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"
33 class ScalarEvolution;
37 /// Optimization analysis message produced during vectorization. Messages inform
38 /// the user why vectorization did not occur.
39 class LoopAccessReport {
41 const Instruction *Instr;
44 LoopAccessReport(const Twine &Message, const Instruction *I)
45 : Message(Message.str()), Instr(I) {}
48 LoopAccessReport(const Instruction *I = nullptr) : Instr(I) {}
50 template <typename A> LoopAccessReport &operator<<(const A &Value) {
51 raw_string_ostream Out(Message);
56 const Instruction *getInstr() const { return Instr; }
58 std::string &str() { return Message; }
59 const std::string &str() const { return Message; }
60 operator Twine() { return Message; }
62 /// \brief Emit an analysis note for \p PassName with the debug location from
63 /// the instruction in \p Message if available. Otherwise use the location of
65 static void emitAnalysis(const LoopAccessReport &Message,
66 const Function *TheFunction,
68 const char *PassName);
71 /// \brief Collection of parameters shared beetween the Loop Vectorizer and the
72 /// Loop Access Analysis.
73 struct VectorizerParams {
74 /// \brief Maximum SIMD width.
75 static const unsigned MaxVectorWidth;
77 /// \brief VF as overridden by the user.
78 static unsigned VectorizationFactor;
79 /// \brief Interleave factor as overridden by the user.
80 static unsigned VectorizationInterleave;
81 /// \brief True if force-vector-interleave was specified by the user.
82 static bool isInterleaveForced();
84 /// \\brief When performing memory disambiguation checks at runtime do not
85 /// make more than this number of comparisons.
86 static unsigned RuntimeMemoryCheckThreshold;
89 /// \brief Checks memory dependences among accesses to the same underlying
90 /// object to determine whether there vectorization is legal or not (and at
91 /// which vectorization factor).
93 /// Note: This class will compute a conservative dependence for access to
94 /// different underlying pointers. Clients, such as the loop vectorizer, will
95 /// sometimes deal these potential dependencies by emitting runtime checks.
97 /// We use the ScalarEvolution framework to symbolically evalutate access
98 /// functions pairs. Since we currently don't restructure the loop we can rely
99 /// on the program order of memory accesses to determine their safety.
100 /// At the moment we will only deem accesses as safe for:
101 /// * A negative constant distance assuming program order.
103 /// Safe: tmp = a[i + 1]; OR a[i + 1] = x;
104 /// a[i] = tmp; y = a[i];
106 /// The latter case is safe because later checks guarantuee that there can't
107 /// be a cycle through a phi node (that is, we check that "x" and "y" is not
108 /// the same variable: a header phi can only be an induction or a reduction, a
109 /// reduction can't have a memory sink, an induction can't have a memory
110 /// source). This is important and must not be violated (or we have to
111 /// resort to checking for cycles through memory).
113 /// * A positive constant distance assuming program order that is bigger
114 /// than the biggest memory access.
116 /// tmp = a[i] OR b[i] = x
117 /// a[i+2] = tmp y = b[i+2];
119 /// Safe distance: 2 x sizeof(a[0]), and 2 x sizeof(b[0]), respectively.
121 /// * Zero distances and all accesses have the same size.
123 class MemoryDepChecker {
125 typedef PointerIntPair<Value *, 1, bool> MemAccessInfo;
126 typedef SmallPtrSet<MemAccessInfo, 8> MemAccessInfoSet;
127 /// \brief Set of potential dependent memory accesses.
128 typedef EquivalenceClasses<MemAccessInfo> DepCandidates;
130 /// \brief Dependece between memory access instructions.
132 /// \brief The type of the dependence.
136 // We couldn't determine the direction or the distance.
138 // Lexically forward.
140 // Forward, but if vectorized, is likely to prevent store-to-load
142 ForwardButPreventsForwarding,
143 // Lexically backward.
145 // Backward, but the distance allows a vectorization factor of
146 // MaxSafeDepDistBytes.
147 BackwardVectorizable,
148 // Same, but may prevent store-to-load forwarding.
149 BackwardVectorizableButPreventsForwarding
152 /// \brief Index of the source of the dependence in the InstMap vector.
154 /// \brief Index of the destination of the dependence in the InstMap vector.
155 unsigned Destination;
156 /// \brief The type of the dependence.
159 Dependence(unsigned Source, unsigned Destination, DepType Type)
160 : Source(Source), Destination(Destination), Type(Type) {}
162 /// \brief Dependence types that don't prevent vectorization.
163 static bool isSafeForVectorization(DepType Type);
165 /// \brief Dependence types that can be queried from the analysis.
166 static bool isInterestingDependence(DepType Type);
168 /// \brief Lexically backward dependence types.
169 bool isPossiblyBackward() const;
172 MemoryDepChecker(ScalarEvolution *Se, const Loop *L)
173 : SE(Se), InnermostLoop(L), AccessIdx(0),
174 ShouldRetryWithRuntimeCheck(false), SafeForVectorization(true),
175 RecordInterestingDependences(true) {}
177 /// \brief Register the location (instructions are given increasing numbers)
178 /// of a write access.
179 void addAccess(StoreInst *SI) {
180 Value *Ptr = SI->getPointerOperand();
181 Accesses[MemAccessInfo(Ptr, true)].push_back(AccessIdx);
182 InstMap.push_back(SI);
186 /// \brief Register the location (instructions are given increasing numbers)
187 /// of a write access.
188 void addAccess(LoadInst *LI) {
189 Value *Ptr = LI->getPointerOperand();
190 Accesses[MemAccessInfo(Ptr, false)].push_back(AccessIdx);
191 InstMap.push_back(LI);
195 /// \brief Check whether the dependencies between the accesses are safe.
197 /// Only checks sets with elements in \p CheckDeps.
198 bool areDepsSafe(DepCandidates &AccessSets, MemAccessInfoSet &CheckDeps,
199 const ValueToValueMap &Strides);
201 /// \brief No memory dependence was encountered that would inhibit
203 bool isSafeForVectorization() const { return SafeForVectorization; }
205 /// \brief The maximum number of bytes of a vector register we can vectorize
206 /// the accesses safely with.
207 unsigned getMaxSafeDepDistBytes() { return MaxSafeDepDistBytes; }
209 /// \brief In same cases when the dependency check fails we can still
210 /// vectorize the loop with a dynamic array access check.
211 bool shouldRetryWithRuntimeCheck() { return ShouldRetryWithRuntimeCheck; }
213 /// \brief Returns the interesting dependences. If null is returned we
214 /// exceeded the MaxInterestingDependence threshold and this information is
216 const SmallVectorImpl<Dependence> *getInterestingDependences() const {
217 return RecordInterestingDependences ? &InterestingDependences : nullptr;
220 /// \brief The vector of memory access instructions. The indices are used as
221 /// instruction identifiers in the Dependence class.
222 const SmallVectorImpl<Instruction *> &getMemoryInstructions() const {
228 const Loop *InnermostLoop;
230 /// \brief Maps access locations (ptr, read/write) to program order.
231 DenseMap<MemAccessInfo, std::vector<unsigned> > Accesses;
233 /// \brief Memory access instructions in program order.
234 SmallVector<Instruction *, 16> InstMap;
236 /// \brief The program order index to be used for the next instruction.
239 // We can access this many bytes in parallel safely.
240 unsigned MaxSafeDepDistBytes;
242 /// \brief If we see a non-constant dependence distance we can still try to
243 /// vectorize this loop with runtime checks.
244 bool ShouldRetryWithRuntimeCheck;
246 /// \brief No memory dependence was encountered that would inhibit
248 bool SafeForVectorization;
250 //// \brief True if InterestingDependences reflects the dependences in the
251 //// loop. If false we exceeded MaxInterestingDependence and
252 //// InterestingDependences is invalid.
253 bool RecordInterestingDependences;
255 /// \brief Interesting memory dependences collected during the analysis as
256 /// defined by isInterestingDependence. Only valid if
257 /// RecordInterestingDependences is true.
258 SmallVector<Dependence, 8> InterestingDependences;
260 /// \brief Check whether there is a plausible dependence between the two
263 /// Access \p A must happen before \p B in program order. The two indices
264 /// identify the index into the program order map.
266 /// This function checks whether there is a plausible dependence (or the
267 /// absence of such can't be proved) between the two accesses. If there is a
268 /// plausible dependence but the dependence distance is bigger than one
269 /// element access it records this distance in \p MaxSafeDepDistBytes (if this
270 /// distance is smaller than any other distance encountered so far).
271 /// Otherwise, this function returns true signaling a possible dependence.
272 Dependence::DepType isDependent(const MemAccessInfo &A, unsigned AIdx,
273 const MemAccessInfo &B, unsigned BIdx,
274 const ValueToValueMap &Strides);
276 /// \brief Check whether the data dependence could prevent store-load
278 bool couldPreventStoreLoadForward(unsigned Distance, unsigned TypeByteSize);
281 /// \brief Drive the analysis of memory accesses in the loop
283 /// This class is responsible for analyzing the memory accesses of a loop. It
284 /// collects the accesses and then its main helper the AccessAnalysis class
285 /// finds and categorizes the dependences in buildDependenceSets.
287 /// For memory dependences that can be analyzed at compile time, it determines
288 /// whether the dependence is part of cycle inhibiting vectorization. This work
289 /// is delegated to the MemoryDepChecker class.
291 /// For memory dependences that cannot be determined at compile time, it
292 /// generates run-time checks to prove independence. This is done by
293 /// AccessAnalysis::canCheckPtrAtRT and the checks are maintained by the
294 /// RuntimePointerCheck class.
295 class LoopAccessInfo {
297 /// This struct holds information about the memory runtime legality check that
298 /// a group of pointers do not overlap.
299 struct RuntimePointerCheck {
300 RuntimePointerCheck() : Need(false) {}
302 /// Reset the state of the pointer runtime information.
309 DependencySetId.clear();
313 /// Insert a pointer and calculate the start and end SCEVs.
314 void insert(ScalarEvolution *SE, Loop *Lp, Value *Ptr, bool WritePtr,
315 unsigned DepSetId, unsigned ASId,
316 const ValueToValueMap &Strides);
318 /// \brief No run-time memory checking is necessary.
319 bool empty() const { return Pointers.empty(); }
321 /// \brief Decide whether we need to issue a run-time check for pointer at
322 /// index \p I and \p J to prove their independence.
323 bool needsChecking(unsigned I, unsigned J) const;
325 /// \brief Print the list run-time memory checks necessary.
326 void print(raw_ostream &OS, unsigned Depth = 0) const;
328 /// This flag indicates if we need to add the runtime check.
330 /// Holds the pointers that we need to check.
331 SmallVector<TrackingVH<Value>, 2> Pointers;
332 /// Holds the pointer value at the beginning of the loop.
333 SmallVector<const SCEV*, 2> Starts;
334 /// Holds the pointer value at the end of the loop.
335 SmallVector<const SCEV*, 2> Ends;
336 /// Holds the information if this pointer is used for writing to memory.
337 SmallVector<bool, 2> IsWritePtr;
338 /// Holds the id of the set of pointers that could be dependent because of a
339 /// shared underlying object.
340 SmallVector<unsigned, 2> DependencySetId;
341 /// Holds the id of the disjoint alias set to which this pointer belongs.
342 SmallVector<unsigned, 2> AliasSetId;
345 LoopAccessInfo(Loop *L, ScalarEvolution *SE, const DataLayout &DL,
346 const TargetLibraryInfo *TLI, AliasAnalysis *AA,
347 DominatorTree *DT, const ValueToValueMap &Strides);
349 /// Return true we can analyze the memory accesses in the loop and there are
350 /// no memory dependence cycles.
351 bool canVectorizeMemory() const { return CanVecMem; }
353 const RuntimePointerCheck *getRuntimePointerCheck() const {
357 /// Return true if the block BB needs to be predicated in order for the loop
358 /// to be vectorized.
359 static bool blockNeedsPredication(BasicBlock *BB, Loop *TheLoop,
362 /// Returns true if the value V is uniform within the loop.
363 bool isUniform(Value *V) const;
365 unsigned getMaxSafeDepDistBytes() const { return MaxSafeDepDistBytes; }
366 unsigned getNumStores() const { return NumStores; }
367 unsigned getNumLoads() const { return NumLoads;}
369 /// \brief Add code that checks at runtime if the accessed arrays overlap.
371 /// Returns a pair of instructions where the first element is the first
372 /// instruction generated in possibly a sequence of instructions and the
373 /// second value is the final comparator value or NULL if no check is needed.
374 std::pair<Instruction *, Instruction *>
375 addRuntimeCheck(Instruction *Loc) const;
377 /// \brief The diagnostics report generated for the analysis. E.g. why we
378 /// couldn't analyze the loop.
379 const Optional<LoopAccessReport> &getReport() const { return Report; }
381 /// \brief the Memory Dependence Checker which can determine the
382 /// loop-independent and loop-carried dependences between memory accesses.
383 const MemoryDepChecker &getDepChecker() const { return DepChecker; }
385 /// \brief Print the information about the memory accesses in the loop.
386 void print(raw_ostream &OS, unsigned Depth = 0) const;
388 /// \brief Used to ensure that if the analysis was run with speculating the
389 /// value of symbolic strides, the client queries it with the same assumption.
390 /// Only used in DEBUG build but we don't want NDEBUG-dependent ABI.
391 unsigned NumSymbolicStrides;
394 /// \brief Analyze the loop. Substitute symbolic strides using Strides.
395 void analyzeLoop(const ValueToValueMap &Strides);
397 /// \brief Check if the structure of the loop allows it to be analyzed by this
399 bool canAnalyzeLoop();
401 void emitAnalysis(LoopAccessReport &Message);
403 /// We need to check that all of the pointers in this list are disjoint
405 RuntimePointerCheck PtrRtCheck;
407 /// \brief the Memory Dependence Checker which can determine the
408 /// loop-independent and loop-carried dependences between memory accesses.
409 MemoryDepChecker DepChecker;
413 const DataLayout &DL;
414 const TargetLibraryInfo *TLI;
421 unsigned MaxSafeDepDistBytes;
423 /// \brief Cache the result of analyzeLoop.
426 /// \brief The diagnostics report generated for the analysis. E.g. why we
427 /// couldn't analyze the loop.
428 Optional<LoopAccessReport> Report;
431 Value *stripIntegerCast(Value *V);
433 ///\brief Return the SCEV corresponding to a pointer with the symbolic stride
434 ///replaced with constant one.
436 /// If \p OrigPtr is not null, use it to look up the stride value instead of \p
437 /// Ptr. \p PtrToStride provides the mapping between the pointer value and its
438 /// stride as collected by LoopVectorizationLegality::collectStridedAccess.
439 const SCEV *replaceSymbolicStrideSCEV(ScalarEvolution *SE,
440 const ValueToValueMap &PtrToStride,
441 Value *Ptr, Value *OrigPtr = nullptr);
443 /// \brief This analysis provides dependence information for the memory accesses
446 /// It runs the analysis for a loop on demand. This can be initiated by
447 /// querying the loop access info via LAA::getInfo. getInfo return a
448 /// LoopAccessInfo object. See this class for the specifics of what information
450 class LoopAccessAnalysis : public FunctionPass {
454 LoopAccessAnalysis() : FunctionPass(ID) {
455 initializeLoopAccessAnalysisPass(*PassRegistry::getPassRegistry());
458 bool runOnFunction(Function &F) override;
460 void getAnalysisUsage(AnalysisUsage &AU) const override;
462 /// \brief Query the result of the loop access information for the loop \p L.
464 /// If the client speculates (and then issues run-time checks) for the values
465 /// of symbolic strides, \p Strides provides the mapping (see
466 /// replaceSymbolicStrideSCEV). If there is no cached result available run
468 const LoopAccessInfo &getInfo(Loop *L, const ValueToValueMap &Strides);
470 void releaseMemory() override {
471 // Invalidate the cache when the pass is freed.
472 LoopAccessInfoMap.clear();
475 /// \brief Print the result of the analysis when invoked with -analyze.
476 void print(raw_ostream &OS, const Module *M = nullptr) const override;
479 /// \brief The cache.
480 DenseMap<Loop *, std::unique_ptr<LoopAccessInfo>> LoopAccessInfoMap;
482 // The used analysis passes.
484 const TargetLibraryInfo *TLI;
488 } // End llvm namespace