283 lines
12 KiB
C
283 lines
12 KiB
C
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//===- llvm/Analysis/LoopCacheAnalysis.h ------------------------*- C++ -*-===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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///
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/// \file
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/// This file defines the interface for the loop cache analysis.
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///
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//===----------------------------------------------------------------------===//
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#ifndef LLVM_ANALYSIS_LOOPCACHEANALYSIS_H
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#define LLVM_ANALYSIS_LOOPCACHEANALYSIS_H
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#include "llvm/Analysis/LoopAnalysisManager.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/PassManager.h"
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#include "llvm/Support/raw_ostream.h"
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namespace llvm {
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class AAResults;
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class DependenceInfo;
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class LPMUpdater;
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class ScalarEvolution;
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class SCEV;
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class TargetTransformInfo;
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using CacheCostTy = int64_t;
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using LoopVectorTy = SmallVector<Loop *, 8>;
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/// Represents a memory reference as a base pointer and a set of indexing
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/// operations. For example given the array reference A[i][2j+1][3k+2] in a
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/// 3-dim loop nest:
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/// for(i=0;i<n;++i)
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/// for(j=0;j<m;++j)
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/// for(k=0;k<o;++k)
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/// ... A[i][2j+1][3k+2] ...
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/// We expect:
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/// BasePointer -> A
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/// Subscripts -> [{0,+,1}<%for.i>][{1,+,2}<%for.j>][{2,+,3}<%for.k>]
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/// Sizes -> [m][o][4]
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class IndexedReference {
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friend raw_ostream &operator<<(raw_ostream &OS, const IndexedReference &R);
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public:
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/// Construct an indexed reference given a \p StoreOrLoadInst instruction.
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IndexedReference(Instruction &StoreOrLoadInst, const LoopInfo &LI,
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ScalarEvolution &SE);
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bool isValid() const { return IsValid; }
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const SCEV *getBasePointer() const { return BasePointer; }
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size_t getNumSubscripts() const { return Subscripts.size(); }
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const SCEV *getSubscript(unsigned SubNum) const {
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assert(SubNum < getNumSubscripts() && "Invalid subscript number");
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return Subscripts[SubNum];
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}
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const SCEV *getFirstSubscript() const {
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assert(!Subscripts.empty() && "Expecting non-empty container");
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return Subscripts.front();
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}
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const SCEV *getLastSubscript() const {
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assert(!Subscripts.empty() && "Expecting non-empty container");
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return Subscripts.back();
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}
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/// Return true/false if the current object and the indexed reference \p Other
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/// are/aren't in the same cache line of size \p CLS. Two references are in
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/// the same chace line iff the distance between them in the innermost
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/// dimension is less than the cache line size. Return None if unsure.
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Optional<bool> hasSpacialReuse(const IndexedReference &Other, unsigned CLS,
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AAResults &AA) const;
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/// Return true if the current object and the indexed reference \p Other
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/// have distance smaller than \p MaxDistance in the dimension associated with
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/// the given loop \p L. Return false if the distance is not smaller than \p
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/// MaxDistance and None if unsure.
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Optional<bool> hasTemporalReuse(const IndexedReference &Other,
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unsigned MaxDistance, const Loop &L,
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DependenceInfo &DI, AAResults &AA) const;
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/// Compute the cost of the reference w.r.t. the given loop \p L when it is
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/// considered in the innermost position in the loop nest.
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/// The cost is defined as:
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/// - equal to one if the reference is loop invariant, or
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/// - equal to '(TripCount * stride) / cache_line_size' if:
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/// + the reference stride is less than the cache line size, and
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/// + the coefficient of this loop's index variable used in all other
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/// subscripts is zero
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/// - or otherwise equal to 'TripCount'.
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CacheCostTy computeRefCost(const Loop &L, unsigned CLS) const;
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private:
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/// Attempt to delinearize the indexed reference.
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bool delinearize(const LoopInfo &LI);
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/// Return true if the index reference is invariant with respect to loop \p L.
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bool isLoopInvariant(const Loop &L) const;
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/// Return true if the indexed reference is 'consecutive' in loop \p L.
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/// An indexed reference is 'consecutive' if the only coefficient that uses
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/// the loop induction variable is the rightmost one, and the access stride is
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/// smaller than the cache line size \p CLS.
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bool isConsecutive(const Loop &L, unsigned CLS) const;
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/// Return the coefficient used in the rightmost dimension.
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const SCEV *getLastCoefficient() const;
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/// Return true if the coefficient corresponding to induction variable of
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/// loop \p L in the given \p Subscript is zero or is loop invariant in \p L.
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bool isCoeffForLoopZeroOrInvariant(const SCEV &Subscript,
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const Loop &L) const;
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/// Verify that the given \p Subscript is 'well formed' (must be a simple add
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/// recurrence).
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bool isSimpleAddRecurrence(const SCEV &Subscript, const Loop &L) const;
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/// Return true if the given reference \p Other is definetely aliased with
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/// the indexed reference represented by this class.
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bool isAliased(const IndexedReference &Other, AAResults &AA) const;
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private:
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/// True if the reference can be delinearized, false otherwise.
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bool IsValid = false;
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/// Represent the memory reference instruction.
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Instruction &StoreOrLoadInst;
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/// The base pointer of the memory reference.
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const SCEV *BasePointer = nullptr;
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/// The subscript (indexes) of the memory reference.
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SmallVector<const SCEV *, 3> Subscripts;
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/// The dimensions of the memory reference.
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SmallVector<const SCEV *, 3> Sizes;
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ScalarEvolution &SE;
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};
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/// A reference group represents a set of memory references that exhibit
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/// temporal or spacial reuse. Two references belong to the same
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/// reference group with respect to a inner loop L iff:
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/// 1. they have a loop independent dependency, or
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/// 2. they have a loop carried dependence with a small dependence distance
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/// (e.g. less than 2) carried by the inner loop, or
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/// 3. they refer to the same array, and the subscript in their innermost
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/// dimension is less than or equal to 'd' (where 'd' is less than the cache
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/// line size)
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///
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/// Intuitively a reference group represents memory references that access
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/// the same cache line. Conditions 1,2 above account for temporal reuse, while
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/// contition 3 accounts for spacial reuse.
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using ReferenceGroupTy = SmallVector<std::unique_ptr<IndexedReference>, 8>;
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using ReferenceGroupsTy = SmallVector<ReferenceGroupTy, 8>;
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/// \c CacheCost represents the estimated cost of a inner loop as the number of
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/// cache lines used by the memory references it contains.
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/// The 'cache cost' of a loop 'L' in a loop nest 'LN' is computed as the sum of
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/// the cache costs of all of its reference groups when the loop is considered
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/// to be in the innermost position in the nest.
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/// A reference group represents memory references that fall into the same cache
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/// line. Each reference group is analysed with respect to the innermost loop in
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/// a loop nest. The cost of a reference is defined as follow:
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/// - one if it is loop invariant w.r.t the innermost loop,
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/// - equal to the loop trip count divided by the cache line times the
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/// reference stride if the reference stride is less than the cache line
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/// size (CLS), and the coefficient of this loop's index variable used in all
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/// other subscripts is zero (e.g. RefCost = TripCount/(CLS/RefStride))
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/// - equal to the innermost loop trip count if the reference stride is greater
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/// or equal to the cache line size CLS.
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class CacheCost {
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friend raw_ostream &operator<<(raw_ostream &OS, const CacheCost &CC);
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using LoopTripCountTy = std::pair<const Loop *, unsigned>;
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using LoopCacheCostTy = std::pair<const Loop *, CacheCostTy>;
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public:
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static CacheCostTy constexpr InvalidCost = -1;
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/// Construct a CacheCost object for the loop nest described by \p Loops.
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/// The optional parameter \p TRT can be used to specify the max. distance
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/// between array elements accessed in a loop so that the elements are
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/// classified to have temporal reuse.
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CacheCost(const LoopVectorTy &Loops, const LoopInfo &LI, ScalarEvolution &SE,
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TargetTransformInfo &TTI, AAResults &AA, DependenceInfo &DI,
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Optional<unsigned> TRT = None);
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/// Create a CacheCost for the loop nest rooted by \p Root.
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/// The optional parameter \p TRT can be used to specify the max. distance
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/// between array elements accessed in a loop so that the elements are
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/// classified to have temporal reuse.
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static std::unique_ptr<CacheCost>
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getCacheCost(Loop &Root, LoopStandardAnalysisResults &AR, DependenceInfo &DI,
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Optional<unsigned> TRT = None);
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/// Return the estimated cost of loop \p L if the given loop is part of the
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/// loop nest associated with this object. Return -1 otherwise.
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CacheCostTy getLoopCost(const Loop &L) const {
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auto IT = llvm::find_if(LoopCosts, [&L](const LoopCacheCostTy &LCC) {
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return LCC.first == &L;
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});
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return (IT != LoopCosts.end()) ? (*IT).second : -1;
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}
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/// Return the estimated ordered loop costs.
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const ArrayRef<LoopCacheCostTy> getLoopCosts() const { return LoopCosts; }
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private:
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/// Calculate the cache footprint of each loop in the nest (when it is
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/// considered to be in the innermost position).
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void calculateCacheFootprint();
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/// Partition store/load instructions in the loop nest into reference groups.
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/// Two or more memory accesses belong in the same reference group if they
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/// share the same cache line.
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bool populateReferenceGroups(ReferenceGroupsTy &RefGroups) const;
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/// Calculate the cost of the given loop \p L assuming it is the innermost
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/// loop in nest.
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CacheCostTy computeLoopCacheCost(const Loop &L,
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const ReferenceGroupsTy &RefGroups) const;
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/// Compute the cost of a representative reference in reference group \p RG
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/// when the given loop \p L is considered as the innermost loop in the nest.
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/// The computed cost is an estimate for the number of cache lines used by the
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/// reference group. The representative reference cost is defined as:
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/// - equal to one if the reference is loop invariant, or
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/// - equal to '(TripCount * stride) / cache_line_size' if (a) loop \p L's
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/// induction variable is used only in the reference subscript associated
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/// with loop \p L, and (b) the reference stride is less than the cache
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/// line size, or
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/// - TripCount otherwise
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CacheCostTy computeRefGroupCacheCost(const ReferenceGroupTy &RG,
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const Loop &L) const;
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/// Sort the LoopCosts vector by decreasing cache cost.
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void sortLoopCosts() {
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sort(LoopCosts, [](const LoopCacheCostTy &A, const LoopCacheCostTy &B) {
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return A.second > B.second;
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});
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}
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private:
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/// Loops in the loop nest associated with this object.
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LoopVectorTy Loops;
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/// Trip counts for the loops in the loop nest associated with this object.
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SmallVector<LoopTripCountTy, 3> TripCounts;
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/// Cache costs for the loops in the loop nest associated with this object.
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SmallVector<LoopCacheCostTy, 3> LoopCosts;
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/// The max. distance between array elements accessed in a loop so that the
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/// elements are classified to have temporal reuse.
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Optional<unsigned> TRT;
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const LoopInfo &LI;
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ScalarEvolution &SE;
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TargetTransformInfo &TTI;
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AAResults &AA;
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DependenceInfo &DI;
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};
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raw_ostream &operator<<(raw_ostream &OS, const IndexedReference &R);
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raw_ostream &operator<<(raw_ostream &OS, const CacheCost &CC);
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/// Printer pass for the \c CacheCost results.
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class LoopCachePrinterPass : public PassInfoMixin<LoopCachePrinterPass> {
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raw_ostream &OS;
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public:
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explicit LoopCachePrinterPass(raw_ostream &OS) : OS(OS) {}
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PreservedAnalyses run(Loop &L, LoopAnalysisManager &AM,
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LoopStandardAnalysisResults &AR, LPMUpdater &U);
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};
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} // namespace llvm
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#endif // LLVM_ANALYSIS_LOOPCACHEANALYSIS_H
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