1//===- NaryReassociate.h - Reassociate n-ary expressions --------*- 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// This pass reassociates n-ary add expressions and eliminates the redundancy
10// exposed by the reassociation.
11//
12// A motivating example:
13//
14// void foo(int a, int b) {
15// bar(a + b);
16// bar((a + 2) + b);
17// }
18//
19// An ideal compiler should reassociate (a + 2) + b to (a + b) + 2 and simplify
20// the above code to
21//
22// int t = a + b;
23// bar(t);
24// bar(t + 2);
25//
26// However, the Reassociate pass is unable to do that because it processes each
27// instruction individually and believes (a + 2) + b is the best form according
28// to its rank system.
29//
30// To address this limitation, NaryReassociate reassociates an expression in a
31// form that reuses existing instructions. As a result, NaryReassociate can
32// reassociate (a + 2) + b in the example to (a + b) + 2 because it detects that
33// (a + b) is computed before.
34//
35// NaryReassociate works as follows. For every instruction in the form of (a +
36// b) + c, it checks whether a + c or b + c is already computed by a dominating
37// instruction. If so, it then reassociates (a + b) + c into (a + c) + b or (b +
38// c) + a and removes the redundancy accordingly. To efficiently look up whether
39// an expression is computed before, we store each instruction seen and its SCEV
40// into an SCEV-to-instruction map.
41//
42// Although the algorithm pattern-matches only ternary additions, it
43// automatically handles many >3-ary expressions by walking through the function
44// in the depth-first order. For example, given
45//
46// (a + c) + d
47// ((a + b) + c) + d
48//
49// NaryReassociate first rewrites (a + b) + c to (a + c) + b, and then rewrites
50// ((a + c) + b) + d into ((a + c) + d) + b.
51//
52// Finally, the above dominator-based algorithm may need to be run multiple
53// iterations before emitting optimal code. One source of this need is that we
54// only split an operand when it is used only once. The above algorithm can
55// eliminate an instruction and decrease the usage count of its operands. As a
56// result, an instruction that previously had multiple uses may become a
57// single-use instruction and thus eligible for split consideration. For
58// example,
59//
60// ac = a + c
61// ab = a + b
62// abc = ab + c
63// ab2 = ab + b
64// ab2c = ab2 + c
65//
66// In the first iteration, we cannot reassociate abc to ac+b because ab is used
67// twice. However, we can reassociate ab2c to abc+b in the first iteration. As a
68// result, ab2 becomes dead and ab will be used only once in the second
69// iteration.
70//
71// Limitations and TODO items:
72//
73// 1) We only considers n-ary adds and muls for now. This should be extended
74// and generalized.
75//
76//===----------------------------------------------------------------------===//
77
78#ifndef LLVM_TRANSFORMS_SCALAR_NARYREASSOCIATE_H
79#define LLVM_TRANSFORMS_SCALAR_NARYREASSOCIATE_H
80
81#include "llvm/ADT/DenseMap.h"
82#include "llvm/ADT/SmallVector.h"
83#include "llvm/Analysis/ScalarEvolution.h"
84#include "llvm/IR/PassManager.h"
85#include "llvm/IR/ValueHandle.h"
86
87namespace llvm {
88
89class AssumptionCache;
90class BinaryOperator;
91class DataLayout;
92class DominatorTree;
93class Function;
94class GetElementPtrInst;
95class Instruction;
96class IntrinsicInst;
97class TargetLibraryInfo;
98class TargetTransformInfo;
99class Type;
100class Value;
101
102class NaryReassociatePass : public OptionalPassInfoMixin<NaryReassociatePass> {
103public:
104 LLVM_ABI PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
105
106 // Glue for old PM.
107 LLVM_ABI bool runImpl(Function &F, AssumptionCache *AC_, DominatorTree *DT_,
108 ScalarEvolution *SE_, TargetLibraryInfo *TLI_,
109 TargetTransformInfo *TTI_);
110
111private:
112 // Runs only one iteration of the dominator-based algorithm. See the header
113 // comments for why we need multiple iterations.
114 bool doOneIteration(Function &F);
115
116 // Reassociates I for better CSE.
117 Instruction *tryReassociate(Instruction *I, SCEVUse &OrigSCEV);
118
119 // Reassociate GEP for better CSE.
120 Instruction *tryReassociateGEP(GetElementPtrInst *GEP);
121
122 // Try splitting GEP at the I-th index and see whether either part can be
123 // CSE'ed. This is a helper function for tryReassociateGEP.
124 //
125 // \p IndexedType The element type indexed by GEP's I-th index. This is
126 // equivalent to
127 // GEP->getIndexedType(GEP->getPointerOperand(), 0-th index,
128 // ..., i-th index).
129 GetElementPtrInst *tryReassociateGEPAtIndex(GetElementPtrInst *GEP,
130 unsigned I, Type *IndexedType);
131
132 // Given GEP's I-th index = LHS + RHS, see whether &Base[..][LHS][..] or
133 // &Base[..][RHS][..] can be CSE'ed and rewrite GEP accordingly.
134 GetElementPtrInst *tryReassociateGEPAtIndex(GetElementPtrInst *GEP,
135 unsigned I, Value *LHS,
136 Value *RHS, Type *IndexedType);
137
138 // Reassociate binary operators for better CSE.
139 Instruction *tryReassociateBinaryOp(BinaryOperator *I);
140
141 // A helper function for tryReassociateBinaryOp. LHS and RHS are explicitly
142 // passed.
143 Instruction *tryReassociateBinaryOp(Value *LHS, Value *RHS,
144 BinaryOperator *I);
145 // Rewrites I to (LHS op RHS) if LHS is computed already.
146 Instruction *tryReassociatedBinaryOp(SCEVUse LHS, Value *RHS,
147 BinaryOperator *I);
148
149 // Tries to match Op1 and Op2 by using V.
150 bool matchTernaryOp(BinaryOperator *I, Value *V, Value *&Op1, Value *&Op2);
151
152 // Gets SCEV for (LHS op RHS).
153 SCEVUse getBinarySCEV(BinaryOperator *I, SCEVUse LHS, SCEVUse RHS);
154
155 // Returns the closest dominator of \c Dominatee that computes
156 // \c CandidateExpr. Returns null if not found.
157 Instruction *findClosestMatchingDominator(SCEVUse CandidateExpr,
158 Instruction *Dominatee);
159
160 // Reassociate Min/Max.
161 Value *tryReassociateMinOrMax(IntrinsicInst *I);
162
163 // GetElementPtrInst implicitly sign-extends an index if the index is shorter
164 // than the pointer size. This function returns whether Index is shorter than
165 // GEP's pointer size, i.e., whether Index needs to be sign-extended in order
166 // to be an index of GEP.
167 bool requiresSignExtension(Value *Index, GetElementPtrInst *GEP);
168
169 AssumptionCache *AC;
170 const DataLayout *DL;
171 DominatorTree *DT;
172 ScalarEvolution *SE;
173 TargetLibraryInfo *TLI;
174 TargetTransformInfo *TTI;
175
176 // A lookup table quickly telling which instructions compute the given SCEV.
177 // Note that there can be multiple instructions at different locations
178 // computing to the same SCEV, so we map a SCEV to an instruction list. For
179 // example,
180 //
181 // if (p1)
182 // foo(a + b);
183 // if (p2)
184 // bar(a + b);
185 DenseMap<const SCEV *, SmallVector<WeakTrackingVH, 2>> SeenExprs;
186};
187
188} // end namespace llvm
189
190#endif // LLVM_TRANSFORMS_SCALAR_NARYREASSOCIATE_H
191