ValueTracking.h revision 0a60fa33210202a38a59ae3ea8681216f234ce51 
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/System/DataTypes.h"
19#include <string>
20
21namespace llvm {
22  template <typename T> class SmallVectorImpl;
23  class Value;
24  class Instruction;
25  class APInt;
26  class TargetData;
27
28  /// ComputeMaskedBits - Determine which of the bits specified in Mask are
29  /// known to be either zero or one and return them in the KnownZero/KnownOne
30  /// bit sets.  This code only analyzes bits in Mask, in order to short-circuit
31  /// processing.
32  ///
33  /// This function is defined on values with integer type, values with pointer
34  /// type (but only if TD is non-null), and vectors of integers.  In the case
35  /// where V is a vector, the mask, known zero, and known one values are the
36  /// same width as the vector element, and the bit is set only if it is true
37  /// for all of the elements in the vector.
38  void ComputeMaskedBits(Value *V, const APInt &Mask, APInt &KnownZero,
39                         APInt &KnownOne, const TargetData *TD = 0,
40                         unsigned Depth = 0);
41
42  /// MaskedValueIsZero - Return true if 'V & Mask' is known to be zero.  We use
43  /// this predicate to simplify operations downstream.  Mask is known to be
44  /// zero for bits that V cannot have.
45  ///
46  /// This function is defined on values with integer type, values with pointer
47  /// type (but only if TD is non-null), and vectors of integers.  In the case
48  /// where V is a vector, the mask, known zero, and known one values are the
49  /// same width as the vector element, and the bit is set only if it is true
50  /// for all of the elements in the vector.
51  bool MaskedValueIsZero(Value *V, const APInt &Mask,
52                         const TargetData *TD = 0, unsigned Depth = 0);
53
54
55  /// ComputeNumSignBits - Return the number of times the sign bit of the
56  /// register is replicated into the other bits.  We know that at least 1 bit
57  /// is always equal to the sign bit (itself), but other cases can give us
58  /// information.  For example, immediately after an "ashr X, 2", we know that
59  /// the top 3 bits are all equal to each other, so we return 3.
60  ///
61  /// 'Op' must have a scalar integer type.
62  ///
63  unsigned ComputeNumSignBits(Value *Op, const TargetData *TD = 0,
64                              unsigned Depth = 0);
65
66  /// ComputeMultiple - This function computes the integer multiple of Base that
67  /// equals V.  If successful, it returns true and returns the multiple in
68  /// Multiple.  If unsuccessful, it returns false.  Also, if V can be
69  /// simplified to an integer, then the simplified V is returned in Val.  Look
70  /// through sext only if LookThroughSExt=true.
71  bool ComputeMultiple(Value *V, unsigned Base, Value *&Multiple,
72                       bool LookThroughSExt = false,
73                       unsigned Depth = 0);
74
75  /// CannotBeNegativeZero - Return true if we can prove that the specified FP
76  /// value is never equal to -0.0.
77  ///
78  bool CannotBeNegativeZero(const Value *V, unsigned Depth = 0);
79
80  /// DecomposeGEPExpression - If V is a symbolic pointer expression, decompose
81  /// it into a base pointer with a constant offset and a number of scaled
82  /// symbolic offsets.
83  ///
84  /// The scaled symbolic offsets (represented by pairs of a Value* and a scale
85  /// in the VarIndices vector) are Value*'s that are known to be scaled by the
86  /// specified amount, but which may have other unrepresented high bits. As
87  /// such, the gep cannot necessarily be reconstructed from its decomposed
88  /// form.
89  ///
90  /// When TargetData is around, this function is capable of analyzing
91  /// everything that Value::getUnderlyingObject() can look through.  When not,
92  /// it just looks through pointer casts.
93  ///
94  const Value *DecomposeGEPExpression(const Value *V, int64_t &BaseOffs,
95                 SmallVectorImpl<std::pair<const Value*, int64_t> > &VarIndices,
96                                      const TargetData *TD);
97
98
99
100  /// FindScalarValue - Given an aggregrate and an sequence of indices, see if
101  /// the scalar value indexed is already around as a register, for example if
102  /// it were inserted directly into the aggregrate.
103  ///
104  /// If InsertBefore is not null, this function will duplicate (modified)
105  /// insertvalues when a part of a nested struct is extracted.
106  Value *FindInsertedValue(Value *V,
107                           const unsigned *idx_begin,
108                           const unsigned *idx_end,
109                           Instruction *InsertBefore = 0);
110
111  /// This is a convenience wrapper for finding values indexed by a single index
112  /// only.
113  inline Value *FindInsertedValue(Value *V, const unsigned Idx,
114                                  Instruction *InsertBefore = 0) {
115    const unsigned Idxs[1] = { Idx };
116    return FindInsertedValue(V, &Idxs[0], &Idxs[1], InsertBefore);
117  }
118
119  /// GetConstantStringInfo - This function computes the length of a
120  /// null-terminated C string pointed to by V.  If successful, it returns true
121  /// and returns the string in Str.  If unsuccessful, it returns false.  If
122  /// StopAtNul is set to true (the default), the returned string is truncated
123  /// by a nul character in the global.  If StopAtNul is false, the nul
124  /// character is included in the result string.
125  bool GetConstantStringInfo(const Value *V, std::string &Str,
126                             uint64_t Offset = 0,
127                             bool StopAtNul = true);
128
129  /// GetStringLength - If we can compute the length of the string pointed to by
130  /// the specified pointer, return 'len+1'.  If we can't, return 0.
131  uint64_t GetStringLength(Value *V);
132} // end namespace llvm
133
134#endif
135