTop);
426 DomTreeNode *IDom = DT->getNode(*BB)->getIDom();
427 // VMap must contain entry for IDom, as the iteration order is RPO.
428 DT->addNewBlock(NewBB, cast<BasicBlock>(VMap[IDom->getBlock()]));
433 // Hook-up the control flow for the newly inserted blocks.
434 // The new header is hooked up directly to the "top", which is either
435 // the original loop preheader (for the first iteration) or the previous
436 // iteration's exiting block (for every other iteration)
437 InsertTop->getTerminator()->setSuccessor(0, cast<BasicBlock>(VMap[Header]));
439 // Similarly, for the latch:
440 // The original exiting edge is still hooked up to the loop exit.
441 // The backedge now goes to the "bottom", which is either the loop's real
442 // header (for the last peeled iteration) or the copied header of the next
443 // iteration (for every other iteration)
444 BasicBlock *NewLatch = cast<BasicBlock>(VMap[Latch]);
445 BranchInst *LatchBR = cast<BranchInst>(NewLatch->getTerminator());
446 unsigned HeaderIdx = (LatchBR->getSuccessor(0) == Header ? 0 : 1);
447 LatchBR->setSuccessor(HeaderIdx, InsertBot);
448 LatchBR->setSuccessor(1 - HeaderIdx, Exit);
450 DT->changeImmediateDominator(InsertBot, NewLatch);
452 // The new copy of the loop body starts with a bunch of PHI nodes
453 // that pick an incoming value from either the preheader, or the previous
454 // loop iteration. Since this copy is no longer part of the loop, we
455 // resolve this statically:
456 // For the first iteration, we use the value from the preheader directly.
457 // For any other iteration, we replace the phi with the value generated by
458 // the immediately preceding clone of the loop body (which represents
459 // the previous iteration).
460 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
461 PHINode *NewPHI = cast<PHINode>(VMap[&*I]);
462 if (IterNumber == 0) {
463 VMap[&*I] = NewPHI->getIncomingValueForBlock(PreHeader);
465 Value *LatchVal = NewPHI->getIncomingValueForBlock(Latch);
466 Instruction *LatchInst = dyn_cast<Instruction>(LatchVal);
467 if (LatchInst && L->contains(LatchInst))
468 VMap[&*I] = LVMap[LatchInst];
470 VMap[&*I] = LatchVal;
472 cast<BasicBlock>(VMap[Header])->getInstList().erase(NewPHI);
475 // Fix up the outgoing values - we need to add a value for the iteration
476 // we've just created. Note that this must happen *after* the incoming
477 // values are adjusted, since the value going out of the latch may also be
478 // a value coming into the header.
479 for (BasicBlock::iterator I = Exit->begin(); isa<PHINode>(I); ++I) {
480 PHINode *PHI = cast<PHINode>(I);
481 Value *LatchVal = PHI->getIncomingValueForBlock(Latch);
482 Instruction *LatchInst = dyn_cast<Instruction>(LatchVal);
483 if (LatchInst && L->contains(LatchInst))
484 LatchVal = VMap[LatchVal];
485 PHI->addIncoming(LatchVal, cast<BasicBlock>(VMap[Latch]));
488 // LastValueMap is updated with the values for the current loop
489 // which are used the next time this function is called.
490 for (const auto &KV : VMap)
491 LVMap[KV.first] = KV.second;
494 /// Peel off the first \p PeelCount iterations of loop \p L.
496 /// Note that this does not peel them off as a single straight-line block.
497 /// Rather, each iteration is peeled off separately, and needs to check the
499 /// For loops that dynamically execute \p PeelCount iterations or less
500 /// this provides a benefit, since the peeled off iterations, which account
501 /// for the bulk of dynamic execution, can be further simplified by scalar
503 bool llvm::peelLoop(Loop *L, unsigned PeelCount, LoopInfo *LI,
504 ScalarEvolution *SE, DominatorTree *DT,
505 AssumptionCache *AC, bool PreserveLCSSA) {
506 assert(PeelCount > 0 && "Attempt to peel out zero iterations?");
507 assert(canPeel(L) && "Attempt to peel a loop which is not peelable?");
509 LoopBlocksDFS LoopBlocks(L);
510 LoopBlocks.perform(LI);
512 BasicBlock *Header = L->getHeader();
513 BasicBlock *PreHeader = L->getLoopPreheader();
514 BasicBlock *Latch = L->getLoopLatch();
515 BasicBlock *Exit = L->getUniqueExitBlock();
517 Function *F = Header->getParent();
519 // Set up all the necessary basic blocks. It is convenient to split the
520 // preheader into 3 parts - two blocks to anchor the peeled copy of the loop
521 // body, and a new preheader for the "real" loop.
523 // Peeling the first iteration transforms.
529 // If (cond) goto Header
536 // If (!cond) goto Exit
542 // If (cond) goto Header
545 // Each following iteration will split the current bottom anchor in two,
546 // and put the new copy of the loop body between these two blocks. That is,
547 // after peeling another iteration from the example above, we'll split
548 // InsertBot, and get:
552 // If (!cond) goto Exit
555 // If (!cond) goto Exit
561 // If (cond) goto Header
564 BasicBlock *InsertTop = SplitEdge(PreHeader, Header, DT, LI);
565 BasicBlock *InsertBot =
566 SplitBlock(InsertTop, InsertTop->getTerminator(), DT, LI);
567 BasicBlock *NewPreHeader =
568 SplitBlock(InsertBot, InsertBot->getTerminator(), DT, LI);
570 InsertTop->setName(Header->getName() + ".peel.begin");
571 InsertBot->setName(Header->getName() + ".peel.next");
572 NewPreHeader->setName(PreHeader->getName() + ".peel.newph");
574 ValueToValueMapTy LVMap;
576 // If we have branch weight information, we'll want to update it for the
577 // newly created branches.
578 BranchInst *LatchBR =
579 cast<BranchInst>(cast<BasicBlock>(Latch)->getTerminator());
580 unsigned HeaderIdx = (LatchBR->getSuccessor(0) == Header ? 0 : 1);
582 uint64_t TrueWeight, FalseWeight;
583 uint64_t ExitWeight = 0, CurHeaderWeight = 0;
584 if (LatchBR->extractProfMetadata(TrueWeight, FalseWeight)) {
585 ExitWeight = HeaderIdx ? TrueWeight : FalseWeight;
586 // The # of times the loop body executes is the sum of the exit block
587 // weight and the # of times the backedges are taken.
588 CurHeaderWeight = TrueWeight + FalseWeight;
591 // For each peeled-off iteration, make a copy of the loop.
592 for (unsigned Iter = 0; Iter < PeelCount; ++Iter) {
593 SmallVector<BasicBlock *, 8> NewBlocks;
594 ValueToValueMapTy VMap;
596 // Subtract the exit weight from the current header weight -- the exit
597 // weight is exactly the weight of the previous iteration's header.
598 // FIXME: due to the way the distribution is constructed, we need a
599 // guard here to make sure we don't end up with non-positive weights.
600 if (ExitWeight < CurHeaderWeight)
601 CurHeaderWeight -= ExitWeight;
605 cloneLoopBlocks(L, Iter, InsertTop, InsertBot, Exit,
606 NewBlocks, LoopBlocks, VMap, LVMap, DT, LI);
608 // Remap to use values from the current iteration instead of the
610 remapInstructionsInBlocks(NewBlocks, VMap);
613 // Latches of the cloned loops dominate over the loop exit, so idom of the
614 // latter is the first cloned loop body, as original PreHeader dominates
615 // the original loop body.
617 DT->changeImmediateDominator(Exit, cast<BasicBlock>(LVMap[Latch]));
618 #ifdef EXPENSIVE_CHECKS
619 assert(DT->verify(DominatorTree::VerificationLevel::Fast));
623 auto *LatchBRCopy = cast<BranchInst>(VMap[LatchBR]);
624 updateBranchWeights(InsertBot, LatchBRCopy, Iter,
625 PeelCount, ExitWeight);
626 // Remove Loop metadata from the latch branch instruction
627 // because it is not the Loop's latch branch anymore.
628 LatchBRCopy->setMetadata(LLVMContext::MD_loop, nullptr);
630 InsertTop = InsertBot;
631 InsertBot = SplitBlock(InsertBot, InsertBot->getTerminator(), DT, LI);
632 InsertBot->setName(Header->getName() + ".peel.next");
634 F->getBasicBlockList().splice(InsertTop->getIterator(),
635 F->getBasicBlockList(),
636 NewBlocks[0]->getIterator(), F->end());
639 // Now adjust the phi nodes in the loop header to get their initial values
640 // from the last peeled-off iteration instead of the preheader.
641 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
642 PHINode *PHI = cast<PHINode>(I);
643 Value *NewVal = PHI->getIncomingValueForBlock(Latch);
644 Instruction *LatchInst = dyn_cast<Instruction>(NewVal);
645 if (LatchInst && L->contains(LatchInst))
646 NewVal = LVMap[LatchInst];
648 PHI->setIncomingValue(PHI->getBasicBlockIndex(NewPreHeader), NewVal);
651 // Adjust the branch weights on the loop exit.
653 // The backedge count is the difference of current header weight and
654 // current loop exit weight. If the current header weight is smaller than
655 // the current loop exit weight, we mark the loop backedge weight as 1.
656 uint64_t BackEdgeWeight = 0;
657 if (ExitWeight < CurHeaderWeight)
658 BackEdgeWeight = CurHeaderWeight - ExitWeight;
661 MDBuilder MDB(LatchBR->getContext());
663 HeaderIdx ? MDB.createBranchWeights(ExitWeight, BackEdgeWeight)
664 : MDB.createBranchWeights(BackEdgeWeight, ExitWeight);
665 LatchBR->setMetadata(LLVMContext::MD_prof, WeightNode);
668 // If the loop is nested, we changed the parent loop, update SE.
669 if (Loop *ParentLoop = L->getParentLoop()) {
670 SE->forgetLoop(ParentLoop);
672 // FIXME: Incrementally update loop-simplify
673 simplifyLoop(ParentLoop, DT, LI, SE, AC, PreserveLCSSA);
675 // FIXME: Incrementally update loop-simplify
676 simplifyLoop(L, DT, LI, SE, AC, PreserveLCSSA);