1 //===---- llvm/Analysis/ScalarEvolutionExpander.h - SCEV Exprs --*- 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 defines the classes used to generate code from scalar expressions.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #ifndef LLVM_ANALYSIS_SCALAREVOLUTIONEXPANDER_H
15 #define LLVM_ANALYSIS_SCALAREVOLUTIONEXPANDER_H
16 
17 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
18 #include "llvm/Analysis/ScalarEvolutionNormalization.h"
19 #include "llvm/Analysis/TargetFolder.h"
20 #include "llvm/IR/IRBuilder.h"
21 #include "llvm/IR/ValueHandle.h"
22 #include <set>
23 
24 namespace llvm {
25   class TargetTransformInfo;
26 
27   /// Return true if the given expression is safe to expand in the sense that
28   /// all materialized values are safe to speculate.
29   bool isSafeToExpand(const SCEV *S, ScalarEvolution &SE);
30 
31   /// This class uses information about analyze scalars to
32   /// rewrite expressions in canonical form.
33   ///
34   /// Clients should create an instance of this class when rewriting is needed,
35   /// and destroy it when finished to allow the release of the associated
36   /// memory.
37   class SCEVExpander : public SCEVVisitor<SCEVExpander, Value*> {
38     ScalarEvolution &SE;
39     const DataLayout &DL;
40 
41     // New instructions receive a name to identifies them with the current pass.
42     const char* IVName;
43 
44     // InsertedExpressions caches Values for reuse, so must track RAUW.
45     std::map<std::pair<const SCEV *, Instruction *>, TrackingVH<Value> >
46       InsertedExpressions;
47     // InsertedValues only flags inserted instructions so needs no RAUW.
48     std::set<AssertingVH<Value> > InsertedValues;
49     std::set<AssertingVH<Value> > InsertedPostIncValues;
50 
51     /// A memoization of the "relevant" loop for a given SCEV.
52     DenseMap<const SCEV *, const Loop *> RelevantLoops;
53 
54     /// \brief Addrecs referring to any of the given loops are expanded
55     /// in post-inc mode. For example, expanding {1,+,1}<L> in post-inc mode
56     /// returns the add instruction that adds one to the phi for {0,+,1}<L>,
57     /// as opposed to a new phi starting at 1. This is only supported in
58     /// non-canonical mode.
59     PostIncLoopSet PostIncLoops;
60 
61     /// \brief When this is non-null, addrecs expanded in the loop it indicates
62     /// should be inserted with increments at IVIncInsertPos.
63     const Loop *IVIncInsertLoop;
64 
65     /// \brief When expanding addrecs in the IVIncInsertLoop loop, insert the IV
66     /// increment at this position.
67     Instruction *IVIncInsertPos;
68 
69     /// \brief Phis that complete an IV chain. Reuse
70     std::set<AssertingVH<PHINode> > ChainedPhis;
71 
72     /// \brief When true, expressions are expanded in "canonical" form. In
73     /// particular, addrecs are expanded as arithmetic based on a canonical
74     /// induction variable. When false, expression are expanded in a more
75     /// literal form.
76     bool CanonicalMode;
77 
78     /// \brief When invoked from LSR, the expander is in "strength reduction"
79     /// mode. The only difference is that phi's are only reused if they are
80     /// already in "expanded" form.
81     bool LSRMode;
82 
83     typedef IRBuilder<true, TargetFolder> BuilderType;
84     BuilderType Builder;
85 
86 #ifndef NDEBUG
87     const char *DebugType;
88 #endif
89 
90     friend struct SCEVVisitor<SCEVExpander, Value*>;
91 
92   public:
93     /// \brief Construct a SCEVExpander in "canonical" mode.
94     explicit SCEVExpander(ScalarEvolution &se, const DataLayout &DL,
95                           const char *name)
96         : SE(se), DL(DL), IVName(name), IVIncInsertLoop(nullptr),
97           IVIncInsertPos(nullptr), CanonicalMode(true), LSRMode(false),
98           Builder(se.getContext(), TargetFolder(DL)) {
99 #ifndef NDEBUG
100       DebugType = "";
101 #endif
102     }
103 
104 #ifndef NDEBUG
105     void setDebugType(const char* s) { DebugType = s; }
106 #endif
107 
108     /// \brief Erase the contents of the InsertedExpressions map so that users
109     /// trying to expand the same expression into multiple BasicBlocks or
110     /// different places within the same BasicBlock can do so.
111     void clear() {
112       InsertedExpressions.clear();
113       InsertedValues.clear();
114       InsertedPostIncValues.clear();
115       ChainedPhis.clear();
116     }
117 
118     /// \brief Return true for expressions that may incur non-trivial cost to
119     /// evaluate at runtime.
120     bool isHighCostExpansion(const SCEV *Expr, Loop *L) {
121       SmallPtrSet<const SCEV *, 8> Processed;
122       return isHighCostExpansionHelper(Expr, L, Processed);
123     }
124 
125     /// \brief This method returns the canonical induction variable of the
126     /// specified type for the specified loop (inserting one if there is none).
127     /// A canonical induction variable starts at zero and steps by one on each
128     /// iteration.
129     PHINode *getOrInsertCanonicalInductionVariable(const Loop *L, Type *Ty);
130 
131     /// \brief Return the induction variable increment's IV operand.
132     Instruction *getIVIncOperand(Instruction *IncV, Instruction *InsertPos,
133                                  bool allowScale);
134 
135     /// \brief Utility for hoisting an IV increment.
136     bool hoistIVInc(Instruction *IncV, Instruction *InsertPos);
137 
138     /// \brief replace congruent phis with their most canonical
139     /// representative. Return the number of phis eliminated.
140     unsigned replaceCongruentIVs(Loop *L, const DominatorTree *DT,
141                                  SmallVectorImpl<WeakVH> &DeadInsts,
142                                  const TargetTransformInfo *TTI = nullptr);
143 
144     /// \brief Insert code to directly compute the specified SCEV expression
145     /// into the program.  The inserted code is inserted into the specified
146     /// block.
147     Value *expandCodeFor(const SCEV *SH, Type *Ty, Instruction *I);
148 
149     /// \brief Set the current IV increment loop and position.
150     void setIVIncInsertPos(const Loop *L, Instruction *Pos) {
151       assert(!CanonicalMode &&
152              "IV increment positions are not supported in CanonicalMode");
153       IVIncInsertLoop = L;
154       IVIncInsertPos = Pos;
155     }
156 
157     /// \brief Enable post-inc expansion for addrecs referring to the given
158     /// loops. Post-inc expansion is only supported in non-canonical mode.
159     void setPostInc(const PostIncLoopSet &L) {
160       assert(!CanonicalMode &&
161              "Post-inc expansion is not supported in CanonicalMode");
162       PostIncLoops = L;
163     }
164 
165     /// \brief Disable all post-inc expansion.
166     void clearPostInc() {
167       PostIncLoops.clear();
168 
169       // When we change the post-inc loop set, cached expansions may no
170       // longer be valid.
171       InsertedPostIncValues.clear();
172     }
173 
174     /// \brief Disable the behavior of expanding expressions in canonical form
175     /// rather than in a more literal form. Non-canonical mode is useful for
176     /// late optimization passes.
177     void disableCanonicalMode() { CanonicalMode = false; }
178 
179     void enableLSRMode() { LSRMode = true; }
180 
181     /// \brief Clear the current insertion point. This is useful if the
182     /// instruction that had been serving as the insertion point may have been
183     /// deleted.
184     void clearInsertPoint() {
185       Builder.ClearInsertionPoint();
186     }
187 
188     /// \brief Return true if the specified instruction was inserted by the code
189     /// rewriter.  If so, the client should not modify the instruction.
190     bool isInsertedInstruction(Instruction *I) const {
191       return InsertedValues.count(I) || InsertedPostIncValues.count(I);
192     }
193 
194     void setChainedPhi(PHINode *PN) { ChainedPhis.insert(PN); }
195 
196   private:
197     LLVMContext &getContext() const { return SE.getContext(); }
198 
199     /// \brief Recursive helper function for isHighCostExpansion.
200     bool isHighCostExpansionHelper(const SCEV *S, Loop *L,
201                                    SmallPtrSetImpl<const SCEV *> &Processed);
202 
203     /// \brief Insert the specified binary operator, doing a small amount
204     /// of work to avoid inserting an obviously redundant operation.
205     Value *InsertBinop(Instruction::BinaryOps Opcode, Value *LHS, Value *RHS);
206 
207     /// \brief Arrange for there to be a cast of V to Ty at IP, reusing an
208     /// existing cast if a suitable one exists, moving an existing cast if a
209     /// suitable one exists but isn't in the right place, or or creating a new
210     /// one.
211     Value *ReuseOrCreateCast(Value *V, Type *Ty,
212                              Instruction::CastOps Op,
213                              BasicBlock::iterator IP);
214 
215     /// \brief Insert a cast of V to the specified type, which must be possible
216     /// with a noop cast, doing what we can to share the casts.
217     Value *InsertNoopCastOfTo(Value *V, Type *Ty);
218 
219     /// \brief Expand a SCEVAddExpr with a pointer type into a GEP
220     /// instead of using ptrtoint+arithmetic+inttoptr.
221     Value *expandAddToGEP(const SCEV *const *op_begin,
222                           const SCEV *const *op_end,
223                           PointerType *PTy, Type *Ty, Value *V);
224 
225     Value *expand(const SCEV *S);
226 
227     /// \brief Insert code to directly compute the specified SCEV expression
228     /// into the program.  The inserted code is inserted into the SCEVExpander's
229     /// current insertion point. If a type is specified, the result will be
230     /// expanded to have that type, with a cast if necessary.
231     Value *expandCodeFor(const SCEV *SH, Type *Ty = nullptr);
232 
233     /// \brief Determine the most "relevant" loop for the given SCEV.
234     const Loop *getRelevantLoop(const SCEV *);
235 
236     Value *visitConstant(const SCEVConstant *S) {
237       return S->getValue();
238     }
239 
240     Value *visitTruncateExpr(const SCEVTruncateExpr *S);
241 
242     Value *visitZeroExtendExpr(const SCEVZeroExtendExpr *S);
243 
244     Value *visitSignExtendExpr(const SCEVSignExtendExpr *S);
245 
246     Value *visitAddExpr(const SCEVAddExpr *S);
247 
248     Value *visitMulExpr(const SCEVMulExpr *S);
249 
250     Value *visitUDivExpr(const SCEVUDivExpr *S);
251 
252     Value *visitAddRecExpr(const SCEVAddRecExpr *S);
253 
254     Value *visitSMaxExpr(const SCEVSMaxExpr *S);
255 
256     Value *visitUMaxExpr(const SCEVUMaxExpr *S);
257 
258     Value *visitUnknown(const SCEVUnknown *S) {
259       return S->getValue();
260     }
261 
262     void rememberInstruction(Value *I);
263 
264     bool isNormalAddRecExprPHI(PHINode *PN, Instruction *IncV, const Loop *L);
265 
266     bool isExpandedAddRecExprPHI(PHINode *PN, Instruction *IncV, const Loop *L);
267 
268     Value *expandAddRecExprLiterally(const SCEVAddRecExpr *);
269     PHINode *getAddRecExprPHILiterally(const SCEVAddRecExpr *Normalized,
270                                        const Loop *L,
271                                        Type *ExpandTy,
272                                        Type *IntTy,
273                                        Type *&TruncTy,
274                                        bool &InvertStep);
275     Value *expandIVInc(PHINode *PN, Value *StepV, const Loop *L,
276                        Type *ExpandTy, Type *IntTy, bool useSubtract);
277   };
278 }
279 
280 #endif
281