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