1//===- LoopUnrollAnalyzer.cpp - Unrolling Effect Estimation -----*- 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 file implements UnrolledInstAnalyzer class. It's used for predicting
10// potential effects that loop unrolling might have, such as enabling constant
11// propagation and other optimizations.
12//
13//===----------------------------------------------------------------------===//
14
15#include "llvm/Analysis/LoopUnrollAnalyzer.h"
16#include "llvm/Analysis/InstructionSimplify.h"
17#include "llvm/Analysis/LoopInfo.h"
18#include "llvm/Analysis/ScalarEvolutionExpressions.h"
19#include "llvm/IR/Operator.h"
20
21using namespace llvm;
22
23/// Try to simplify instruction \param I using its SCEV expression.
24///
25/// The idea is that some AddRec expressions become constants, which then
26/// could trigger folding of other instructions. However, that only happens
27/// for expressions whose start value is also constant, which isn't always the
28/// case. In another common and important case the start value is just some
29/// address (i.e. SCEVUnknown) - in this case we compute the offset and save
30/// it along with the base address instead.
31bool UnrolledInstAnalyzer::simplifyInstWithSCEV(Instruction *I) {
32 if (!SE.isSCEVable(Ty: I->getType()))
33 return false;
34
35 const SCEV *S = SE.getSCEV(V: I);
36 if (auto *SC = dyn_cast<SCEVConstant>(Val: S)) {
37 SimplifiedValues[I] = SC->getValue();
38 return true;
39 }
40
41 // If we have a loop invariant computation, we only need to compute it once.
42 // Given that, all but the first occurance are free.
43 if (!IterationNumber->isZero() && SE.isLoopInvariant(S, L))
44 return true;
45
46 auto *AR = dyn_cast<SCEVAddRecExpr>(Val: S);
47 if (!AR || AR->getLoop() != L)
48 return false;
49
50 const SCEV *ValueAtIteration = AR->evaluateAtIteration(It: IterationNumber, SE);
51 // Check if the AddRec expression becomes a constant.
52 if (auto *SC = dyn_cast<SCEVConstant>(Val: ValueAtIteration)) {
53 SimplifiedValues[I] = SC->getValue();
54 return true;
55 }
56
57 // Check if the offset from the base address becomes a constant.
58 auto *Base = dyn_cast<SCEVUnknown>(Val: SE.getPointerBase(V: S));
59 if (!Base)
60 return false;
61 auto *Offset =
62 dyn_cast<SCEVConstant>(Val: SE.getMinusSCEV(LHS: ValueAtIteration, RHS: Base));
63 if (!Offset)
64 return false;
65 SimplifiedAddress Address;
66 Address.Base = Base->getValue();
67 Address.Offset = Offset->getValue();
68 SimplifiedAddresses[I] = Address;
69 return false;
70}
71
72/// Try to simplify binary operator I.
73///
74/// TODO: Probably it's worth to hoist the code for estimating the
75/// simplifications effects to a separate class, since we have a very similar
76/// code in InlineCost already.
77bool UnrolledInstAnalyzer::visitBinaryOperator(BinaryOperator &I) {
78 Value *LHS = I.getOperand(i_nocapture: 0), *RHS = I.getOperand(i_nocapture: 1);
79 if (!isa<Constant>(Val: LHS))
80 if (Value *SimpleLHS = SimplifiedValues.lookup(Val: LHS))
81 LHS = SimpleLHS;
82 if (!isa<Constant>(Val: RHS))
83 if (Value *SimpleRHS = SimplifiedValues.lookup(Val: RHS))
84 RHS = SimpleRHS;
85
86 Value *SimpleV = nullptr;
87 const DataLayout &DL = I.getDataLayout();
88 if (auto FI = dyn_cast<FPMathOperator>(Val: &I))
89 SimpleV =
90 simplifyBinOp(Opcode: I.getOpcode(), LHS, RHS, FMF: FI->getFastMathFlags(), Q: DL);
91 else
92 SimpleV = simplifyBinOp(Opcode: I.getOpcode(), LHS, RHS, Q: DL);
93
94 if (SimpleV) {
95 SimplifiedValues[&I] = SimpleV;
96 return true;
97 }
98 return Base::visitBinaryOperator(I);
99}
100
101/// Try to fold load I.
102bool UnrolledInstAnalyzer::visitLoad(LoadInst &I) {
103 Value *AddrOp = I.getPointerOperand();
104
105 auto AddressIt = SimplifiedAddresses.find(Val: AddrOp);
106 if (AddressIt == SimplifiedAddresses.end())
107 return false;
108 ConstantInt *SimplifiedAddrOp = AddressIt->second.Offset;
109
110 auto *GV = dyn_cast<GlobalVariable>(Val: AddressIt->second.Base);
111 // We're only interested in loads that can be completely folded to a
112 // constant.
113 if (!GV || !GV->hasDefinitiveInitializer() || !GV->isConstant())
114 return false;
115
116 ConstantDataSequential *CDS =
117 dyn_cast<ConstantDataSequential>(Val: GV->getInitializer());
118 if (!CDS)
119 return false;
120
121 // We might have a vector load from an array. FIXME: for now we just bail
122 // out in this case, but we should be able to resolve and simplify such
123 // loads.
124 if (CDS->getElementType() != I.getType())
125 return false;
126
127 unsigned ElemSize = CDS->getElementType()->getPrimitiveSizeInBits() / 8U;
128 if (SimplifiedAddrOp->getValue().getActiveBits() > 64)
129 return false;
130 int64_t SimplifiedAddrOpV = SimplifiedAddrOp->getSExtValue();
131 if (SimplifiedAddrOpV < 0) {
132 // FIXME: For now we conservatively ignore out of bound accesses, but
133 // we're allowed to perform the optimization in this case.
134 return false;
135 }
136 uint64_t Index = static_cast<uint64_t>(SimplifiedAddrOpV) / ElemSize;
137 if (Index >= CDS->getNumElements()) {
138 // FIXME: For now we conservatively ignore out of bound accesses, but
139 // we're allowed to perform the optimization in this case.
140 return false;
141 }
142
143 Constant *CV = CDS->getElementAsConstant(i: Index);
144 assert(CV && "Constant expected.");
145 SimplifiedValues[&I] = CV;
146
147 return true;
148}
149
150/// Try to simplify cast instruction.
151bool UnrolledInstAnalyzer::visitCastInst(CastInst &I) {
152 Value *Op = I.getOperand(i_nocapture: 0);
153 if (Value *Simplified = SimplifiedValues.lookup(Val: Op))
154 Op = Simplified;
155
156 // The cast can be invalid, because SimplifiedValues contains results of SCEV
157 // analysis, which operates on integers (and, e.g., might convert i8* null to
158 // i32 0).
159 if (CastInst::castIsValid(op: I.getOpcode(), S: Op, DstTy: I.getType())) {
160 const DataLayout &DL = I.getDataLayout();
161 if (Value *V = simplifyCastInst(CastOpc: I.getOpcode(), Op, Ty: I.getType(), Q: DL)) {
162 SimplifiedValues[&I] = V;
163 return true;
164 }
165 }
166
167 return Base::visitCastInst(I);
168}
169
170/// Try to simplify cmp instruction.
171bool UnrolledInstAnalyzer::visitCmpInst(CmpInst &I) {
172 Value *LHS = I.getOperand(i_nocapture: 0), *RHS = I.getOperand(i_nocapture: 1);
173
174 // First try to handle simplified comparisons.
175 if (!isa<Constant>(Val: LHS))
176 if (Value *SimpleLHS = SimplifiedValues.lookup(Val: LHS))
177 LHS = SimpleLHS;
178 if (!isa<Constant>(Val: RHS))
179 if (Value *SimpleRHS = SimplifiedValues.lookup(Val: RHS))
180 RHS = SimpleRHS;
181
182 if (!isa<Constant>(Val: LHS) && !isa<Constant>(Val: RHS)) {
183 auto SimplifiedLHS = SimplifiedAddresses.find(Val: LHS);
184 if (SimplifiedLHS != SimplifiedAddresses.end()) {
185 auto SimplifiedRHS = SimplifiedAddresses.find(Val: RHS);
186 if (SimplifiedRHS != SimplifiedAddresses.end()) {
187 SimplifiedAddress &LHSAddr = SimplifiedLHS->second;
188 SimplifiedAddress &RHSAddr = SimplifiedRHS->second;
189 if (LHSAddr.Base == RHSAddr.Base) {
190 LHS = LHSAddr.Offset;
191 RHS = RHSAddr.Offset;
192 }
193 }
194 }
195 }
196
197 const DataLayout &DL = I.getDataLayout();
198 if (Value *V = simplifyCmpInst(Predicate: I.getPredicate(), LHS, RHS, Q: DL)) {
199 SimplifiedValues[&I] = V;
200 return true;
201 }
202
203 return Base::visitCmpInst(I);
204}
205
206bool UnrolledInstAnalyzer::visitPHINode(PHINode &PN) {
207 // Run base visitor first. This way we can gather some useful for later
208 // analysis information.
209 if (Base::visitPHINode(I&: PN))
210 return true;
211
212 // The loop induction PHI nodes are definitionally free.
213 return PN.getParent() == L->getHeader();
214}
215
216bool UnrolledInstAnalyzer::visitInstruction(Instruction &I) {
217 return simplifyInstWithSCEV(I: &I);
218}
219