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Diffstat (limited to 'include/llvm/Analysis/LoopAccessAnalysis.h')
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diff --git a/include/llvm/Analysis/LoopAccessAnalysis.h b/include/llvm/Analysis/LoopAccessAnalysis.h new file mode 100644 index 0000000000000..c14e1451f338b --- /dev/null +++ b/include/llvm/Analysis/LoopAccessAnalysis.h @@ -0,0 +1,552 @@ +//===- llvm/Analysis/LoopAccessAnalysis.h -----------------------*- C++ -*-===// +// +// The LLVM Compiler Infrastructure +// +// This file is distributed under the University of Illinois Open Source +// License. See LICENSE.TXT for details. +// +//===----------------------------------------------------------------------===// +// +// This file defines the interface for the loop memory dependence framework that +// was originally developed for the Loop Vectorizer. +// +//===----------------------------------------------------------------------===// + +#ifndef LLVM_ANALYSIS_LOOPACCESSANALYSIS_H +#define LLVM_ANALYSIS_LOOPACCESSANALYSIS_H + +#include "llvm/ADT/EquivalenceClasses.h" +#include "llvm/ADT/Optional.h" +#include "llvm/ADT/SetVector.h" +#include "llvm/Analysis/AliasAnalysis.h" +#include "llvm/Analysis/AliasSetTracker.h" +#include "llvm/Analysis/ScalarEvolutionExpressions.h" +#include "llvm/IR/ValueHandle.h" +#include "llvm/Pass.h" +#include "llvm/Support/raw_ostream.h" + +namespace llvm { + +class Value; +class DataLayout; +class AliasAnalysis; +class ScalarEvolution; +class Loop; +class SCEV; + +/// Optimization analysis message produced during vectorization. Messages inform +/// the user why vectorization did not occur. +class LoopAccessReport { + std::string Message; + const Instruction *Instr; + +protected: + LoopAccessReport(const Twine &Message, const Instruction *I) + : Message(Message.str()), Instr(I) {} + +public: + LoopAccessReport(const Instruction *I = nullptr) : Instr(I) {} + + template <typename A> LoopAccessReport &operator<<(const A &Value) { + raw_string_ostream Out(Message); + Out << Value; + return *this; + } + + const Instruction *getInstr() const { return Instr; } + + std::string &str() { return Message; } + const std::string &str() const { return Message; } + operator Twine() { return Message; } + + /// \brief Emit an analysis note for \p PassName with the debug location from + /// the instruction in \p Message if available. Otherwise use the location of + /// \p TheLoop. + static void emitAnalysis(const LoopAccessReport &Message, + const Function *TheFunction, + const Loop *TheLoop, + const char *PassName); +}; + +/// \brief Collection of parameters shared beetween the Loop Vectorizer and the +/// Loop Access Analysis. +struct VectorizerParams { + /// \brief Maximum SIMD width. + static const unsigned MaxVectorWidth; + + /// \brief VF as overridden by the user. + static unsigned VectorizationFactor; + /// \brief Interleave factor as overridden by the user. + static unsigned VectorizationInterleave; + /// \brief True if force-vector-interleave was specified by the user. + static bool isInterleaveForced(); + + /// \\brief When performing memory disambiguation checks at runtime do not + /// make more than this number of comparisons. + static unsigned RuntimeMemoryCheckThreshold; +}; + +/// \brief Checks memory dependences among accesses to the same underlying +/// object to determine whether there vectorization is legal or not (and at +/// which vectorization factor). +/// +/// Note: This class will compute a conservative dependence for access to +/// different underlying pointers. Clients, such as the loop vectorizer, will +/// sometimes deal these potential dependencies by emitting runtime checks. +/// +/// We use the ScalarEvolution framework to symbolically evalutate access +/// functions pairs. Since we currently don't restructure the loop we can rely +/// on the program order of memory accesses to determine their safety. +/// At the moment we will only deem accesses as safe for: +/// * A negative constant distance assuming program order. +/// +/// Safe: tmp = a[i + 1]; OR a[i + 1] = x; +/// a[i] = tmp; y = a[i]; +/// +/// The latter case is safe because later checks guarantuee that there can't +/// be a cycle through a phi node (that is, we check that "x" and "y" is not +/// the same variable: a header phi can only be an induction or a reduction, a +/// reduction can't have a memory sink, an induction can't have a memory +/// source). This is important and must not be violated (or we have to +/// resort to checking for cycles through memory). +/// +/// * A positive constant distance assuming program order that is bigger +/// than the biggest memory access. +/// +/// tmp = a[i] OR b[i] = x +/// a[i+2] = tmp y = b[i+2]; +/// +/// Safe distance: 2 x sizeof(a[0]), and 2 x sizeof(b[0]), respectively. +/// +/// * Zero distances and all accesses have the same size. +/// +class MemoryDepChecker { +public: + typedef PointerIntPair<Value *, 1, bool> MemAccessInfo; + typedef SmallPtrSet<MemAccessInfo, 8> MemAccessInfoSet; + /// \brief Set of potential dependent memory accesses. + typedef EquivalenceClasses<MemAccessInfo> DepCandidates; + + /// \brief Dependece between memory access instructions. + struct Dependence { + /// \brief The type of the dependence. + enum DepType { + // No dependence. + NoDep, + // We couldn't determine the direction or the distance. + Unknown, + // Lexically forward. + Forward, + // Forward, but if vectorized, is likely to prevent store-to-load + // forwarding. + ForwardButPreventsForwarding, + // Lexically backward. + Backward, + // Backward, but the distance allows a vectorization factor of + // MaxSafeDepDistBytes. + BackwardVectorizable, + // Same, but may prevent store-to-load forwarding. + BackwardVectorizableButPreventsForwarding + }; + + /// \brief String version of the types. + static const char *DepName[]; + + /// \brief Index of the source of the dependence in the InstMap vector. + unsigned Source; + /// \brief Index of the destination of the dependence in the InstMap vector. + unsigned Destination; + /// \brief The type of the dependence. + DepType Type; + + Dependence(unsigned Source, unsigned Destination, DepType Type) + : Source(Source), Destination(Destination), Type(Type) {} + + /// \brief Dependence types that don't prevent vectorization. + static bool isSafeForVectorization(DepType Type); + + /// \brief Dependence types that can be queried from the analysis. + static bool isInterestingDependence(DepType Type); + + /// \brief Lexically backward dependence types. + bool isPossiblyBackward() const; + + /// \brief Print the dependence. \p Instr is used to map the instruction + /// indices to instructions. + void print(raw_ostream &OS, unsigned Depth, + const SmallVectorImpl<Instruction *> &Instrs) const; + }; + + MemoryDepChecker(ScalarEvolution *Se, const Loop *L) + : SE(Se), InnermostLoop(L), AccessIdx(0), + ShouldRetryWithRuntimeCheck(false), SafeForVectorization(true), + RecordInterestingDependences(true) {} + + /// \brief Register the location (instructions are given increasing numbers) + /// of a write access. + void addAccess(StoreInst *SI) { + Value *Ptr = SI->getPointerOperand(); + Accesses[MemAccessInfo(Ptr, true)].push_back(AccessIdx); + InstMap.push_back(SI); + ++AccessIdx; + } + + /// \brief Register the location (instructions are given increasing numbers) + /// of a write access. + void addAccess(LoadInst *LI) { + Value *Ptr = LI->getPointerOperand(); + Accesses[MemAccessInfo(Ptr, false)].push_back(AccessIdx); + InstMap.push_back(LI); + ++AccessIdx; + } + + /// \brief Check whether the dependencies between the accesses are safe. + /// + /// Only checks sets with elements in \p CheckDeps. + bool areDepsSafe(DepCandidates &AccessSets, MemAccessInfoSet &CheckDeps, + const ValueToValueMap &Strides); + + /// \brief No memory dependence was encountered that would inhibit + /// vectorization. + bool isSafeForVectorization() const { return SafeForVectorization; } + + /// \brief The maximum number of bytes of a vector register we can vectorize + /// the accesses safely with. + unsigned getMaxSafeDepDistBytes() { return MaxSafeDepDistBytes; } + + /// \brief In same cases when the dependency check fails we can still + /// vectorize the loop with a dynamic array access check. + bool shouldRetryWithRuntimeCheck() { return ShouldRetryWithRuntimeCheck; } + + /// \brief Returns the interesting dependences. If null is returned we + /// exceeded the MaxInterestingDependence threshold and this information is + /// not available. + const SmallVectorImpl<Dependence> *getInterestingDependences() const { + return RecordInterestingDependences ? &InterestingDependences : nullptr; + } + + void clearInterestingDependences() { InterestingDependences.clear(); } + + /// \brief The vector of memory access instructions. The indices are used as + /// instruction identifiers in the Dependence class. + const SmallVectorImpl<Instruction *> &getMemoryInstructions() const { + return InstMap; + } + + /// \brief Find the set of instructions that read or write via \p Ptr. + SmallVector<Instruction *, 4> getInstructionsForAccess(Value *Ptr, + bool isWrite) const; + +private: + ScalarEvolution *SE; + const Loop *InnermostLoop; + + /// \brief Maps access locations (ptr, read/write) to program order. + DenseMap<MemAccessInfo, std::vector<unsigned> > Accesses; + + /// \brief Memory access instructions in program order. + SmallVector<Instruction *, 16> InstMap; + + /// \brief The program order index to be used for the next instruction. + unsigned AccessIdx; + + // We can access this many bytes in parallel safely. + unsigned MaxSafeDepDistBytes; + + /// \brief If we see a non-constant dependence distance we can still try to + /// vectorize this loop with runtime checks. + bool ShouldRetryWithRuntimeCheck; + + /// \brief No memory dependence was encountered that would inhibit + /// vectorization. + bool SafeForVectorization; + + //// \brief True if InterestingDependences reflects the dependences in the + //// loop. If false we exceeded MaxInterestingDependence and + //// InterestingDependences is invalid. + bool RecordInterestingDependences; + + /// \brief Interesting memory dependences collected during the analysis as + /// defined by isInterestingDependence. Only valid if + /// RecordInterestingDependences is true. + SmallVector<Dependence, 8> InterestingDependences; + + /// \brief Check whether there is a plausible dependence between the two + /// accesses. + /// + /// Access \p A must happen before \p B in program order. The two indices + /// identify the index into the program order map. + /// + /// This function checks whether there is a plausible dependence (or the + /// absence of such can't be proved) between the two accesses. If there is a + /// plausible dependence but the dependence distance is bigger than one + /// element access it records this distance in \p MaxSafeDepDistBytes (if this + /// distance is smaller than any other distance encountered so far). + /// Otherwise, this function returns true signaling a possible dependence. + Dependence::DepType isDependent(const MemAccessInfo &A, unsigned AIdx, + const MemAccessInfo &B, unsigned BIdx, + const ValueToValueMap &Strides); + + /// \brief Check whether the data dependence could prevent store-load + /// forwarding. + bool couldPreventStoreLoadForward(unsigned Distance, unsigned TypeByteSize); +}; + +/// \brief Drive the analysis of memory accesses in the loop +/// +/// This class is responsible for analyzing the memory accesses of a loop. It +/// collects the accesses and then its main helper the AccessAnalysis class +/// finds and categorizes the dependences in buildDependenceSets. +/// +/// For memory dependences that can be analyzed at compile time, it determines +/// whether the dependence is part of cycle inhibiting vectorization. This work +/// is delegated to the MemoryDepChecker class. +/// +/// For memory dependences that cannot be determined at compile time, it +/// generates run-time checks to prove independence. This is done by +/// AccessAnalysis::canCheckPtrAtRT and the checks are maintained by the +/// RuntimePointerCheck class. +class LoopAccessInfo { +public: + /// This struct holds information about the memory runtime legality check that + /// a group of pointers do not overlap. + struct RuntimePointerCheck { + RuntimePointerCheck() : Need(false) {} + + /// Reset the state of the pointer runtime information. + void reset() { + Need = false; + Pointers.clear(); + Starts.clear(); + Ends.clear(); + IsWritePtr.clear(); + DependencySetId.clear(); + AliasSetId.clear(); + } + + /// Insert a pointer and calculate the start and end SCEVs. + void insert(ScalarEvolution *SE, Loop *Lp, Value *Ptr, bool WritePtr, + unsigned DepSetId, unsigned ASId, + const ValueToValueMap &Strides); + + /// \brief No run-time memory checking is necessary. + bool empty() const { return Pointers.empty(); } + + /// \brief Decide whether we need to issue a run-time check for pointer at + /// index \p I and \p J to prove their independence. + /// + /// If \p PtrPartition is set, it contains the partition number for + /// pointers (-1 if the pointer belongs to multiple partitions). In this + /// case omit checks between pointers belonging to the same partition. + bool needsChecking(unsigned I, unsigned J, + const SmallVectorImpl<int> *PtrPartition) const; + + /// \brief Return true if any pointer requires run-time checking according + /// to needsChecking. + bool needsAnyChecking(const SmallVectorImpl<int> *PtrPartition) const; + + /// \brief Print the list run-time memory checks necessary. + /// + /// If \p PtrPartition is set, it contains the partition number for + /// pointers (-1 if the pointer belongs to multiple partitions). In this + /// case omit checks between pointers belonging to the same partition. + void print(raw_ostream &OS, unsigned Depth = 0, + const SmallVectorImpl<int> *PtrPartition = nullptr) const; + + /// This flag indicates if we need to add the runtime check. + bool Need; + /// Holds the pointers that we need to check. + SmallVector<TrackingVH<Value>, 2> Pointers; + /// Holds the pointer value at the beginning of the loop. + SmallVector<const SCEV*, 2> Starts; + /// Holds the pointer value at the end of the loop. + SmallVector<const SCEV*, 2> Ends; + /// Holds the information if this pointer is used for writing to memory. + SmallVector<bool, 2> IsWritePtr; + /// Holds the id of the set of pointers that could be dependent because of a + /// shared underlying object. + SmallVector<unsigned, 2> DependencySetId; + /// Holds the id of the disjoint alias set to which this pointer belongs. + SmallVector<unsigned, 2> AliasSetId; + }; + + LoopAccessInfo(Loop *L, ScalarEvolution *SE, const DataLayout &DL, + const TargetLibraryInfo *TLI, AliasAnalysis *AA, + DominatorTree *DT, LoopInfo *LI, + const ValueToValueMap &Strides); + + /// Return true we can analyze the memory accesses in the loop and there are + /// no memory dependence cycles. + bool canVectorizeMemory() const { return CanVecMem; } + + const RuntimePointerCheck *getRuntimePointerCheck() const { + return &PtrRtCheck; + } + + /// \brief Number of memchecks required to prove independence of otherwise + /// may-alias pointers. + unsigned getNumRuntimePointerChecks() const { return NumComparisons; } + + /// Return true if the block BB needs to be predicated in order for the loop + /// to be vectorized. + static bool blockNeedsPredication(BasicBlock *BB, Loop *TheLoop, + DominatorTree *DT); + + /// Returns true if the value V is uniform within the loop. + bool isUniform(Value *V) const; + + unsigned getMaxSafeDepDistBytes() const { return MaxSafeDepDistBytes; } + unsigned getNumStores() const { return NumStores; } + unsigned getNumLoads() const { return NumLoads;} + + /// \brief Add code that checks at runtime if the accessed arrays overlap. + /// + /// Returns a pair of instructions where the first element is the first + /// instruction generated in possibly a sequence of instructions and the + /// second value is the final comparator value or NULL if no check is needed. + /// + /// If \p PtrPartition is set, it contains the partition number for pointers + /// (-1 if the pointer belongs to multiple partitions). In this case omit + /// checks between pointers belonging to the same partition. + std::pair<Instruction *, Instruction *> + addRuntimeCheck(Instruction *Loc, + const SmallVectorImpl<int> *PtrPartition = nullptr) const; + + /// \brief The diagnostics report generated for the analysis. E.g. why we + /// couldn't analyze the loop. + const Optional<LoopAccessReport> &getReport() const { return Report; } + + /// \brief the Memory Dependence Checker which can determine the + /// loop-independent and loop-carried dependences between memory accesses. + const MemoryDepChecker &getDepChecker() const { return DepChecker; } + + /// \brief Return the list of instructions that use \p Ptr to read or write + /// memory. + SmallVector<Instruction *, 4> getInstructionsForAccess(Value *Ptr, + bool isWrite) const { + return DepChecker.getInstructionsForAccess(Ptr, isWrite); + } + + /// \brief Print the information about the memory accesses in the loop. + void print(raw_ostream &OS, unsigned Depth = 0) const; + + /// \brief Used to ensure that if the analysis was run with speculating the + /// value of symbolic strides, the client queries it with the same assumption. + /// Only used in DEBUG build but we don't want NDEBUG-dependent ABI. + unsigned NumSymbolicStrides; + + /// \brief Checks existence of store to invariant address inside loop. + /// If the loop has any store to invariant address, then it returns true, + /// else returns false. + bool hasStoreToLoopInvariantAddress() const { + return StoreToLoopInvariantAddress; + } + +private: + /// \brief Analyze the loop. Substitute symbolic strides using Strides. + void analyzeLoop(const ValueToValueMap &Strides); + + /// \brief Check if the structure of the loop allows it to be analyzed by this + /// pass. + bool canAnalyzeLoop(); + + void emitAnalysis(LoopAccessReport &Message); + + /// We need to check that all of the pointers in this list are disjoint + /// at runtime. + RuntimePointerCheck PtrRtCheck; + + /// \brief the Memory Dependence Checker which can determine the + /// loop-independent and loop-carried dependences between memory accesses. + MemoryDepChecker DepChecker; + + /// \brief Number of memchecks required to prove independence of otherwise + /// may-alias pointers + unsigned NumComparisons; + + Loop *TheLoop; + ScalarEvolution *SE; + const DataLayout &DL; + const TargetLibraryInfo *TLI; + AliasAnalysis *AA; + DominatorTree *DT; + LoopInfo *LI; + + unsigned NumLoads; + unsigned NumStores; + + unsigned MaxSafeDepDistBytes; + + /// \brief Cache the result of analyzeLoop. + bool CanVecMem; + + /// \brief Indicator for storing to uniform addresses. + /// If a loop has write to a loop invariant address then it should be true. + bool StoreToLoopInvariantAddress; + + /// \brief The diagnostics report generated for the analysis. E.g. why we + /// couldn't analyze the loop. + Optional<LoopAccessReport> Report; +}; + +Value *stripIntegerCast(Value *V); + +///\brief Return the SCEV corresponding to a pointer with the symbolic stride +///replaced with constant one. +/// +/// If \p OrigPtr is not null, use it to look up the stride value instead of \p +/// Ptr. \p PtrToStride provides the mapping between the pointer value and its +/// stride as collected by LoopVectorizationLegality::collectStridedAccess. +const SCEV *replaceSymbolicStrideSCEV(ScalarEvolution *SE, + const ValueToValueMap &PtrToStride, + Value *Ptr, Value *OrigPtr = nullptr); + +/// \brief This analysis provides dependence information for the memory accesses +/// of a loop. +/// +/// It runs the analysis for a loop on demand. This can be initiated by +/// querying the loop access info via LAA::getInfo. getInfo return a +/// LoopAccessInfo object. See this class for the specifics of what information +/// is provided. +class LoopAccessAnalysis : public FunctionPass { +public: + static char ID; + + LoopAccessAnalysis() : FunctionPass(ID) { + initializeLoopAccessAnalysisPass(*PassRegistry::getPassRegistry()); + } + + bool runOnFunction(Function &F) override; + + void getAnalysisUsage(AnalysisUsage &AU) const override; + + /// \brief Query the result of the loop access information for the loop \p L. + /// + /// If the client speculates (and then issues run-time checks) for the values + /// of symbolic strides, \p Strides provides the mapping (see + /// replaceSymbolicStrideSCEV). If there is no cached result available run + /// the analysis. + const LoopAccessInfo &getInfo(Loop *L, const ValueToValueMap &Strides); + + void releaseMemory() override { + // Invalidate the cache when the pass is freed. + LoopAccessInfoMap.clear(); + } + + /// \brief Print the result of the analysis when invoked with -analyze. + void print(raw_ostream &OS, const Module *M = nullptr) const override; + +private: + /// \brief The cache. + DenseMap<Loop *, std::unique_ptr<LoopAccessInfo>> LoopAccessInfoMap; + + // The used analysis passes. + ScalarEvolution *SE; + const TargetLibraryInfo *TLI; + AliasAnalysis *AA; + DominatorTree *DT; + LoopInfo *LI; +}; +} // End llvm namespace + +#endif |