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comparison lib/Analysis/ScalarEvolutionNormalization.cpp @ 0:95c75e76d11b LLVM3.4

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LLVM 3.4
author Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
date Thu, 12 Dec 2013 13:56:28 +0900
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children 54457678186b
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-1:000000000000 0:95c75e76d11b
1 //===- ScalarEvolutionNormalization.cpp - See below -------------*- C++ -*-===//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file implements utilities for working with "normalized" expressions.
11 // See the comments at the top of ScalarEvolutionNormalization.h for details.
12 //
13 //===----------------------------------------------------------------------===//
14
15 #include "llvm/Analysis/Dominators.h"
16 #include "llvm/Analysis/LoopInfo.h"
17 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
18 #include "llvm/Analysis/ScalarEvolutionNormalization.h"
19 using namespace llvm;
20
21 /// IVUseShouldUsePostIncValue - We have discovered a "User" of an IV expression
22 /// and now we need to decide whether the user should use the preinc or post-inc
23 /// value. If this user should use the post-inc version of the IV, return true.
24 ///
25 /// Choosing wrong here can break dominance properties (if we choose to use the
26 /// post-inc value when we cannot) or it can end up adding extra live-ranges to
27 /// the loop, resulting in reg-reg copies (if we use the pre-inc value when we
28 /// should use the post-inc value).
29 static bool IVUseShouldUsePostIncValue(Instruction *User, Value *Operand,
30 const Loop *L, DominatorTree *DT) {
31 // If the user is in the loop, use the preinc value.
32 if (L->contains(User)) return false;
33
34 BasicBlock *LatchBlock = L->getLoopLatch();
35 if (!LatchBlock)
36 return false;
37
38 // Ok, the user is outside of the loop. If it is dominated by the latch
39 // block, use the post-inc value.
40 if (DT->dominates(LatchBlock, User->getParent()))
41 return true;
42
43 // There is one case we have to be careful of: PHI nodes. These little guys
44 // can live in blocks that are not dominated by the latch block, but (since
45 // their uses occur in the predecessor block, not the block the PHI lives in)
46 // should still use the post-inc value. Check for this case now.
47 PHINode *PN = dyn_cast<PHINode>(User);
48 if (!PN || !Operand) return false; // not a phi, not dominated by latch block.
49
50 // Look at all of the uses of Operand by the PHI node. If any use corresponds
51 // to a block that is not dominated by the latch block, give up and use the
52 // preincremented value.
53 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
54 if (PN->getIncomingValue(i) == Operand &&
55 !DT->dominates(LatchBlock, PN->getIncomingBlock(i)))
56 return false;
57
58 // Okay, all uses of Operand by PN are in predecessor blocks that really are
59 // dominated by the latch block. Use the post-incremented value.
60 return true;
61 }
62
63 namespace {
64
65 /// Hold the state used during post-inc expression transformation, including a
66 /// map of transformed expressions.
67 class PostIncTransform {
68 TransformKind Kind;
69 PostIncLoopSet &Loops;
70 ScalarEvolution &SE;
71 DominatorTree &DT;
72
73 DenseMap<const SCEV*, const SCEV*> Transformed;
74
75 public:
76 PostIncTransform(TransformKind kind, PostIncLoopSet &loops,
77 ScalarEvolution &se, DominatorTree &dt):
78 Kind(kind), Loops(loops), SE(se), DT(dt) {}
79
80 const SCEV *TransformSubExpr(const SCEV *S, Instruction *User,
81 Value *OperandValToReplace);
82
83 protected:
84 const SCEV *TransformImpl(const SCEV *S, Instruction *User,
85 Value *OperandValToReplace);
86 };
87
88 } // namespace
89
90 /// Implement post-inc transformation for all valid expression types.
91 const SCEV *PostIncTransform::
92 TransformImpl(const SCEV *S, Instruction *User, Value *OperandValToReplace) {
93
94 if (const SCEVCastExpr *X = dyn_cast<SCEVCastExpr>(S)) {
95 const SCEV *O = X->getOperand();
96 const SCEV *N = TransformSubExpr(O, User, OperandValToReplace);
97 if (O != N)
98 switch (S->getSCEVType()) {
99 case scZeroExtend: return SE.getZeroExtendExpr(N, S->getType());
100 case scSignExtend: return SE.getSignExtendExpr(N, S->getType());
101 case scTruncate: return SE.getTruncateExpr(N, S->getType());
102 default: llvm_unreachable("Unexpected SCEVCastExpr kind!");
103 }
104 return S;
105 }
106
107 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
108 // An addrec. This is the interesting part.
109 SmallVector<const SCEV *, 8> Operands;
110 const Loop *L = AR->getLoop();
111 // The addrec conceptually uses its operands at loop entry.
112 Instruction *LUser = L->getHeader()->begin();
113 // Transform each operand.
114 for (SCEVNAryExpr::op_iterator I = AR->op_begin(), E = AR->op_end();
115 I != E; ++I) {
116 Operands.push_back(TransformSubExpr(*I, LUser, 0));
117 }
118 // Conservatively use AnyWrap until/unless we need FlagNW.
119 const SCEV *Result = SE.getAddRecExpr(Operands, L, SCEV::FlagAnyWrap);
120 switch (Kind) {
121 case NormalizeAutodetect:
122 // Normalize this SCEV by subtracting the expression for the final step.
123 // We only allow affine AddRecs to be normalized, otherwise we would not
124 // be able to correctly denormalize.
125 // e.g. {1,+,3,+,2} == {-2,+,1,+,2} + {3,+,2}
126 // Normalized form: {-2,+,1,+,2}
127 // Denormalized form: {1,+,3,+,2}
128 //
129 // However, denormalization would use the a different step expression than
130 // normalization (see getPostIncExpr), generating the wrong final
131 // expression: {-2,+,1,+,2} + {1,+,2} => {-1,+,3,+,2}
132 if (AR->isAffine() &&
133 IVUseShouldUsePostIncValue(User, OperandValToReplace, L, &DT)) {
134 Result = SE.getMinusSCEV(Result, AR->getStepRecurrence(SE));
135 Loops.insert(L);
136 }
137 #if 0
138 // This assert is conceptually correct, but ScalarEvolution currently
139 // sometimes fails to canonicalize two equal SCEVs to exactly the same
140 // form. It's possibly a pessimization when this happens, but it isn't a
141 // correctness problem, so disable this assert for now.
142 assert(S == TransformSubExpr(Result, User, OperandValToReplace) &&
143 "SCEV normalization is not invertible!");
144 #endif
145 break;
146 case Normalize:
147 if (Loops.count(L)) {
148 const SCEV *TransformedStep =
149 TransformSubExpr(AR->getStepRecurrence(SE),
150 User, OperandValToReplace);
151 Result = SE.getMinusSCEV(Result, TransformedStep);
152 }
153 #if 0
154 // See the comment on the assert above.
155 assert(S == TransformSubExpr(Result, User, OperandValToReplace) &&
156 "SCEV normalization is not invertible!");
157 #endif
158 break;
159 case Denormalize:
160 if (Loops.count(L))
161 Result = cast<SCEVAddRecExpr>(Result)->getPostIncExpr(SE);
162 break;
163 }
164 return Result;
165 }
166
167 if (const SCEVNAryExpr *X = dyn_cast<SCEVNAryExpr>(S)) {
168 SmallVector<const SCEV *, 8> Operands;
169 bool Changed = false;
170 // Transform each operand.
171 for (SCEVNAryExpr::op_iterator I = X->op_begin(), E = X->op_end();
172 I != E; ++I) {
173 const SCEV *O = *I;
174 const SCEV *N = TransformSubExpr(O, User, OperandValToReplace);
175 Changed |= N != O;
176 Operands.push_back(N);
177 }
178 // If any operand actually changed, return a transformed result.
179 if (Changed)
180 switch (S->getSCEVType()) {
181 case scAddExpr: return SE.getAddExpr(Operands);
182 case scMulExpr: return SE.getMulExpr(Operands);
183 case scSMaxExpr: return SE.getSMaxExpr(Operands);
184 case scUMaxExpr: return SE.getUMaxExpr(Operands);
185 default: llvm_unreachable("Unexpected SCEVNAryExpr kind!");
186 }
187 return S;
188 }
189
190 if (const SCEVUDivExpr *X = dyn_cast<SCEVUDivExpr>(S)) {
191 const SCEV *LO = X->getLHS();
192 const SCEV *RO = X->getRHS();
193 const SCEV *LN = TransformSubExpr(LO, User, OperandValToReplace);
194 const SCEV *RN = TransformSubExpr(RO, User, OperandValToReplace);
195 if (LO != LN || RO != RN)
196 return SE.getUDivExpr(LN, RN);
197 return S;
198 }
199
200 llvm_unreachable("Unexpected SCEV kind!");
201 }
202
203 /// Manage recursive transformation across an expression DAG. Revisiting
204 /// expressions would lead to exponential recursion.
205 const SCEV *PostIncTransform::
206 TransformSubExpr(const SCEV *S, Instruction *User, Value *OperandValToReplace) {
207
208 if (isa<SCEVConstant>(S) || isa<SCEVUnknown>(S))
209 return S;
210
211 const SCEV *Result = Transformed.lookup(S);
212 if (Result)
213 return Result;
214
215 Result = TransformImpl(S, User, OperandValToReplace);
216 Transformed[S] = Result;
217 return Result;
218 }
219
220 /// Top level driver for transforming an expression DAG into its requested
221 /// post-inc form (either "Normalized" or "Denormalized".
222 const SCEV *llvm::TransformForPostIncUse(TransformKind Kind,
223 const SCEV *S,
224 Instruction *User,
225 Value *OperandValToReplace,
226 PostIncLoopSet &Loops,
227 ScalarEvolution &SE,
228 DominatorTree &DT) {
229 PostIncTransform Transform(Kind, Loops, SE, DT);
230 return Transform.TransformSubExpr(S, User, OperandValToReplace);
231 }