AliasAnalysis.h revision ab8c565768ff7485f40cbb65e2914f9046e743d4 
1//===- llvm/Analysis/AliasAnalysis.h - Alias Analysis Interface -*- C++ -*-===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file was developed by the LLVM research group and is distributed under
6// the University of Illinois Open Source License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This file defines the generic AliasAnalysis interface, which is used as the
11// common interface used by all clients of alias analysis information, and
12// implemented by all alias analysis implementations.  Mod/Ref information is
13// also captured by this interface.
14//
15// Implementations of this interface must implement the various virtual methods,
16// which automatically provides functionality for the entire suite of client
17// APIs.
18//
19// This API represents memory as a (Pointer, Size) pair.  The Pointer component
20// specifies the base memory address of the region, the Size specifies how large
21// of an area is being queried.  If Size is 0, two pointers only alias if they
22// are exactly equal.  If size is greater than zero, but small, the two pointers
23// alias if the areas pointed to overlap.  If the size is very large (ie, ~0U),
24// then the two pointers alias if they may be pointing to components of the same
25// memory object.  Pointers that point to two completely different objects in
26// memory never alias, regardless of the value of the Size component.
27//
28//===----------------------------------------------------------------------===//
29
30#ifndef LLVM_ANALYSIS_ALIAS_ANALYSIS_H
31#define LLVM_ANALYSIS_ALIAS_ANALYSIS_H
32
33#include "llvm/Support/CallSite.h"
34#include "llvm/Pass.h"    // Need this for IncludeFile
35
36namespace llvm {
37
38class LoadInst;
39class StoreInst;
40class TargetData;
41
42class AliasAnalysis {
43protected:
44  const TargetData *TD;
45  AliasAnalysis *AA;       // Previous Alias Analysis to chain to.
46
47  /// InitializeAliasAnalysis - Subclasses must call this method to initialize
48  /// the AliasAnalysis interface before any other methods are called.  This is
49  /// typically called by the run* methods of these subclasses.  This may be
50  /// called multiple times.
51  ///
52  void InitializeAliasAnalysis(Pass *P);
53
54  // getAnalysisUsage - All alias analysis implementations should invoke this
55  // directly (using AliasAnalysis::getAnalysisUsage(AU)) to make sure that
56  // TargetData is required by the pass.
57  virtual void getAnalysisUsage(AnalysisUsage &AU) const;
58
59public:
60  AliasAnalysis() : TD(0), AA(0) {}
61  virtual ~AliasAnalysis();  // We want to be subclassed
62
63  /// getTargetData - Every alias analysis implementation depends on the size of
64  /// data items in the current Target.  This provides a uniform way to handle
65  /// it.
66  ///
67  const TargetData &getTargetData() const { return *TD; }
68
69  //===--------------------------------------------------------------------===//
70  /// Alias Queries...
71  ///
72
73  /// Alias analysis result - Either we know for sure that it does not alias, we
74  /// know for sure it must alias, or we don't know anything: The two pointers
75  /// _might_ alias.  This enum is designed so you can do things like:
76  ///     if (AA.alias(P1, P2)) { ... }
77  /// to check to see if two pointers might alias.
78  ///
79  enum AliasResult { NoAlias = 0, MayAlias = 1, MustAlias = 2 };
80
81  /// alias - The main low level interface to the alias analysis implementation.
82  /// Returns a Result indicating whether the two pointers are aliased to each
83  /// other.  This is the interface that must be implemented by specific alias
84  /// analysis implementations.
85  ///
86  virtual AliasResult alias(const Value *V1, unsigned V1Size,
87                            const Value *V2, unsigned V2Size);
88
89  /// getMustAliases - If there are any pointers known that must alias this
90  /// pointer, return them now.  This allows alias-set based alias analyses to
91  /// perform a form a value numbering (which is exposed by load-vn).  If an
92  /// alias analysis supports this, it should ADD any must aliased pointers to
93  /// the specified vector.
94  ///
95  virtual void getMustAliases(Value *P, std::vector<Value*> &RetVals);
96
97  /// pointsToConstantMemory - If the specified pointer is known to point into
98  /// constant global memory, return true.  This allows disambiguation of store
99  /// instructions from constant pointers.
100  ///
101  virtual bool pointsToConstantMemory(const Value *P);
102
103  /// doesNotAccessMemory - If the specified function is known to never read or
104  /// write memory, return true.  If the function only reads from known-constant
105  /// memory, it is also legal to return true.  Functions that unwind the stack
106  /// are not legal for this predicate.
107  ///
108  /// Many optimizations (such as CSE and LICM) can be performed on calls to it,
109  /// without worrying about aliasing properties, and many functions have this
110  /// property (e.g. 'sin' and 'cos').
111  ///
112  /// This property corresponds to the GCC 'const' attribute.
113  ///
114  virtual bool doesNotAccessMemory(Function *F);
115
116  /// onlyReadsMemory - If the specified function is known to only read from
117  /// non-volatile memory (or not access memory at all), return true.  Functions
118  /// that unwind the stack are not legal for this predicate.
119  ///
120  /// This property allows many common optimizations to be performed in the
121  /// absence of interfering store instructions, such as CSE of strlen calls.
122  ///
123  /// This property corresponds to the GCC 'pure' attribute.
124  ///
125  virtual bool onlyReadsMemory(Function *F);
126
127
128  //===--------------------------------------------------------------------===//
129  /// Simple mod/ref information...
130  ///
131
132  /// ModRefResult - Represent the result of a mod/ref query.  Mod and Ref are
133  /// bits which may be or'd together.
134  ///
135  enum ModRefResult { NoModRef = 0, Ref = 1, Mod = 2, ModRef = 3 };
136
137  /// getModRefInfo - Return information about whether or not an instruction may
138  /// read or write memory specified by the pointer operand.  An instruction
139  /// that doesn't read or write memory may be trivially LICM'd for example.
140
141  /// getModRefInfo (for call sites) - Return whether information about whether
142  /// a particular call site modifies or reads the memory specified by the
143  /// pointer.
144  ///
145  virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
146
147  /// getModRefInfo - Return information about whether two call sites may refer
148  /// to the same set of memory locations.  This function returns NoModRef if
149  /// the two calls refer to disjoint memory locations, Ref if CS1 reads memory
150  /// written by CS2, Mod if CS1 writes to memory read or written by CS2, or
151  /// ModRef if CS1 might read or write memory accessed by CS2.
152  ///
153  virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2);
154
155  /// hasNoModRefInfoForCalls - Return true if the analysis has no mod/ref
156  /// information for pairs of function calls (other than "pure" and "const"
157  /// functions).  This can be used by clients to avoid many pointless queries.
158  /// Remember that if you override this and chain to another analysis, you must
159  /// make sure that it doesn't have mod/ref info either.
160  ///
161  virtual bool hasNoModRefInfoForCalls() const;
162
163  /// Convenience functions...
164  ModRefResult getModRefInfo(LoadInst *L, Value *P, unsigned Size);
165  ModRefResult getModRefInfo(StoreInst *S, Value *P, unsigned Size);
166  ModRefResult getModRefInfo(CallInst *C, Value *P, unsigned Size) {
167    return getModRefInfo(CallSite(C), P, Size);
168  }
169  ModRefResult getModRefInfo(InvokeInst *I, Value *P, unsigned Size) {
170    return getModRefInfo(CallSite(I), P, Size);
171  }
172  ModRefResult getModRefInfo(Instruction *I, Value *P, unsigned Size) {
173    switch (I->getOpcode()) {
174    case Instruction::Load:   return getModRefInfo((LoadInst*)I, P, Size);
175    case Instruction::Store:  return getModRefInfo((StoreInst*)I, P, Size);
176    case Instruction::Call:   return getModRefInfo((CallInst*)I, P, Size);
177    case Instruction::Invoke: return getModRefInfo((InvokeInst*)I, P, Size);
178    default:                  return NoModRef;
179    }
180  }
181
182  //===--------------------------------------------------------------------===//
183  /// Higher level methods for querying mod/ref information.
184  ///
185
186  /// canBasicBlockModify - Return true if it is possible for execution of the
187  /// specified basic block to modify the value pointed to by Ptr.
188  ///
189  bool canBasicBlockModify(const BasicBlock &BB, const Value *P, unsigned Size);
190
191  /// canInstructionRangeModify - Return true if it is possible for the
192  /// execution of the specified instructions to modify the value pointed to by
193  /// Ptr.  The instructions to consider are all of the instructions in the
194  /// range of [I1,I2] INCLUSIVE.  I1 and I2 must be in the same basic block.
195  ///
196  bool canInstructionRangeModify(const Instruction &I1, const Instruction &I2,
197                                 const Value *Ptr, unsigned Size);
198
199  //===--------------------------------------------------------------------===//
200  /// Methods that clients should call when they transform the program to allow
201  /// alias analyses to update their internal data structures.  Note that these
202  /// methods may be called on any instruction, regardless of whether or not
203  /// they have pointer-analysis implications.
204  ///
205
206  /// deleteValue - This method should be called whenever an LLVM Value is
207  /// deleted from the program, for example when an instruction is found to be
208  /// redundant and is eliminated.
209  ///
210  virtual void deleteValue(Value *V);
211
212  /// copyValue - This method should be used whenever a preexisting value in the
213  /// program is copied or cloned, introducing a new value.  Note that analysis
214  /// implementations should tolerate clients that use this method to introduce
215  /// the same value multiple times: if the analysis already knows about a
216  /// value, it should ignore the request.
217  ///
218  virtual void copyValue(Value *From, Value *To);
219
220  /// replaceWithNewValue - This method is the obvious combination of the two
221  /// above, and it provided as a helper to simplify client code.
222  ///
223  void replaceWithNewValue(Value *Old, Value *New) {
224    copyValue(Old, New);
225    deleteValue(Old);
226  }
227};
228
229// Because of the way .a files work, we must force the BasicAA implementation to
230// be pulled in if the AliasAnalysis header is included.  Otherwise we run
231// the risk of AliasAnalysis being used, but the default implementation not
232// being linked into the tool that uses it.
233//
234extern void BasicAAStub();
235static IncludeFile HDR_INCLUDE_BASICAA_CPP((void*)&BasicAAStub);
236
237} // End llvm namespace
238
239#endif
240