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
22namespace llvm {
23
24class AAResults;
25class DependenceInfo;
26class Instruction;
27class LPMUpdater;
28class raw_ostream;
29class LoopInfo;
30class Loop;
31class ScalarEvolution;
32class SCEV;
33class TargetTransformInfo;
34
35using CacheCostTy = InstructionCost;
36using 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]
49class IndexedReference {
50 friend raw_ostream &operator<<(raw_ostream &OS, const IndexedReference &R);
51
52public:
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
99private:
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
140private:
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.
172using ReferenceGroupTy = SmallVector<std::unique_ptr<IndexedReference>, 8>;
173using 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.
190class 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
195public:
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(Range: LoopCosts, P: [&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
224private:
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(Range&: LoopCosts,
255 C: [](const LoopCacheCostTy &A, const LoopCacheCostTy &B) {
256 return A.second > B.second;
257 });
258 }
259
260private:
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
281raw_ostream &operator<<(raw_ostream &OS, const IndexedReference &R);
282raw_ostream &operator<<(raw_ostream &OS, const CacheCost &CC);
283
284/// Printer pass for the \c CacheCost results.
285class LoopCachePrinterPass : public PassInfoMixin<LoopCachePrinterPass> {
286 raw_ostream &OS;
287
288public:
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