1 //===- LoopDistribute.cpp - Loop Distribution Pass ------------------------===//
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 the Loop Distribution Pass. Its main focus is to
11 // distribute loops that cannot be vectorized due to dependence cycles. It
12 // tries to isolate the offending dependences into a new loop allowing
13 // vectorization of the remaining parts.
15 // For dependence analysis, the pass uses the LoopVectorizer's
16 // LoopAccessAnalysis. Because this analysis presumes no change in the order of
17 // memory operations, special care is taken to preserve the lexical order of
20 // Similarly to the Vectorizer, the pass also supports loop versioning to
21 // run-time disambiguate potentially overlapping arrays.
23 //===----------------------------------------------------------------------===//
25 #include "llvm/ADT/DepthFirstIterator.h"
26 #include "llvm/ADT/EquivalenceClasses.h"
27 #include "llvm/ADT/STLExtras.h"
28 #include "llvm/ADT/Statistic.h"
29 #include "llvm/Analysis/LoopAccessAnalysis.h"
30 #include "llvm/Analysis/LoopInfo.h"
31 #include "llvm/IR/Dominators.h"
32 #include "llvm/Pass.h"
33 #include "llvm/Support/CommandLine.h"
34 #include "llvm/Support/Debug.h"
35 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
36 #include "llvm/Transforms/Utils/Cloning.h"
39 #define LDIST_NAME "loop-distribute"
40 #define DEBUG_TYPE LDIST_NAME
45 LDistVerify("loop-distribute-verify", cl::Hidden,
46 cl::desc("Turn on DominatorTree and LoopInfo verification "
47 "after Loop Distribution"),
50 static cl::opt<bool> DistributeNonIfConvertible(
51 "loop-distribute-non-if-convertible", cl::Hidden,
52 cl::desc("Whether to distribute into a loop that may not be "
53 "if-convertible by the loop vectorizer"),
56 STATISTIC(NumLoopsDistributed, "Number of loops distributed");
58 /// \brief Remaps instructions in a loop including the preheader.
59 static void remapInstructionsInLoop(const SmallVectorImpl<BasicBlock *> &Blocks,
60 ValueToValueMapTy &VMap) {
61 // Rewrite the code to refer to itself.
62 for (auto *BB : Blocks)
63 for (auto &Inst : *BB)
64 RemapInstruction(&Inst, VMap,
65 RF_NoModuleLevelChanges | RF_IgnoreMissingEntries);
68 /// \brief Clones a loop \p OrigLoop. Returns the loop and the blocks in \p
71 /// Updates LoopInfo and DominatorTree assuming the loop is dominated by block
72 /// \p LoopDomBB. Insert the new blocks before block specified in \p Before.
73 static Loop *cloneLoopWithPreheader(BasicBlock *Before, BasicBlock *LoopDomBB,
74 Loop *OrigLoop, ValueToValueMapTy &VMap,
75 const Twine &NameSuffix, LoopInfo *LI,
77 SmallVectorImpl<BasicBlock *> &Blocks) {
78 Function *F = OrigLoop->getHeader()->getParent();
79 Loop *ParentLoop = OrigLoop->getParentLoop();
81 Loop *NewLoop = new Loop();
83 ParentLoop->addChildLoop(NewLoop);
85 LI->addTopLevelLoop(NewLoop);
87 BasicBlock *OrigPH = OrigLoop->getLoopPreheader();
88 BasicBlock *NewPH = CloneBasicBlock(OrigPH, VMap, NameSuffix, F);
89 // To rename the loop PHIs.
91 Blocks.push_back(NewPH);
95 ParentLoop->addBasicBlockToLoop(NewPH, *LI);
97 // Update DominatorTree.
98 DT->addNewBlock(NewPH, LoopDomBB);
100 for (BasicBlock *BB : OrigLoop->getBlocks()) {
101 BasicBlock *NewBB = CloneBasicBlock(BB, VMap, NameSuffix, F);
105 NewLoop->addBasicBlockToLoop(NewBB, *LI);
107 // Update DominatorTree.
108 BasicBlock *IDomBB = DT->getNode(BB)->getIDom()->getBlock();
109 DT->addNewBlock(NewBB, cast<BasicBlock>(VMap[IDomBB]));
111 Blocks.push_back(NewBB);
114 // Move them physically from the end of the block list.
115 F->getBasicBlockList().splice(Before, F->getBasicBlockList(), NewPH);
116 F->getBasicBlockList().splice(Before, F->getBasicBlockList(),
117 NewLoop->getHeader(), F->end());
123 /// \brief Maintains the set of instructions of the loop for a partition before
124 /// cloning. After cloning, it hosts the new loop.
125 class InstPartition {
126 typedef SmallPtrSet<Instruction *, 8> InstructionSet;
129 InstPartition(Instruction *I, Loop *L, bool DepCycle = false)
130 : DepCycle(DepCycle), OrigLoop(L), ClonedLoop(nullptr) {
134 /// \brief Returns whether this partition contains a dependence cycle.
135 bool hasDepCycle() const { return DepCycle; }
137 /// \brief Adds an instruction to this partition.
138 void add(Instruction *I) { Set.insert(I); }
140 /// \brief Collection accessors.
141 InstructionSet::iterator begin() { return Set.begin(); }
142 InstructionSet::iterator end() { return Set.end(); }
143 InstructionSet::const_iterator begin() const { return Set.begin(); }
144 InstructionSet::const_iterator end() const { return Set.end(); }
145 bool empty() const { return Set.empty(); }
147 /// \brief Moves this partition into \p Other. This partition becomes empty
149 void moveTo(InstPartition &Other) {
150 Other.Set.insert(Set.begin(), Set.end());
152 Other.DepCycle |= DepCycle;
155 /// \brief Populates the partition with a transitive closure of all the
156 /// instructions that the seeded instructions dependent on.
157 void populateUsedSet() {
158 // FIXME: We currently don't use control-dependence but simply include all
159 // blocks (possibly empty at the end) and let simplifycfg mostly clean this
161 for (auto *B : OrigLoop->getBlocks())
162 Set.insert(B->getTerminator());
164 // Follow the use-def chains to form a transitive closure of all the
165 // instructions that the originally seeded instructions depend on.
166 SmallVector<Instruction *, 8> Worklist(Set.begin(), Set.end());
167 while (!Worklist.empty()) {
168 Instruction *I = Worklist.pop_back_val();
169 // Insert instructions from the loop that we depend on.
170 for (Value *V : I->operand_values()) {
171 auto *I = dyn_cast<Instruction>(V);
172 if (I && OrigLoop->contains(I->getParent()) && Set.insert(I).second)
173 Worklist.push_back(I);
178 /// \brief Clones the original loop.
180 /// Updates LoopInfo and DominatorTree using the information that block \p
181 /// LoopDomBB dominates the loop.
182 Loop *cloneLoopWithPreheader(BasicBlock *InsertBefore, BasicBlock *LoopDomBB,
183 unsigned Index, LoopInfo *LI,
185 ClonedLoop = ::cloneLoopWithPreheader(InsertBefore, LoopDomBB, OrigLoop,
186 VMap, Twine(".ldist") + Twine(Index),
187 LI, DT, ClonedLoopBlocks);
191 /// \brief The cloned loop. If this partition is mapped to the original loop,
193 const Loop *getClonedLoop() const { return ClonedLoop; }
195 /// \brief Returns the loop where this partition ends up after distribution.
196 /// If this partition is mapped to the original loop then use the block from
198 const Loop *getDistributedLoop() const {
199 return ClonedLoop ? ClonedLoop : OrigLoop;
202 /// \brief The VMap that is populated by cloning and then used in
203 /// remapinstruction to remap the cloned instructions.
204 ValueToValueMapTy &getVMap() { return VMap; }
206 /// \brief Remaps the cloned instructions using VMap.
207 void remapInstructions() { remapInstructionsInLoop(ClonedLoopBlocks, VMap); }
209 /// \brief Based on the set of instructions selected for this partition,
210 /// removes the unnecessary ones.
211 void removeUnusedInsts() {
212 SmallVector<Instruction *, 8> Unused;
214 for (auto *Block : OrigLoop->getBlocks())
215 for (auto &Inst : *Block)
216 if (!Set.count(&Inst)) {
217 Instruction *NewInst = &Inst;
219 NewInst = cast<Instruction>(VMap[NewInst]);
221 assert(!isa<BranchInst>(NewInst) &&
222 "Branches are marked used early on");
223 Unused.push_back(NewInst);
226 // Delete the instructions backwards, as it has a reduced likelihood of
227 // having to update as many def-use and use-def chains.
228 for (auto I = Unused.rbegin(), E = Unused.rend(); I != E; ++I) {
231 if (!Inst->use_empty())
232 Inst->replaceAllUsesWith(UndefValue::get(Inst->getType()));
233 Inst->eraseFromParent();
239 dbgs() << " (cycle)\n";
241 // Prefix with the block name.
242 dbgs() << " " << I->getParent()->getName() << ":" << *I << "\n";
245 void printBlocks() const {
246 for (auto *BB : getDistributedLoop()->getBlocks())
251 /// \brief Instructions from OrigLoop selected for this partition.
254 /// \brief Whether this partition contains a dependence cycle.
257 /// \brief The original loop.
260 /// \brief The cloned loop. If this partition is mapped to the original loop,
264 /// \brief The blocks of ClonedLoop including the preheader. If this
265 /// partition is mapped to the original loop, this is empty.
266 SmallVector<BasicBlock *, 8> ClonedLoopBlocks;
268 /// \brief These gets populated once the set of instructions have been
269 /// finalized. If this partition is mapped to the original loop, these are not
271 ValueToValueMapTy VMap;
274 /// \brief Holds the set of Partitions. It populates them, merges them and then
275 /// clones the loops.
276 class InstPartitionContainer {
277 typedef DenseMap<Instruction *, int> InstToPartitionIdT;
280 InstPartitionContainer(Loop *L, LoopInfo *LI, DominatorTree *DT)
281 : L(L), LI(LI), DT(DT) {}
283 /// \brief Returns the number of partitions.
284 unsigned getSize() const { return PartitionContainer.size(); }
286 /// \brief Adds \p Inst into the current partition if that is marked to
287 /// contain cycles. Otherwise start a new partition for it.
288 void addToCyclicPartition(Instruction *Inst) {
289 // If the current partition is non-cyclic. Start a new one.
290 if (PartitionContainer.empty() || !PartitionContainer.back()->hasDepCycle())
291 PartitionContainer.push_back(
292 llvm::make_unique<InstPartition>(Inst, L, true));
294 PartitionContainer.back()->add(Inst);
297 /// \brief Adds \p Inst into a partition that is not marked to contain
298 /// dependence cycles.
300 // Initially we isolate memory instructions into as many partitions as
301 // possible, then later we may merge them back together.
302 void addToNewNonCyclicPartition(Instruction *Inst) {
303 PartitionContainer.push_back(llvm::make_unique<InstPartition>(Inst, L));
306 /// \brief Merges adjacent non-cyclic partitions.
308 /// The idea is that we currently only want to isolate the non-vectorizable
309 /// partition. We could later allow more distribution among these partition
311 void mergeAdjacentNonCyclic() {
312 mergeAdjacentPartitionsIf(
313 [](const InstPartition *P) { return !P->hasDepCycle(); });
316 /// \brief If a partition contains only conditional stores, we won't vectorize
317 /// it. Try to merge it with a previous cyclic partition.
318 void mergeNonIfConvertible() {
319 mergeAdjacentPartitionsIf([&](const InstPartition *Partition) {
320 if (Partition->hasDepCycle())
323 // Now, check if all stores are conditional in this partition.
324 bool seenStore = false;
326 for (auto *Inst : *Partition)
327 if (isa<StoreInst>(Inst)) {
329 if (!LoopAccessInfo::blockNeedsPredication(Inst->getParent(), L, DT))
336 /// \brief Merges the partitions according to various heuristics.
337 void mergeBeforePopulating() {
338 mergeAdjacentNonCyclic();
339 if (!DistributeNonIfConvertible)
340 mergeNonIfConvertible();
343 /// \brief Merges partitions in order to ensure that no loads are duplicated.
345 /// We can't duplicate loads because that could potentially reorder them.
346 /// LoopAccessAnalysis provides dependency information with the context that
347 /// the order of memory operation is preserved.
349 /// Return if any partitions were merged.
350 bool mergeToAvoidDuplicatedLoads() {
351 typedef DenseMap<Instruction *, InstPartition *> LoadToPartitionT;
352 typedef EquivalenceClasses<InstPartition *> ToBeMergedT;
354 LoadToPartitionT LoadToPartition;
355 ToBeMergedT ToBeMerged;
357 // Step through the partitions and create equivalence between partitions
358 // that contain the same load. Also put partitions in between them in the
359 // same equivalence class to avoid reordering of memory operations.
360 for (PartitionContainerT::iterator I = PartitionContainer.begin(),
361 E = PartitionContainer.end();
363 auto *PartI = I->get();
365 // If a load occurs in two partitions PartI and PartJ, merge all
366 // partitions (PartI, PartJ] into PartI.
367 for (Instruction *Inst : *PartI)
368 if (isa<LoadInst>(Inst)) {
370 LoadToPartitionT::iterator LoadToPart;
372 std::tie(LoadToPart, NewElt) =
373 LoadToPartition.insert(std::make_pair(Inst, PartI));
375 DEBUG(dbgs() << "Merging partitions due to this load in multiple "
376 << "partitions: " << PartI << ", "
377 << LoadToPart->second << "\n" << *Inst << "\n");
382 ToBeMerged.unionSets(PartI, PartJ->get());
383 } while (PartJ->get() != LoadToPart->second);
387 if (ToBeMerged.empty())
390 // Merge the member of an equivalence class into its class leader. This
391 // makes the members empty.
392 for (ToBeMergedT::iterator I = ToBeMerged.begin(), E = ToBeMerged.end();
397 auto PartI = I->getData();
398 for (auto PartJ : make_range(std::next(ToBeMerged.member_begin(I)),
399 ToBeMerged.member_end())) {
400 PartJ->moveTo(*PartI);
404 // Remove the empty partitions.
405 for (PartitionContainerT::iterator PartI = PartitionContainer.begin(),
406 E = PartitionContainer.end();
408 if ((*PartI)->empty())
409 PartI = PartitionContainer.erase(PartI);
416 /// \brief Sets up the mapping between instructions to partitions. If the
417 /// instruction is duplicated across multiple partitions, set the entry to -1.
418 void setupPartitionIdOnInstructions() {
420 for (auto &PartitionPtr : PartitionContainer) {
421 for (Instruction *Inst : *PartitionPtr) {
423 InstToPartitionIdT::iterator Iter;
425 std::tie(Iter, NewElt) =
426 InstToPartitionId.insert(std::make_pair(Inst, PartitionID));
434 /// \brief Populates the partition with everything that the seeding
435 /// instructions require.
436 void populateUsedSet() {
437 for (auto &P : PartitionContainer)
438 P->populateUsedSet();
441 /// \brief This performs the main chunk of the work of cloning the loops for
443 void cloneLoops(Pass *P) {
444 BasicBlock *OrigPH = L->getLoopPreheader();
445 // At this point the predecessor of the preheader is either the memcheck
446 // block or the top part of the original preheader.
447 BasicBlock *Pred = OrigPH->getSinglePredecessor();
448 assert(Pred && "Preheader does not have a single predecessor");
449 BasicBlock *ExitBlock = L->getExitBlock();
450 assert(ExitBlock && "No single exit block");
453 assert(!PartitionContainer.empty() && "at least two partitions expected");
454 // We're cloning the preheader along with the loop so we already made sure
456 assert(&*OrigPH->begin() == OrigPH->getTerminator() &&
457 "preheader not empty");
459 // Create a loop for each partition except the last. Clone the original
460 // loop before PH along with adding a preheader for the cloned loop. Then
461 // update PH to point to the newly added preheader.
462 BasicBlock *TopPH = OrigPH;
463 unsigned Index = getSize() - 1;
464 for (auto I = std::next(PartitionContainer.crbegin()),
465 E = PartitionContainer.crend();
466 I != E; ++I, --Index, TopPH = NewLoop->getLoopPreheader()) {
469 NewLoop = Part->cloneLoopWithPreheader(TopPH, Pred, Index, LI, DT);
471 Part->getVMap()[ExitBlock] = TopPH;
472 Part->remapInstructions();
474 Pred->getTerminator()->replaceUsesOfWith(OrigPH, TopPH);
476 // Now go in forward order and update the immediate dominator for the
477 // preheaders with the exiting block of the previous loop. Dominance
478 // within the loop is updated in cloneLoopWithPreheader.
479 for (auto Curr = PartitionContainer.cbegin(),
480 Next = std::next(PartitionContainer.cbegin()),
481 E = PartitionContainer.cend();
482 Next != E; ++Curr, ++Next)
483 DT->changeImmediateDominator(
484 (*Next)->getDistributedLoop()->getLoopPreheader(),
485 (*Curr)->getDistributedLoop()->getExitingBlock());
488 /// \brief Removes the dead instructions from the cloned loops.
489 void removeUnusedInsts() {
490 for (auto &PartitionPtr : PartitionContainer)
491 PartitionPtr->removeUnusedInsts();
494 /// \brief For each memory pointer, it computes the partitionId the pointer is
497 /// This returns an array of int where the I-th entry corresponds to I-th
498 /// entry in LAI.getRuntimePointerCheck(). If the pointer is used in multiple
499 /// partitions its entry is set to -1.
501 computePartitionSetForPointers(const LoopAccessInfo &LAI) {
502 const LoopAccessInfo::RuntimePointerCheck *RtPtrCheck =
503 LAI.getRuntimePointerCheck();
505 unsigned N = RtPtrCheck->Pointers.size();
506 SmallVector<int, 8> PtrToPartitions(N);
507 for (unsigned I = 0; I < N; ++I) {
508 Value *Ptr = RtPtrCheck->Pointers[I];
510 LAI.getInstructionsForAccess(Ptr, RtPtrCheck->IsWritePtr[I]);
512 int &Partition = PtrToPartitions[I];
513 // First set it to uninitialized.
515 for (Instruction *Inst : Instructions) {
516 // Note that this could be -1 if Inst is duplicated across multiple
518 int ThisPartition = this->InstToPartitionId[Inst];
520 Partition = ThisPartition;
521 // -1 means belonging to multiple partitions.
522 else if (Partition == -1)
524 else if (Partition != (int)ThisPartition)
527 assert(Partition != -2 && "Pointer not belonging to any partition");
530 return PtrToPartitions;
533 void print(raw_ostream &OS) const {
535 for (auto &P : PartitionContainer) {
536 OS << "Partition " << Index++ << " (" << P.get() << "):\n";
541 void dump() const { print(dbgs()); }
544 friend raw_ostream &operator<<(raw_ostream &OS,
545 const InstPartitionContainer &Partitions) {
546 Partitions.print(OS);
551 void printBlocks() const {
553 for (auto &P : PartitionContainer) {
554 dbgs() << "\nPartition " << Index++ << " (" << P.get() << "):\n";
560 typedef std::list<std::unique_ptr<InstPartition>> PartitionContainerT;
562 /// \brief List of partitions.
563 PartitionContainerT PartitionContainer;
565 /// \brief Mapping from Instruction to partition Id. If the instruction
566 /// belongs to multiple partitions the entry contains -1.
567 InstToPartitionIdT InstToPartitionId;
573 /// \brief The control structure to merge adjacent partitions if both satisfy
574 /// the \p Predicate.
575 template <class UnaryPredicate>
576 void mergeAdjacentPartitionsIf(UnaryPredicate Predicate) {
577 InstPartition *PrevMatch = nullptr;
578 for (auto I = PartitionContainer.begin(); I != PartitionContainer.end();) {
579 auto DoesMatch = Predicate(I->get());
580 if (PrevMatch == nullptr && DoesMatch) {
581 PrevMatch = I->get();
583 } else if (PrevMatch != nullptr && DoesMatch) {
584 (*I)->moveTo(*PrevMatch);
585 I = PartitionContainer.erase(I);
594 /// \brief For each memory instruction, this class maintains difference of the
595 /// number of unsafe dependences that start out from this instruction minus
596 /// those that end here.
598 /// By traversing the memory instructions in program order and accumulating this
599 /// number, we know whether any unsafe dependence crosses over a program point.
600 class MemoryInstructionDependences {
601 typedef MemoryDepChecker::Dependence Dependence;
606 unsigned NumUnsafeDependencesStartOrEnd;
608 Entry(Instruction *Inst) : Inst(Inst), NumUnsafeDependencesStartOrEnd(0) {}
611 typedef SmallVector<Entry, 8> AccessesType;
613 AccessesType::const_iterator begin() const { return Accesses.begin(); }
614 AccessesType::const_iterator end() const { return Accesses.end(); }
616 MemoryInstructionDependences(
617 const SmallVectorImpl<Instruction *> &Instructions,
618 const SmallVectorImpl<Dependence> &InterestingDependences) {
619 std::transform(Instructions.begin(), Instructions.end(),
620 std::back_inserter(Accesses),
621 [](Instruction *Inst) { return Entry(Inst); });
623 DEBUG(dbgs() << "Backward dependences:\n");
624 for (auto &Dep : InterestingDependences)
625 if (Dep.isPossiblyBackward()) {
626 // Note that the designations source and destination follow the program
627 // order, i.e. source is always first. (The direction is given by the
629 ++Accesses[Dep.Source].NumUnsafeDependencesStartOrEnd;
630 --Accesses[Dep.Destination].NumUnsafeDependencesStartOrEnd;
632 DEBUG(Dep.print(dbgs(), 2, Instructions));
637 AccessesType Accesses;
640 /// \brief Handles the loop versioning based on memchecks.
641 class RuntimeCheckEmitter {
643 RuntimeCheckEmitter(const LoopAccessInfo &LAI, Loop *L, LoopInfo *LI,
645 : OrigLoop(L), NonDistributedLoop(nullptr), LAI(LAI), LI(LI), DT(DT) {}
647 /// \brief Given the \p Partitions formed by Loop Distribution, it determines
648 /// in which partition each pointer is used.
649 void partitionPointers(InstPartitionContainer &Partitions) {
650 // Set up partition id in PtrRtChecks. Ptr -> Access -> Intruction ->
652 PtrToPartition = Partitions.computePartitionSetForPointers(LAI);
654 DEBUG(dbgs() << "\nPointers:\n");
655 DEBUG(LAI.getRuntimePointerCheck()->print(dbgs(), 0, &PtrToPartition));
658 /// \brief Returns true if we need memchecks to distribute the loop.
659 bool needsRuntimeChecks() const {
660 return LAI.getRuntimePointerCheck()->needsAnyChecking(&PtrToPartition);
663 /// \brief Performs the CFG manipulation part of versioning the loop including
664 /// the DominatorTree and LoopInfo updates.
665 void versionLoop(Pass *P) {
666 Instruction *FirstCheckInst;
667 Instruction *MemRuntimeCheck;
668 // Add the memcheck in the original preheader (this is empty initially).
669 BasicBlock *MemCheckBB = OrigLoop->getLoopPreheader();
670 std::tie(FirstCheckInst, MemRuntimeCheck) =
671 LAI.addRuntimeCheck(MemCheckBB->getTerminator(), &PtrToPartition);
672 assert(MemRuntimeCheck && "called even though needsAnyChecking = false");
674 // Rename the block to make the IR more readable.
675 MemCheckBB->setName(OrigLoop->getHeader()->getName() + ".ldist.memcheck");
677 // Create empty preheader for the loop (and after cloning for the
678 // original/nondist loop).
680 SplitBlock(MemCheckBB, MemCheckBB->getTerminator(), DT, LI);
681 PH->setName(OrigLoop->getHeader()->getName() + ".ph");
683 // Clone the loop including the preheader.
685 // FIXME: This does not currently preserve SimplifyLoop because the exit
686 // block is a join between the two loops.
687 SmallVector<BasicBlock *, 8> NonDistributedLoopBlocks;
689 cloneLoopWithPreheader(PH, MemCheckBB, OrigLoop, VMap, ".ldist.nondist",
690 LI, DT, NonDistributedLoopBlocks);
691 remapInstructionsInLoop(NonDistributedLoopBlocks, VMap);
693 // Insert the conditional branch based on the result of the memchecks.
694 Instruction *OrigTerm = MemCheckBB->getTerminator();
695 BranchInst::Create(NonDistributedLoop->getLoopPreheader(),
696 OrigLoop->getLoopPreheader(), MemRuntimeCheck, OrigTerm);
697 OrigTerm->eraseFromParent();
699 // The loops merge in the original exit block. This is now dominated by the
700 // memchecking block.
701 DT->changeImmediateDominator(OrigLoop->getExitBlock(), MemCheckBB);
704 /// \brief Adds the necessary PHI nodes for the versioned loops based on the
705 /// loop-defined values used outside of the loop.
706 void addPHINodes(const SmallVectorImpl<Instruction *> &DefsUsedOutside) {
707 BasicBlock *PHIBlock = OrigLoop->getExitBlock();
708 assert(PHIBlock && "No single successor to loop exit block");
710 for (auto *Inst : DefsUsedOutside) {
711 auto *NonDistInst = cast<Instruction>(VMap[Inst]);
713 BasicBlock::iterator I;
715 // First see if we have a single-operand PHI with the value defined by the
717 for (I = PHIBlock->begin(); (PN = dyn_cast<PHINode>(I)); ++I) {
718 assert(PN->getNumOperands() == 1 &&
719 "Exit block should only have on predecessor");
720 if (PN->getIncomingValue(0) == Inst)
725 PN = PHINode::Create(Inst->getType(), 2, Inst->getName() + ".ldist",
727 for (auto *User : Inst->users())
728 if (!OrigLoop->contains(cast<Instruction>(User)->getParent()))
729 User->replaceUsesOfWith(Inst, PN);
730 PN->addIncoming(Inst, OrigLoop->getExitingBlock());
732 // Add the new incoming value from the non-distributed loop.
733 PN->addIncoming(NonDistInst, NonDistributedLoop->getExitingBlock());
738 /// \brief The original loop. This becomes the "versioned" one, i.e. control
739 /// goes if the memchecks all pass.
741 /// \brief The fall-back loop, i.e. if any of the memchecks fail.
742 Loop *NonDistributedLoop;
744 /// \brief For each memory pointer it contains the partitionId it is used in.
746 /// The I-th entry corresponds to I-th entry in LAI.getRuntimePointerCheck().
747 /// If the pointer is used in multiple partitions the entry is set to -1.
748 SmallVector<int, 8> PtrToPartition;
750 /// \brief This maps the instructions from OrigLoop to their counterpart in
751 /// NonDistributedLoop.
752 ValueToValueMapTy VMap;
754 /// \brief Analyses used.
755 const LoopAccessInfo &LAI;
760 /// \brief Returns the instructions that use values defined in the loop.
761 static SmallVector<Instruction *, 8> findDefsUsedOutsideOfLoop(Loop *L) {
762 SmallVector<Instruction *, 8> UsedOutside;
764 for (auto *Block : L->getBlocks())
765 // FIXME: I believe that this could use copy_if if the Inst reference could
766 // be adapted into a pointer.
767 for (auto &Inst : *Block) {
768 auto Users = Inst.users();
769 if (std::any_of(Users.begin(), Users.end(), [&](User *U) {
770 auto *Use = cast<Instruction>(U);
771 return !L->contains(Use->getParent());
773 UsedOutside.push_back(&Inst);
779 /// \brief The pass class.
780 class LoopDistribute : public FunctionPass {
782 LoopDistribute() : FunctionPass(ID) {
783 initializeLoopDistributePass(*PassRegistry::getPassRegistry());
786 bool runOnFunction(Function &F) override {
787 LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
788 LAA = &getAnalysis<LoopAccessAnalysis>();
789 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
791 // Build up a worklist of inner-loops to vectorize. This is necessary as the
792 // act of distributing a loop creates new loops and can invalidate iterators
794 SmallVector<Loop *, 8> Worklist;
796 for (Loop *TopLevelLoop : *LI)
797 for (Loop *L : depth_first(TopLevelLoop))
798 // We only handle inner-most loops.
800 Worklist.push_back(L);
802 // Now walk the identified inner loops.
803 bool Changed = false;
804 for (Loop *L : Worklist)
805 Changed |= processLoop(L);
807 // Process each loop nest in the function.
811 void getAnalysisUsage(AnalysisUsage &AU) const override {
812 AU.addRequired<LoopInfoWrapperPass>();
813 AU.addPreserved<LoopInfoWrapperPass>();
814 AU.addRequired<LoopAccessAnalysis>();
815 AU.addRequired<DominatorTreeWrapperPass>();
816 AU.addPreserved<DominatorTreeWrapperPass>();
822 /// \brief Try to distribute an inner-most loop.
823 bool processLoop(Loop *L) {
824 assert(L->empty() && "Only process inner loops.");
826 DEBUG(dbgs() << "\nLDist: In \"" << L->getHeader()->getParent()->getName()
827 << "\" checking " << *L << "\n");
829 BasicBlock *PH = L->getLoopPreheader();
831 DEBUG(dbgs() << "Skipping; no preheader");
834 if (!L->getExitBlock()) {
835 DEBUG(dbgs() << "Skipping; multiple exit blocks");
838 // LAA will check that we only have a single exiting block.
840 const LoopAccessInfo &LAI = LAA->getInfo(L, ValueToValueMap());
842 // Currently, we only distribute to isolate the part of the loop with
843 // dependence cycles to enable partial vectorization.
844 if (LAI.canVectorizeMemory()) {
845 DEBUG(dbgs() << "Skipping; memory operations are safe for vectorization");
848 auto *InterestingDependences =
849 LAI.getDepChecker().getInterestingDependences();
850 if (!InterestingDependences || InterestingDependences->empty()) {
851 DEBUG(dbgs() << "Skipping; No unsafe dependences to isolate");
855 InstPartitionContainer Partitions(L, LI, DT);
857 // First, go through each memory operation and assign them to consecutive
858 // partitions (the order of partitions follows program order). Put those
859 // with unsafe dependences into "cyclic" partition otherwise put each store
860 // in its own "non-cyclic" partition (we'll merge these later).
862 // Note that a memory operation (e.g. Load2 below) at a program point that
863 // has an unsafe dependence (Store3->Load1) spanning over it must be
864 // included in the same cyclic partition as the dependent operations. This
865 // is to preserve the original program order after distribution. E.g.:
867 // NumUnsafeDependencesStartOrEnd NumUnsafeDependencesActive
869 // Load2 | /Unsafe/ 0 1
873 // NumUnsafeDependencesActive > 0 indicates this situation and in this case
874 // we just keep assigning to the same cyclic partition until
875 // NumUnsafeDependencesActive reaches 0.
876 const MemoryDepChecker &DepChecker = LAI.getDepChecker();
877 MemoryInstructionDependences MID(DepChecker.getMemoryInstructions(),
878 *InterestingDependences);
880 int NumUnsafeDependencesActive = 0;
881 for (auto &InstDep : MID) {
882 Instruction *I = InstDep.Inst;
883 // We update NumUnsafeDependencesActive post-instruction, catch the
884 // start of a dependence directly via NumUnsafeDependencesStartOrEnd.
885 if (NumUnsafeDependencesActive ||
886 InstDep.NumUnsafeDependencesStartOrEnd > 0)
887 Partitions.addToCyclicPartition(I);
889 Partitions.addToNewNonCyclicPartition(I);
890 NumUnsafeDependencesActive += InstDep.NumUnsafeDependencesStartOrEnd;
891 assert(NumUnsafeDependencesActive >= 0 &&
892 "Negative number of dependences active");
895 // Add partitions for values used outside. These partitions can be out of
896 // order from the original program order. This is OK because if the
897 // partition uses a load we will merge this partition with the original
898 // partition of the load that we set up in the previous loop (see
899 // mergeToAvoidDuplicatedLoads).
900 auto DefsUsedOutside = findDefsUsedOutsideOfLoop(L);
901 for (auto *Inst : DefsUsedOutside)
902 Partitions.addToNewNonCyclicPartition(Inst);
904 DEBUG(dbgs() << "Seeded partitions:\n" << Partitions);
905 if (Partitions.getSize() < 2)
908 // Run the merge heuristics: Merge non-cyclic adjacent partitions since we
909 // should be able to vectorize these together.
910 Partitions.mergeBeforePopulating();
911 DEBUG(dbgs() << "\nMerged partitions:\n" << Partitions);
912 if (Partitions.getSize() < 2)
915 // Now, populate the partitions with non-memory operations.
916 Partitions.populateUsedSet();
917 DEBUG(dbgs() << "\nPopulated partitions:\n" << Partitions);
919 // In order to preserve original lexical order for loads, keep them in the
920 // partition that we set up in the MemoryInstructionDependences loop.
921 if (Partitions.mergeToAvoidDuplicatedLoads()) {
922 DEBUG(dbgs() << "\nPartitions merged to ensure unique loads:\n"
924 if (Partitions.getSize() < 2)
928 DEBUG(dbgs() << "\nDistributing loop: " << *L << "\n");
929 // We're done forming the partitions set up the reverse mapping from
930 // instructions to partitions.
931 Partitions.setupPartitionIdOnInstructions();
933 // To keep things simple have an empty preheader before we version or clone
934 // the loop. (Also split if this has no predecessor, i.e. entry, because we
935 // rely on PH having a predecessor.)
936 if (!PH->getSinglePredecessor() || &*PH->begin() != PH->getTerminator())
937 SplitBlock(PH, PH->getTerminator(), DT, LI);
939 // If we need run-time checks to disambiguate pointers are run-time, version
941 RuntimeCheckEmitter RtCheckEmitter(LAI, L, LI, DT);
942 RtCheckEmitter.partitionPointers(Partitions);
943 if (RtCheckEmitter.needsRuntimeChecks()) {
944 RtCheckEmitter.versionLoop(this);
945 RtCheckEmitter.addPHINodes(DefsUsedOutside);
948 // Create identical copies of the original loop for each partition and hook
949 // them up sequentially.
950 Partitions.cloneLoops(this);
952 // Now, we remove the instruction from each loop that don't belong to that
954 Partitions.removeUnusedInsts();
955 DEBUG(dbgs() << "\nAfter removing unused Instrs:\n");
956 DEBUG(Partitions.printBlocks());
963 ++NumLoopsDistributed;
969 LoopAccessAnalysis *LAA;
972 } // anonymous namespace
974 char LoopDistribute::ID;
975 static const char ldist_name[] = "Loop Distribition";
977 INITIALIZE_PASS_BEGIN(LoopDistribute, LDIST_NAME, ldist_name, false, false)
978 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
979 INITIALIZE_PASS_DEPENDENCY(LoopAccessAnalysis)
980 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
981 INITIALIZE_PASS_END(LoopDistribute, LDIST_NAME, ldist_name, false, false)
984 FunctionPass *createLoopDistributePass() { return new LoopDistribute(); }