ValueTracking.h revision 95d594cac3737ae1594a391276942a443cac426b 
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
21namespace llvm {
22  class Value;
23  class Instruction;
24  class APInt;
25  class TargetData;
26  class StringRef;
27  class MDNode;
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  void computeMaskedBitsLoad(const MDNode &Ranges, const APInt &Mask,
43                             APInt &KnownZero);
44
45  /// ComputeSignBit - Determine whether the sign bit is known to be zero or
46  /// one.  Convenience wrapper around ComputeMaskedBits.
47  void ComputeSignBit(Value *V, bool &KnownZero, bool &KnownOne,
48                      const TargetData *TD = 0, unsigned Depth = 0);
49
50  /// isPowerOfTwo - Return true if the given value is known to have exactly one
51  /// bit set when defined. For vectors return true if every element is known to
52  /// be a power of two when defined.  Supports values with integer or pointer
53  /// type and vectors of integers.  If 'OrZero' is set then returns true if the
54  /// given value is either a power of two or zero.
55  bool isPowerOfTwo(Value *V, const TargetData *TD = 0, bool OrZero = false,
56                    unsigned Depth = 0);
57
58  /// isKnownNonZero - Return true if the given value is known to be non-zero
59  /// when defined.  For vectors return true if every element is known to be
60  /// non-zero when defined.  Supports values with integer or pointer type and
61  /// vectors of integers.
62  bool isKnownNonZero(Value *V, const TargetData *TD = 0, unsigned Depth = 0);
63
64  /// MaskedValueIsZero - Return true if 'V & Mask' is known to be zero.  We use
65  /// this predicate to simplify operations downstream.  Mask is known to be
66  /// zero for bits that V cannot have.
67  ///
68  /// This function is defined on values with integer type, values with pointer
69  /// type (but only if TD is non-null), and vectors of integers.  In the case
70  /// where V is a vector, the mask, known zero, and known one values are the
71  /// same width as the vector element, and the bit is set only if it is true
72  /// for all of the elements in the vector.
73  bool MaskedValueIsZero(Value *V, const APInt &Mask,
74                         const TargetData *TD = 0, unsigned Depth = 0);
75
76
77  /// ComputeNumSignBits - Return the number of times the sign bit of the
78  /// register is replicated into the other bits.  We know that at least 1 bit
79  /// is always equal to the sign bit (itself), but other cases can give us
80  /// information.  For example, immediately after an "ashr X, 2", we know that
81  /// the top 3 bits are all equal to each other, so we return 3.
82  ///
83  /// 'Op' must have a scalar integer type.
84  ///
85  unsigned ComputeNumSignBits(Value *Op, const TargetData *TD = 0,
86                              unsigned Depth = 0);
87
88  /// ComputeMultiple - This function computes the integer multiple of Base that
89  /// equals V.  If successful, it returns true and returns the multiple in
90  /// Multiple.  If unsuccessful, it returns false.  Also, if V can be
91  /// simplified to an integer, then the simplified V is returned in Val.  Look
92  /// through sext only if LookThroughSExt=true.
93  bool ComputeMultiple(Value *V, unsigned Base, Value *&Multiple,
94                       bool LookThroughSExt = false,
95                       unsigned Depth = 0);
96
97  /// CannotBeNegativeZero - Return true if we can prove that the specified FP
98  /// value is never equal to -0.0.
99  ///
100  bool CannotBeNegativeZero(const Value *V, unsigned Depth = 0);
101
102  /// isBytewiseValue - If the specified value can be set by repeating the same
103  /// byte in memory, return the i8 value that it is represented with.  This is
104  /// true for all i8 values obviously, but is also true for i32 0, i32 -1,
105  /// i16 0xF0F0, double 0.0 etc.  If the value can't be handled with a repeated
106  /// byte store (e.g. i16 0x1234), return null.
107  Value *isBytewiseValue(Value *V);
108
109  /// FindInsertedValue - Given an aggregrate and an sequence of indices, see if
110  /// the scalar value indexed is already around as a register, for example if
111  /// it were inserted directly into the aggregrate.
112  ///
113  /// If InsertBefore is not null, this function will duplicate (modified)
114  /// insertvalues when a part of a nested struct is extracted.
115  Value *FindInsertedValue(Value *V,
116                           ArrayRef<unsigned> idx_range,
117                           Instruction *InsertBefore = 0);
118
119  /// GetPointerBaseWithConstantOffset - Analyze the specified pointer to see if
120  /// it can be expressed as a base pointer plus a constant offset.  Return the
121  /// base and offset to the caller.
122  Value *GetPointerBaseWithConstantOffset(Value *Ptr, int64_t &Offset,
123                                          const TargetData &TD);
124  static inline const Value *
125  GetPointerBaseWithConstantOffset(const Value *Ptr, int64_t &Offset,
126                                   const TargetData &TD) {
127    return GetPointerBaseWithConstantOffset(const_cast<Value*>(Ptr), Offset,TD);
128  }
129
130  /// getConstantStringInfo - This function computes the length of a
131  /// null-terminated C string pointed to by V.  If successful, it returns true
132  /// and returns the string in Str.  If unsuccessful, it returns false.  This
133  /// does not include the trailing nul character by default.  If TrimAtNul is
134  /// set to false, then this returns any trailing nul characters as well as any
135  /// other characters that come after it.
136  bool getConstantStringInfo(const Value *V, StringRef &Str,
137                             uint64_t Offset = 0, bool TrimAtNul = true);
138
139  /// GetStringLength - If we can compute the length of the string pointed to by
140  /// the specified pointer, return 'len+1'.  If we can't, return 0.
141  uint64_t GetStringLength(Value *V);
142
143  /// GetUnderlyingObject - This method strips off any GEP address adjustments
144  /// and pointer casts from the specified value, returning the original object
145  /// being addressed.  Note that the returned value has pointer type if the
146  /// specified value does.  If the MaxLookup value is non-zero, it limits the
147  /// number of instructions to be stripped off.
148  Value *GetUnderlyingObject(Value *V, const TargetData *TD = 0,
149                             unsigned MaxLookup = 6);
150  static inline const Value *
151  GetUnderlyingObject(const Value *V, const TargetData *TD = 0,
152                      unsigned MaxLookup = 6) {
153    return GetUnderlyingObject(const_cast<Value *>(V), TD, MaxLookup);
154  }
155
156  /// onlyUsedByLifetimeMarkers - Return true if the only users of this pointer
157  /// are lifetime markers.
158  bool onlyUsedByLifetimeMarkers(const Value *V);
159
160  /// isSafeToSpeculativelyExecute - Return true if the instruction does not
161  /// have any effects besides calculating the result and does not have
162  /// undefined behavior.
163  ///
164  /// This method never returns true for an instruction that returns true for
165  /// mayHaveSideEffects; however, this method also does some other checks in
166  /// addition. It checks for undefined behavior, like dividing by zero or
167  /// loading from an invalid pointer (but not for undefined results, like a
168  /// shift with a shift amount larger than the width of the result). It checks
169  /// for malloc and alloca because speculatively executing them might cause a
170  /// memory leak. It also returns false for instructions related to control
171  /// flow, specifically terminators and PHI nodes.
172  ///
173  /// This method only looks at the instruction itself and its operands, so if
174  /// this method returns true, it is safe to move the instruction as long as
175  /// the correct dominance relationships for the operands and users hold.
176  /// However, this method can return true for instructions that read memory;
177  /// for such instructions, moving them may change the resulting value.
178  bool isSafeToSpeculativelyExecute(const Value *V,
179                                    const TargetData *TD = 0);
180
181} // end namespace llvm
182
183#endif
184