ValueTracking.h revision f0426601977c3e386d2d26c72a2cca691dc42072 
1//===- llvm/Analysis/ValueTracking.h - Walk computations --------*- 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 contains routines that help analyze properties that chains of
11// computations have.
12//
13//===----------------------------------------------------------------------===//
14
15#ifndef LLVM_ANALYSIS_VALUETRACKING_H
16#define LLVM_ANALYSIS_VALUETRACKING_H
17
18#include "llvm/ADT/ArrayRef.h"
19#include "llvm/Support/DataTypes.h"
20#include <string>
21
22namespace llvm {
23  template <typename T> class SmallVectorImpl;
24  class Value;
25  class Instruction;
26  class APInt;
27  class TargetData;
28
29  /// ComputeMaskedBits - Determine which of the bits specified in Mask are
30  /// known to be either zero or one and return them in the KnownZero/KnownOne
31  /// bit sets.  This code only analyzes bits in Mask, in order to short-circuit
32  /// processing.
33  ///
34  /// This function is defined on values with integer type, values with pointer
35  /// type (but only if TD is non-null), and vectors of integers.  In the case
36  /// where V is a vector, the mask, known zero, and known one values are the
37  /// same width as the vector element, and the bit is set only if it is true
38  /// for all of the elements in the vector.
39  void ComputeMaskedBits(Value *V, const APInt &Mask, APInt &KnownZero,
40                         APInt &KnownOne, const TargetData *TD = 0,
41                         unsigned Depth = 0);
42
43  /// ComputeSignBit - Determine whether the sign bit is known to be zero or
44  /// one.  Convenience wrapper around ComputeMaskedBits.
45  void ComputeSignBit(Value *V, bool &KnownZero, bool &KnownOne,
46                      const TargetData *TD = 0, unsigned Depth = 0);
47
48  /// isPowerOfTwo - Return true if the given value is known to have exactly one
49  /// bit set when defined. For vectors return true if every element is known to
50  /// be a power of two when defined.  Supports values with integer or pointer
51  /// type and vectors of integers.  If 'OrZero' is set then returns true if the
52  /// given value is either a power of two or zero.
53  bool isPowerOfTwo(Value *V, const TargetData *TD = 0, bool OrZero = false,
54                    unsigned Depth = 0);
55
56  /// isKnownNonZero - Return true if the given value is known to be non-zero
57  /// when defined.  For vectors return true if every element is known to be
58  /// non-zero when defined.  Supports values with integer or pointer type and
59  /// vectors of integers.
60  bool isKnownNonZero(Value *V, const TargetData *TD = 0, unsigned Depth = 0);
61
62  /// MaskedValueIsZero - Return true if 'V & Mask' is known to be zero.  We use
63  /// this predicate to simplify operations downstream.  Mask is known to be
64  /// zero for bits that V cannot have.
65  ///
66  /// This function is defined on values with integer type, values with pointer
67  /// type (but only if TD is non-null), and vectors of integers.  In the case
68  /// where V is a vector, the mask, known zero, and known one values are the
69  /// same width as the vector element, and the bit is set only if it is true
70  /// for all of the elements in the vector.
71  bool MaskedValueIsZero(Value *V, const APInt &Mask,
72                         const TargetData *TD = 0, unsigned Depth = 0);
73
74
75  /// ComputeNumSignBits - Return the number of times the sign bit of the
76  /// register is replicated into the other bits.  We know that at least 1 bit
77  /// is always equal to the sign bit (itself), but other cases can give us
78  /// information.  For example, immediately after an "ashr X, 2", we know that
79  /// the top 3 bits are all equal to each other, so we return 3.
80  ///
81  /// 'Op' must have a scalar integer type.
82  ///
83  unsigned ComputeNumSignBits(Value *Op, const TargetData *TD = 0,
84                              unsigned Depth = 0);
85
86  /// ComputeMultiple - This function computes the integer multiple of Base that
87  /// equals V.  If successful, it returns true and returns the multiple in
88  /// Multiple.  If unsuccessful, it returns false.  Also, if V can be
89  /// simplified to an integer, then the simplified V is returned in Val.  Look
90  /// through sext only if LookThroughSExt=true.
91  bool ComputeMultiple(Value *V, unsigned Base, Value *&Multiple,
92                       bool LookThroughSExt = false,
93                       unsigned Depth = 0);
94
95  /// CannotBeNegativeZero - Return true if we can prove that the specified FP
96  /// value is never equal to -0.0.
97  ///
98  bool CannotBeNegativeZero(const Value *V, unsigned Depth = 0);
99
100  /// isBytewiseValue - If the specified value can be set by repeating the same
101  /// byte in memory, return the i8 value that it is represented with.  This is
102  /// true for all i8 values obviously, but is also true for i32 0, i32 -1,
103  /// i16 0xF0F0, double 0.0 etc.  If the value can't be handled with a repeated
104  /// byte store (e.g. i16 0x1234), return null.
105  Value *isBytewiseValue(Value *V);
106
107  /// FindInsertedValue - Given an aggregrate and an sequence of indices, see if
108  /// the scalar value indexed is already around as a register, for example if
109  /// it were inserted directly into the aggregrate.
110  ///
111  /// If InsertBefore is not null, this function will duplicate (modified)
112  /// insertvalues when a part of a nested struct is extracted.
113  Value *FindInsertedValue(Value *V,
114                           ArrayRef<unsigned> idx_range,
115                           Instruction *InsertBefore = 0);
116
117  /// GetPointerBaseWithConstantOffset - Analyze the specified pointer to see if
118  /// it can be expressed as a base pointer plus a constant offset.  Return the
119  /// base and offset to the caller.
120  Value *GetPointerBaseWithConstantOffset(Value *Ptr, int64_t &Offset,
121                                          const TargetData &TD);
122  static inline const Value *
123  GetPointerBaseWithConstantOffset(const Value *Ptr, int64_t &Offset,
124                                   const TargetData &TD) {
125    return GetPointerBaseWithConstantOffset(const_cast<Value*>(Ptr), Offset,TD);
126  }
127
128  /// GetConstantStringInfo - This function computes the length of a
129  /// null-terminated C string pointed to by V.  If successful, it returns true
130  /// and returns the string in Str.  If unsuccessful, it returns false.  If
131  /// StopAtNul is set to true (the default), the returned string is truncated
132  /// by a nul character in the global.  If StopAtNul is false, the nul
133  /// character is included in the result string.
134  bool GetConstantStringInfo(const Value *V, std::string &Str,
135                             uint64_t Offset = 0,
136                             bool StopAtNul = true);
137
138  /// GetStringLength - If we can compute the length of the string pointed to by
139  /// the specified pointer, return 'len+1'.  If we can't, return 0.
140  uint64_t GetStringLength(Value *V);
141
142  /// GetUnderlyingObject - This method strips off any GEP address adjustments
143  /// and pointer casts from the specified value, returning the original object
144  /// being addressed.  Note that the returned value has pointer type if the
145  /// specified value does.  If the MaxLookup value is non-zero, it limits the
146  /// number of instructions to be stripped off.
147  Value *GetUnderlyingObject(Value *V, const TargetData *TD = 0,
148                             unsigned MaxLookup = 6);
149  static inline const Value *
150  GetUnderlyingObject(const Value *V, const TargetData *TD = 0,
151                      unsigned MaxLookup = 6) {
152    return GetUnderlyingObject(const_cast<Value *>(V), TD, MaxLookup);
153  }
154
155  /// onlyUsedByLifetimeMarkers - Return true if the only users of this pointer
156  /// are lifetime markers.
157  bool onlyUsedByLifetimeMarkers(const Value *V);
158
159  /// isSafeToSpeculativelyExecute - Return true if the instruction does not
160  /// have any effects besides calculating the result and does not have
161  /// undefined behavior.
162  ///
163  /// This method never returns true for an instruction that returns true for
164  /// mayHaveSideEffects; however, this method also does some other checks in
165  /// addition. It checks for undefined behavior, like dividing by zero or
166  /// loading from an invalid pointer (but not for undefined results, like a
167  /// shift with a shift amount larger than the width of the result). It checks
168  /// for malloc and alloca because speculatively executing them might cause a
169  /// memory leak. It also returns false for instructions related to control
170  /// flow, specifically terminators and PHI nodes.
171  ///
172  /// This method only looks at the instruction itself and its operands, so if
173  /// this method returns true, it is safe to move the instruction as long as
174  /// the correct dominance relationships for the operands and users hold.
175  /// However, this method can return true for instructions that read memory;
176  /// for such instructions, moving them may change the resulting value.
177  bool isSafeToSpeculativelyExecute(const Instruction *Inst,
178                                    const TargetData *TD = 0);
179
180} // end namespace llvm
181
182#endif
183