1 //===- llvm/Analysis/LoopCacheAnalysis.h ------------------------*- C++ -*-===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 /// 9 /// \file 10 /// This file defines the interface for the loop cache analysis. 11 /// 12 //===----------------------------------------------------------------------===// 13 14 #ifndef LLVM_ANALYSIS_LOOPCACHEANALYSIS_H 15 #define LLVM_ANALYSIS_LOOPCACHEANALYSIS_H 16 17 #include "llvm/Analysis/LoopAnalysisManager.h" 18 #include "llvm/IR/PassManager.h" 19 #include "llvm/Support/InstructionCost.h" 20 #include <optional> 21 22 namespace llvm { 23 24 class AAResults; 25 class DependenceInfo; 26 class Instruction; 27 class LPMUpdater; 28 class raw_ostream; 29 class LoopInfo; 30 class Loop; 31 class ScalarEvolution; 32 class SCEV; 33 class TargetTransformInfo; 34 35 using CacheCostTy = InstructionCost; 36 using LoopVectorTy = SmallVector<Loop *, 8>; 37 38 /// Represents a memory reference as a base pointer and a set of indexing 39 /// operations. For example given the array reference A[i][2j+1][3k+2] in a 40 /// 3-dim loop nest: 41 /// for(i=0;i<n;++i) 42 /// for(j=0;j<m;++j) 43 /// for(k=0;k<o;++k) 44 /// ... A[i][2j+1][3k+2] ... 45 /// We expect: 46 /// BasePointer -> A 47 /// Subscripts -> [{0,+,1}<%for.i>][{1,+,2}<%for.j>][{2,+,3}<%for.k>] 48 /// Sizes -> [m][o][4] 49 class IndexedReference { 50 friend raw_ostream &operator<<(raw_ostream &OS, const IndexedReference &R); 51 52 public: 53 /// Construct an indexed reference given a \p StoreOrLoadInst instruction. 54 IndexedReference(Instruction &StoreOrLoadInst, const LoopInfo &LI, 55 ScalarEvolution &SE); 56 57 bool isValid() const { return IsValid; } 58 const SCEV *getBasePointer() const { return BasePointer; } 59 size_t getNumSubscripts() const { return Subscripts.size(); } 60 const SCEV *getSubscript(unsigned SubNum) const { 61 assert(SubNum < getNumSubscripts() && "Invalid subscript number"); 62 return Subscripts[SubNum]; 63 } 64 const SCEV *getFirstSubscript() const { 65 assert(!Subscripts.empty() && "Expecting non-empty container"); 66 return Subscripts.front(); 67 } 68 const SCEV *getLastSubscript() const { 69 assert(!Subscripts.empty() && "Expecting non-empty container"); 70 return Subscripts.back(); 71 } 72 73 /// Return true/false if the current object and the indexed reference \p Other 74 /// are/aren't in the same cache line of size \p CLS. Two references are in 75 /// the same chace line iff the distance between them in the innermost 76 /// dimension is less than the cache line size. Return std::nullopt if unsure. 77 std::optional<bool> hasSpacialReuse(const IndexedReference &Other, 78 unsigned CLS, AAResults &AA) const; 79 80 /// Return true if the current object and the indexed reference \p Other 81 /// have distance smaller than \p MaxDistance in the dimension associated with 82 /// the given loop \p L. Return false if the distance is not smaller than \p 83 /// MaxDistance and std::nullopt if unsure. 84 std::optional<bool> hasTemporalReuse(const IndexedReference &Other, 85 unsigned MaxDistance, const Loop &L, 86 DependenceInfo &DI, AAResults &AA) const; 87 88 /// Compute the cost of the reference w.r.t. the given loop \p L when it is 89 /// considered in the innermost position in the loop nest. 90 /// The cost is defined as: 91 /// - equal to one if the reference is loop invariant, or 92 /// - equal to '(TripCount * stride) / cache_line_size' if: 93 /// + the reference stride is less than the cache line size, and 94 /// + the coefficient of this loop's index variable used in all other 95 /// subscripts is zero 96 /// - or otherwise equal to 'TripCount'. 97 CacheCostTy computeRefCost(const Loop &L, unsigned CLS) const; 98 99 private: 100 /// Attempt to delinearize the indexed reference. 101 bool delinearize(const LoopInfo &LI); 102 103 /// Attempt to delinearize \p AccessFn for fixed-size arrays. 104 bool tryDelinearizeFixedSize(const SCEV *AccessFn, 105 SmallVectorImpl<const SCEV *> &Subscripts); 106 107 /// Return true if the index reference is invariant with respect to loop \p L. 108 bool isLoopInvariant(const Loop &L) const; 109 110 /// Return true if the indexed reference is 'consecutive' in loop \p L. 111 /// An indexed reference is 'consecutive' if the only coefficient that uses 112 /// the loop induction variable is the rightmost one, and the access stride is 113 /// smaller than the cache line size \p CLS. Provide a valid \p Stride value 114 /// if the indexed reference is 'consecutive'. 115 bool isConsecutive(const Loop &L, const SCEV *&Stride, unsigned CLS) const; 116 117 /// Retrieve the index of the subscript corresponding to the given loop \p 118 /// L. Return a zero-based positive index if the subscript index is 119 /// succesfully located and a negative value otherwise. For example given the 120 /// indexed reference 'A[i][2j+1][3k+2]', the call 121 /// 'getSubscriptIndex(loop-k)' would return value 2. 122 int getSubscriptIndex(const Loop &L) const; 123 124 /// Return the coefficient used in the rightmost dimension. 125 const SCEV *getLastCoefficient() const; 126 127 /// Return true if the coefficient corresponding to induction variable of 128 /// loop \p L in the given \p Subscript is zero or is loop invariant in \p L. 129 bool isCoeffForLoopZeroOrInvariant(const SCEV &Subscript, 130 const Loop &L) const; 131 132 /// Verify that the given \p Subscript is 'well formed' (must be a simple add 133 /// recurrence). 134 bool isSimpleAddRecurrence(const SCEV &Subscript, const Loop &L) const; 135 136 /// Return true if the given reference \p Other is definetely aliased with 137 /// the indexed reference represented by this class. 138 bool isAliased(const IndexedReference &Other, AAResults &AA) const; 139 140 private: 141 /// True if the reference can be delinearized, false otherwise. 142 bool IsValid = false; 143 144 /// Represent the memory reference instruction. 145 Instruction &StoreOrLoadInst; 146 147 /// The base pointer of the memory reference. 148 const SCEV *BasePointer = nullptr; 149 150 /// The subscript (indexes) of the memory reference. 151 SmallVector<const SCEV *, 3> Subscripts; 152 153 /// The dimensions of the memory reference. 154 SmallVector<const SCEV *, 3> Sizes; 155 156 ScalarEvolution &SE; 157 }; 158 159 /// A reference group represents a set of memory references that exhibit 160 /// temporal or spacial reuse. Two references belong to the same 161 /// reference group with respect to a inner loop L iff: 162 /// 1. they have a loop independent dependency, or 163 /// 2. they have a loop carried dependence with a small dependence distance 164 /// (e.g. less than 2) carried by the inner loop, or 165 /// 3. they refer to the same array, and the subscript in their innermost 166 /// dimension is less than or equal to 'd' (where 'd' is less than the cache 167 /// line size) 168 /// 169 /// Intuitively a reference group represents memory references that access 170 /// the same cache line. Conditions 1,2 above account for temporal reuse, while 171 /// contition 3 accounts for spacial reuse. 172 using ReferenceGroupTy = SmallVector<std::unique_ptr<IndexedReference>, 8>; 173 using ReferenceGroupsTy = SmallVector<ReferenceGroupTy, 8>; 174 175 /// \c CacheCost represents the estimated cost of a inner loop as the number of 176 /// cache lines used by the memory references it contains. 177 /// The 'cache cost' of a loop 'L' in a loop nest 'LN' is computed as the sum of 178 /// the cache costs of all of its reference groups when the loop is considered 179 /// to be in the innermost position in the nest. 180 /// A reference group represents memory references that fall into the same cache 181 /// line. Each reference group is analysed with respect to the innermost loop in 182 /// a loop nest. The cost of a reference is defined as follow: 183 /// - one if it is loop invariant w.r.t the innermost loop, 184 /// - equal to the loop trip count divided by the cache line times the 185 /// reference stride if the reference stride is less than the cache line 186 /// size (CLS), and the coefficient of this loop's index variable used in all 187 /// other subscripts is zero (e.g. RefCost = TripCount/(CLS/RefStride)) 188 /// - equal to the innermost loop trip count if the reference stride is greater 189 /// or equal to the cache line size CLS. 190 class CacheCost { 191 friend raw_ostream &operator<<(raw_ostream &OS, const CacheCost &CC); 192 using LoopTripCountTy = std::pair<const Loop *, unsigned>; 193 using LoopCacheCostTy = std::pair<const Loop *, CacheCostTy>; 194 195 public: 196 /// Construct a CacheCost object for the loop nest described by \p Loops. 197 /// The optional parameter \p TRT can be used to specify the max. distance 198 /// between array elements accessed in a loop so that the elements are 199 /// classified to have temporal reuse. 200 CacheCost(const LoopVectorTy &Loops, const LoopInfo &LI, ScalarEvolution &SE, 201 TargetTransformInfo &TTI, AAResults &AA, DependenceInfo &DI, 202 std::optional<unsigned> TRT = std::nullopt); 203 204 /// Create a CacheCost for the loop nest rooted by \p Root. 205 /// The optional parameter \p TRT can be used to specify the max. distance 206 /// between array elements accessed in a loop so that the elements are 207 /// classified to have temporal reuse. 208 static std::unique_ptr<CacheCost> 209 getCacheCost(Loop &Root, LoopStandardAnalysisResults &AR, DependenceInfo &DI, 210 std::optional<unsigned> TRT = std::nullopt); 211 212 /// Return the estimated cost of loop \p L if the given loop is part of the 213 /// loop nest associated with this object. Return -1 otherwise. 214 CacheCostTy getLoopCost(const Loop &L) const { 215 auto IT = llvm::find_if(LoopCosts, [&L](const LoopCacheCostTy &LCC) { 216 return LCC.first == &L; 217 }); 218 return (IT != LoopCosts.end()) ? (*IT).second : -1; 219 } 220 221 /// Return the estimated ordered loop costs. 222 ArrayRef<LoopCacheCostTy> getLoopCosts() const { return LoopCosts; } 223 224 private: 225 /// Calculate the cache footprint of each loop in the nest (when it is 226 /// considered to be in the innermost position). 227 void calculateCacheFootprint(); 228 229 /// Partition store/load instructions in the loop nest into reference groups. 230 /// Two or more memory accesses belong in the same reference group if they 231 /// share the same cache line. 232 bool populateReferenceGroups(ReferenceGroupsTy &RefGroups) const; 233 234 /// Calculate the cost of the given loop \p L assuming it is the innermost 235 /// loop in nest. 236 CacheCostTy computeLoopCacheCost(const Loop &L, 237 const ReferenceGroupsTy &RefGroups) const; 238 239 /// Compute the cost of a representative reference in reference group \p RG 240 /// when the given loop \p L is considered as the innermost loop in the nest. 241 /// The computed cost is an estimate for the number of cache lines used by the 242 /// reference group. The representative reference cost is defined as: 243 /// - equal to one if the reference is loop invariant, or 244 /// - equal to '(TripCount * stride) / cache_line_size' if (a) loop \p L's 245 /// induction variable is used only in the reference subscript associated 246 /// with loop \p L, and (b) the reference stride is less than the cache 247 /// line size, or 248 /// - TripCount otherwise 249 CacheCostTy computeRefGroupCacheCost(const ReferenceGroupTy &RG, 250 const Loop &L) const; 251 252 /// Sort the LoopCosts vector by decreasing cache cost. 253 void sortLoopCosts() { 254 stable_sort(LoopCosts, 255 [](const LoopCacheCostTy &A, const LoopCacheCostTy &B) { 256 return A.second > B.second; 257 }); 258 } 259 260 private: 261 /// Loops in the loop nest associated with this object. 262 LoopVectorTy Loops; 263 264 /// Trip counts for the loops in the loop nest associated with this object. 265 SmallVector<LoopTripCountTy, 3> TripCounts; 266 267 /// Cache costs for the loops in the loop nest associated with this object. 268 SmallVector<LoopCacheCostTy, 3> LoopCosts; 269 270 /// The max. distance between array elements accessed in a loop so that the 271 /// elements are classified to have temporal reuse. 272 std::optional<unsigned> TRT; 273 274 const LoopInfo &LI; 275 ScalarEvolution &SE; 276 TargetTransformInfo &TTI; 277 AAResults &AA; 278 DependenceInfo &DI; 279 }; 280 281 raw_ostream &operator<<(raw_ostream &OS, const IndexedReference &R); 282 raw_ostream &operator<<(raw_ostream &OS, const CacheCost &CC); 283 284 /// Printer pass for the \c CacheCost results. 285 class LoopCachePrinterPass : public PassInfoMixin<LoopCachePrinterPass> { 286 raw_ostream &OS; 287 288 public: 289 explicit LoopCachePrinterPass(raw_ostream &OS) : OS(OS) {} 290 291 PreservedAnalyses run(Loop &L, LoopAnalysisManager &AM, 292 LoopStandardAnalysisResults &AR, LPMUpdater &U); 293 294 static bool isRequired() { return true; } 295 }; 296 297 } // namespace llvm 298 299 #endif // LLVM_ANALYSIS_LOOPCACHEANALYSIS_H 300