1 //===- llvm/Analysis/LoopAccessAnalysis.h -----------------------*- C++ -*-===//
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 defines the interface for the loop memory dependence framework that
11 // was originally developed for the Loop Vectorizer.
13 //===----------------------------------------------------------------------===//
15 #ifndef LLVM_ANALYSIS_LOOPACCESSANALYSIS_H
16 #define LLVM_ANALYSIS_LOOPACCESSANALYSIS_H
18 #include "llvm/ADT/EquivalenceClasses.h"
19 #include "llvm/ADT/Optional.h"
20 #include "llvm/ADT/SetVector.h"
21 #include "llvm/Analysis/AliasAnalysis.h"
22 #include "llvm/Analysis/AliasSetTracker.h"
23 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
24 #include "llvm/IR/ValueHandle.h"
25 #include "llvm/Pass.h"
26 #include "llvm/Support/raw_ostream.h"
32 class ScalarEvolution;
36 /// Optimization analysis message produced during vectorization. Messages inform
37 /// the user why vectorization did not occur.
38 class LoopAccessReport {
40 const Instruction *Instr;
43 LoopAccessReport(const Twine &Message, const Instruction *I)
44 : Message(Message.str()), Instr(I) {}
47 LoopAccessReport(const Instruction *I = nullptr) : Instr(I) {}
49 template <typename A> LoopAccessReport &operator<<(const A &Value) {
50 raw_string_ostream Out(Message);
55 const Instruction *getInstr() const { return Instr; }
57 std::string &str() { return Message; }
58 const std::string &str() const { return Message; }
59 operator Twine() { return Message; }
61 /// \brief Emit an analysis note for \p PassName with the debug location from
62 /// the instruction in \p Message if available. Otherwise use the location of
64 static void emitAnalysis(const LoopAccessReport &Message,
65 const Function *TheFunction,
67 const char *PassName);
70 /// \brief Collection of parameters shared beetween the Loop Vectorizer and the
71 /// Loop Access Analysis.
72 struct VectorizerParams {
73 /// \brief Maximum SIMD width.
74 static const unsigned MaxVectorWidth;
76 /// \brief VF as overridden by the user.
77 static unsigned VectorizationFactor;
78 /// \brief Interleave factor as overridden by the user.
79 static unsigned VectorizationInterleave;
80 /// \brief True if force-vector-interleave was specified by the user.
81 static bool isInterleaveForced();
83 /// \\brief When performing memory disambiguation checks at runtime do not
84 /// make more than this number of comparisons.
85 static unsigned RuntimeMemoryCheckThreshold;
88 /// \brief Checks memory dependences among accesses to the same underlying
89 /// object to determine whether there vectorization is legal or not (and at
90 /// which vectorization factor).
92 /// Note: This class will compute a conservative dependence for access to
93 /// different underlying pointers. Clients, such as the loop vectorizer, will
94 /// sometimes deal these potential dependencies by emitting runtime checks.
96 /// We use the ScalarEvolution framework to symbolically evalutate access
97 /// functions pairs. Since we currently don't restructure the loop we can rely
98 /// on the program order of memory accesses to determine their safety.
99 /// At the moment we will only deem accesses as safe for:
100 /// * A negative constant distance assuming program order.
102 /// Safe: tmp = a[i + 1]; OR a[i + 1] = x;
103 /// a[i] = tmp; y = a[i];
105 /// The latter case is safe because later checks guarantuee that there can't
106 /// be a cycle through a phi node (that is, we check that "x" and "y" is not
107 /// the same variable: a header phi can only be an induction or a reduction, a
108 /// reduction can't have a memory sink, an induction can't have a memory
109 /// source). This is important and must not be violated (or we have to
110 /// resort to checking for cycles through memory).
112 /// * A positive constant distance assuming program order that is bigger
113 /// than the biggest memory access.
115 /// tmp = a[i] OR b[i] = x
116 /// a[i+2] = tmp y = b[i+2];
118 /// Safe distance: 2 x sizeof(a[0]), and 2 x sizeof(b[0]), respectively.
120 /// * Zero distances and all accesses have the same size.
122 class MemoryDepChecker {
124 typedef PointerIntPair<Value *, 1, bool> MemAccessInfo;
125 typedef SmallPtrSet<MemAccessInfo, 8> MemAccessInfoSet;
126 /// \brief Set of potential dependent memory accesses.
127 typedef EquivalenceClasses<MemAccessInfo> DepCandidates;
129 /// \brief Dependece between memory access instructions.
131 /// \brief The type of the dependence.
135 // We couldn't determine the direction or the distance.
137 // Lexically forward.
139 // Forward, but if vectorized, is likely to prevent store-to-load
141 ForwardButPreventsForwarding,
142 // Lexically backward.
144 // Backward, but the distance allows a vectorization factor of
145 // MaxSafeDepDistBytes.
146 BackwardVectorizable,
147 // Same, but may prevent store-to-load forwarding.
148 BackwardVectorizableButPreventsForwarding
151 /// \brief String version of the types.
152 static const char *DepName[];
154 /// \brief Index of the source of the dependence in the InstMap vector.
156 /// \brief Index of the destination of the dependence in the InstMap vector.
157 unsigned Destination;
158 /// \brief The type of the dependence.
161 Dependence(unsigned Source, unsigned Destination, DepType Type)
162 : Source(Source), Destination(Destination), Type(Type) {}
164 /// \brief Dependence types that don't prevent vectorization.
165 static bool isSafeForVectorization(DepType Type);
167 /// \brief Dependence types that can be queried from the analysis.
168 static bool isInterestingDependence(DepType Type);
170 /// \brief Lexically backward dependence types.
171 bool isPossiblyBackward() const;
173 /// \brief Print the dependence. \p Instr is used to map the instruction
174 /// indices to instructions.
175 void print(raw_ostream &OS, unsigned Depth,
176 const SmallVectorImpl<Instruction *> &Instrs) const;
179 MemoryDepChecker(ScalarEvolution *Se, const Loop *L)
180 : SE(Se), InnermostLoop(L), AccessIdx(0),
181 ShouldRetryWithRuntimeCheck(false), SafeForVectorization(true),
182 RecordInterestingDependences(true) {}
184 /// \brief Register the location (instructions are given increasing numbers)
185 /// of a write access.
186 void addAccess(StoreInst *SI) {
187 Value *Ptr = SI->getPointerOperand();
188 Accesses[MemAccessInfo(Ptr, true)].push_back(AccessIdx);
189 InstMap.push_back(SI);
193 /// \brief Register the location (instructions are given increasing numbers)
194 /// of a write access.
195 void addAccess(LoadInst *LI) {
196 Value *Ptr = LI->getPointerOperand();
197 Accesses[MemAccessInfo(Ptr, false)].push_back(AccessIdx);
198 InstMap.push_back(LI);
202 /// \brief Check whether the dependencies between the accesses are safe.
204 /// Only checks sets with elements in \p CheckDeps.
205 bool areDepsSafe(DepCandidates &AccessSets, MemAccessInfoSet &CheckDeps,
206 const ValueToValueMap &Strides);
208 /// \brief No memory dependence was encountered that would inhibit
210 bool isSafeForVectorization() const { return SafeForVectorization; }
212 /// \brief The maximum number of bytes of a vector register we can vectorize
213 /// the accesses safely with.
214 unsigned getMaxSafeDepDistBytes() { return MaxSafeDepDistBytes; }
216 /// \brief In same cases when the dependency check fails we can still
217 /// vectorize the loop with a dynamic array access check.
218 bool shouldRetryWithRuntimeCheck() { return ShouldRetryWithRuntimeCheck; }
220 /// \brief Returns the interesting dependences. If null is returned we
221 /// exceeded the MaxInterestingDependence threshold and this information is
223 const SmallVectorImpl<Dependence> *getInterestingDependences() const {
224 return RecordInterestingDependences ? &InterestingDependences : nullptr;
227 void clearInterestingDependences() { InterestingDependences.clear(); }
229 /// \brief The vector of memory access instructions. The indices are used as
230 /// instruction identifiers in the Dependence class.
231 const SmallVectorImpl<Instruction *> &getMemoryInstructions() const {
235 /// \brief Find the set of instructions that read or write via \p Ptr.
236 SmallVector<Instruction *, 4> getInstructionsForAccess(Value *Ptr,
241 const Loop *InnermostLoop;
243 /// \brief Maps access locations (ptr, read/write) to program order.
244 DenseMap<MemAccessInfo, std::vector<unsigned> > Accesses;
246 /// \brief Memory access instructions in program order.
247 SmallVector<Instruction *, 16> InstMap;
249 /// \brief The program order index to be used for the next instruction.
252 // We can access this many bytes in parallel safely.
253 unsigned MaxSafeDepDistBytes;
255 /// \brief If we see a non-constant dependence distance we can still try to
256 /// vectorize this loop with runtime checks.
257 bool ShouldRetryWithRuntimeCheck;
259 /// \brief No memory dependence was encountered that would inhibit
261 bool SafeForVectorization;
263 //// \brief True if InterestingDependences reflects the dependences in the
264 //// loop. If false we exceeded MaxInterestingDependence and
265 //// InterestingDependences is invalid.
266 bool RecordInterestingDependences;
268 /// \brief Interesting memory dependences collected during the analysis as
269 /// defined by isInterestingDependence. Only valid if
270 /// RecordInterestingDependences is true.
271 SmallVector<Dependence, 8> InterestingDependences;
273 /// \brief Check whether there is a plausible dependence between the two
276 /// Access \p A must happen before \p B in program order. The two indices
277 /// identify the index into the program order map.
279 /// This function checks whether there is a plausible dependence (or the
280 /// absence of such can't be proved) between the two accesses. If there is a
281 /// plausible dependence but the dependence distance is bigger than one
282 /// element access it records this distance in \p MaxSafeDepDistBytes (if this
283 /// distance is smaller than any other distance encountered so far).
284 /// Otherwise, this function returns true signaling a possible dependence.
285 Dependence::DepType isDependent(const MemAccessInfo &A, unsigned AIdx,
286 const MemAccessInfo &B, unsigned BIdx,
287 const ValueToValueMap &Strides);
289 /// \brief Check whether the data dependence could prevent store-load
291 bool couldPreventStoreLoadForward(unsigned Distance, unsigned TypeByteSize);
294 /// \brief Holds information about the memory runtime legality checks to verify
295 /// that a group of pointers do not overlap.
296 class RuntimePointerChecking {
299 /// Holds the pointer value that we need to check.
300 TrackingVH<Value> PointerValue;
301 /// Holds the pointer value at the beginning of the loop.
303 /// Holds the pointer value at the end of the loop.
305 /// Holds the information if this pointer is used for writing to memory.
307 /// Holds the id of the set of pointers that could be dependent because of a
308 /// shared underlying object.
309 unsigned DependencySetId;
310 /// Holds the id of the disjoint alias set to which this pointer belongs.
312 /// SCEV for the access.
315 PointerInfo(Value *PointerValue, const SCEV *Start, const SCEV *End,
316 bool IsWritePtr, unsigned DependencySetId, unsigned AliasSetId,
318 : PointerValue(PointerValue), Start(Start), End(End),
319 IsWritePtr(IsWritePtr), DependencySetId(DependencySetId),
320 AliasSetId(AliasSetId), Expr(Expr) {}
323 RuntimePointerChecking(ScalarEvolution *SE) : Need(false), SE(SE) {}
325 /// Reset the state of the pointer runtime information.
332 /// Insert a pointer and calculate the start and end SCEVs.
333 void insert(Loop *Lp, Value *Ptr, bool WritePtr, unsigned DepSetId,
334 unsigned ASId, const ValueToValueMap &Strides);
336 /// \brief No run-time memory checking is necessary.
337 bool empty() const { return Pointers.empty(); }
339 /// A grouping of pointers. A single memcheck is required between
341 struct CheckingPtrGroup {
342 /// \brief Create a new pointer checking group containing a single
343 /// pointer, with index \p Index in RtCheck.
344 CheckingPtrGroup(unsigned Index, RuntimePointerChecking &RtCheck)
345 : RtCheck(RtCheck), High(RtCheck.Pointers[Index].End),
346 Low(RtCheck.Pointers[Index].Start) {
347 Members.push_back(Index);
350 /// \brief Tries to add the pointer recorded in RtCheck at index
351 /// \p Index to this pointer checking group. We can only add a pointer
352 /// to a checking group if we will still be able to get
353 /// the upper and lower bounds of the check. Returns true in case
354 /// of success, false otherwise.
355 bool addPointer(unsigned Index);
357 /// Constitutes the context of this pointer checking group. For each
358 /// pointer that is a member of this group we will retain the index
359 /// at which it appears in RtCheck.
360 RuntimePointerChecking &RtCheck;
361 /// The SCEV expression which represents the upper bound of all the
362 /// pointers in this group.
364 /// The SCEV expression which represents the lower bound of all the
365 /// pointers in this group.
367 /// Indices of all the pointers that constitute this grouping.
368 SmallVector<unsigned, 2> Members;
371 /// \brief A memcheck which made up of a pair of grouped pointers.
373 /// These *have* to be const for now, since checks are generated from
374 /// CheckingPtrGroups in LAI::addRuntimeChecks which is a const member
375 /// function. FIXME: once check-generation is moved inside this class (after
376 /// the PtrPartition hack is removed), we could drop const.
377 typedef std::pair<const CheckingPtrGroup *, const CheckingPtrGroup *>
380 /// \brief Generate the checks and store it. This also performs the grouping
381 /// of pointers to reduce the number of memchecks necessary.
382 void generateChecks(MemoryDepChecker::DepCandidates &DepCands,
383 bool UseDependencies);
385 /// \brief Returns the checks that generateChecks created.
386 const SmallVector<PointerCheck, 4> &getChecks() const { return Checks; }
388 /// \brief Decide if we need to add a check between two groups of pointers,
389 /// according to needsChecking.
390 bool needsChecking(const CheckingPtrGroup &M,
391 const CheckingPtrGroup &N) const;
393 /// \brief Returns the number of run-time checks required according to
395 unsigned getNumberOfChecks() const { return Checks.size(); }
397 /// \brief Print the list run-time memory checks necessary.
398 void print(raw_ostream &OS, unsigned Depth = 0) const;
401 void printChecks(raw_ostream &OS, const SmallVectorImpl<PointerCheck> &Checks,
402 unsigned Depth = 0) const;
404 /// This flag indicates if we need to add the runtime check.
407 /// Information about the pointers that may require checking.
408 SmallVector<PointerInfo, 2> Pointers;
410 /// Holds a partitioning of pointers into "check groups".
411 SmallVector<CheckingPtrGroup, 2> CheckingGroups;
413 /// \brief Check if pointers are in the same partition
415 /// \p PtrToPartition contains the partition number for pointers (-1 if the
416 /// pointer belongs to multiple partitions).
418 arePointersInSamePartition(const SmallVectorImpl<int> &PtrToPartition,
419 unsigned PtrIdx1, unsigned PtrIdx2);
421 /// \brief Decide whether we need to issue a run-time check for pointer at
422 /// index \p I and \p J to prove their independence.
423 bool needsChecking(unsigned I, unsigned J) const;
426 /// \brief Groups pointers such that a single memcheck is required
427 /// between two different groups. This will clear the CheckingGroups vector
428 /// and re-compute it. We will only group dependecies if \p UseDependencies
429 /// is true, otherwise we will create a separate group for each pointer.
430 void groupChecks(MemoryDepChecker::DepCandidates &DepCands,
431 bool UseDependencies);
433 /// Generate the checks and return them.
434 SmallVector<PointerCheck, 4>
435 generateChecks() const;
437 /// Holds a pointer to the ScalarEvolution analysis.
440 /// \brief Set of run-time checks required to establish independence of
441 /// otherwise may-aliasing pointers in the loop.
442 SmallVector<PointerCheck, 4> Checks;
445 /// \brief Drive the analysis of memory accesses in the loop
447 /// This class is responsible for analyzing the memory accesses of a loop. It
448 /// collects the accesses and then its main helper the AccessAnalysis class
449 /// finds and categorizes the dependences in buildDependenceSets.
451 /// For memory dependences that can be analyzed at compile time, it determines
452 /// whether the dependence is part of cycle inhibiting vectorization. This work
453 /// is delegated to the MemoryDepChecker class.
455 /// For memory dependences that cannot be determined at compile time, it
456 /// generates run-time checks to prove independence. This is done by
457 /// AccessAnalysis::canCheckPtrAtRT and the checks are maintained by the
458 /// RuntimePointerCheck class.
459 class LoopAccessInfo {
461 LoopAccessInfo(Loop *L, ScalarEvolution *SE, const DataLayout &DL,
462 const TargetLibraryInfo *TLI, AliasAnalysis *AA,
463 DominatorTree *DT, LoopInfo *LI,
464 const ValueToValueMap &Strides);
466 /// Return true we can analyze the memory accesses in the loop and there are
467 /// no memory dependence cycles.
468 bool canVectorizeMemory() const { return CanVecMem; }
470 const RuntimePointerChecking *getRuntimePointerChecking() const {
471 return &PtrRtChecking;
474 /// \brief Number of memchecks required to prove independence of otherwise
475 /// may-alias pointers.
476 unsigned getNumRuntimePointerChecks() const {
477 return PtrRtChecking.getNumberOfChecks();
480 /// Return true if the block BB needs to be predicated in order for the loop
481 /// to be vectorized.
482 static bool blockNeedsPredication(BasicBlock *BB, Loop *TheLoop,
485 /// Returns true if the value V is uniform within the loop.
486 bool isUniform(Value *V) const;
488 unsigned getMaxSafeDepDistBytes() const { return MaxSafeDepDistBytes; }
489 unsigned getNumStores() const { return NumStores; }
490 unsigned getNumLoads() const { return NumLoads;}
492 /// \brief Add code that checks at runtime if the accessed arrays overlap.
494 /// Returns a pair of instructions where the first element is the first
495 /// instruction generated in possibly a sequence of instructions and the
496 /// second value is the final comparator value or NULL if no check is needed.
497 std::pair<Instruction *, Instruction *>
498 addRuntimeChecks(Instruction *Loc) const;
500 /// \brief Generete the instructions for the checks in \p PointerChecks.
502 /// Returns a pair of instructions where the first element is the first
503 /// instruction generated in possibly a sequence of instructions and the
504 /// second value is the final comparator value or NULL if no check is needed.
505 std::pair<Instruction *, Instruction *>
506 addRuntimeChecks(Instruction *Loc,
507 const SmallVectorImpl<RuntimePointerChecking::PointerCheck>
508 &PointerChecks) const;
510 /// \brief The diagnostics report generated for the analysis. E.g. why we
511 /// couldn't analyze the loop.
512 const Optional<LoopAccessReport> &getReport() const { return Report; }
514 /// \brief the Memory Dependence Checker which can determine the
515 /// loop-independent and loop-carried dependences between memory accesses.
516 const MemoryDepChecker &getDepChecker() const { return DepChecker; }
518 /// \brief Return the list of instructions that use \p Ptr to read or write
520 SmallVector<Instruction *, 4> getInstructionsForAccess(Value *Ptr,
521 bool isWrite) const {
522 return DepChecker.getInstructionsForAccess(Ptr, isWrite);
525 /// \brief Print the information about the memory accesses in the loop.
526 void print(raw_ostream &OS, unsigned Depth = 0) const;
528 /// \brief Used to ensure that if the analysis was run with speculating the
529 /// value of symbolic strides, the client queries it with the same assumption.
530 /// Only used in DEBUG build but we don't want NDEBUG-dependent ABI.
531 unsigned NumSymbolicStrides;
533 /// \brief Checks existence of store to invariant address inside loop.
534 /// If the loop has any store to invariant address, then it returns true,
535 /// else returns false.
536 bool hasStoreToLoopInvariantAddress() const {
537 return StoreToLoopInvariantAddress;
541 /// \brief Analyze the loop. Substitute symbolic strides using Strides.
542 void analyzeLoop(const ValueToValueMap &Strides);
544 /// \brief Check if the structure of the loop allows it to be analyzed by this
546 bool canAnalyzeLoop();
548 void emitAnalysis(LoopAccessReport &Message);
550 /// We need to check that all of the pointers in this list are disjoint
552 RuntimePointerChecking PtrRtChecking;
554 /// \brief the Memory Dependence Checker which can determine the
555 /// loop-independent and loop-carried dependences between memory accesses.
556 MemoryDepChecker DepChecker;
560 const DataLayout &DL;
561 const TargetLibraryInfo *TLI;
569 unsigned MaxSafeDepDistBytes;
571 /// \brief Cache the result of analyzeLoop.
574 /// \brief Indicator for storing to uniform addresses.
575 /// If a loop has write to a loop invariant address then it should be true.
576 bool StoreToLoopInvariantAddress;
578 /// \brief The diagnostics report generated for the analysis. E.g. why we
579 /// couldn't analyze the loop.
580 Optional<LoopAccessReport> Report;
583 Value *stripIntegerCast(Value *V);
585 ///\brief Return the SCEV corresponding to a pointer with the symbolic stride
586 ///replaced with constant one.
588 /// If \p OrigPtr is not null, use it to look up the stride value instead of \p
589 /// Ptr. \p PtrToStride provides the mapping between the pointer value and its
590 /// stride as collected by LoopVectorizationLegality::collectStridedAccess.
591 const SCEV *replaceSymbolicStrideSCEV(ScalarEvolution *SE,
592 const ValueToValueMap &PtrToStride,
593 Value *Ptr, Value *OrigPtr = nullptr);
595 /// \brief Check the stride of the pointer and ensure that it does not wrap in
596 /// the address space.
597 int isStridedPtr(ScalarEvolution *SE, Value *Ptr, const Loop *Lp,
598 const ValueToValueMap &StridesMap);
600 /// \brief This analysis provides dependence information for the memory accesses
603 /// It runs the analysis for a loop on demand. This can be initiated by
604 /// querying the loop access info via LAA::getInfo. getInfo return a
605 /// LoopAccessInfo object. See this class for the specifics of what information
607 class LoopAccessAnalysis : public FunctionPass {
611 LoopAccessAnalysis() : FunctionPass(ID) {
612 initializeLoopAccessAnalysisPass(*PassRegistry::getPassRegistry());
615 bool runOnFunction(Function &F) override;
617 void getAnalysisUsage(AnalysisUsage &AU) const override;
619 /// \brief Query the result of the loop access information for the loop \p L.
621 /// If the client speculates (and then issues run-time checks) for the values
622 /// of symbolic strides, \p Strides provides the mapping (see
623 /// replaceSymbolicStrideSCEV). If there is no cached result available run
625 const LoopAccessInfo &getInfo(Loop *L, const ValueToValueMap &Strides);
627 void releaseMemory() override {
628 // Invalidate the cache when the pass is freed.
629 LoopAccessInfoMap.clear();
632 /// \brief Print the result of the analysis when invoked with -analyze.
633 void print(raw_ostream &OS, const Module *M = nullptr) const override;
636 /// \brief The cache.
637 DenseMap<Loop *, std::unique_ptr<LoopAccessInfo>> LoopAccessInfoMap;
639 // The used analysis passes.
641 const TargetLibraryInfo *TLI;
646 } // End llvm namespace