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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 
22 namespace 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.
52   bool isPowerOfTwo(Value *V, const TargetData *TD = 0, unsigned Depth = 0);
53 
54   /// isKnownNonZero - Return true if the given value is known to be non-zero
55   /// when defined.  For vectors return true if every element is known to be
56   /// non-zero when defined.  Supports values with integer or pointer type and
57   /// vectors of integers.
58   bool isKnownNonZero(Value *V, const TargetData *TD = 0, unsigned Depth = 0);
59 
60   /// MaskedValueIsZero - Return true if 'V & Mask' is known to be zero.  We use
61   /// this predicate to simplify operations downstream.  Mask is known to be
62   /// zero for bits that V cannot have.
63   ///
64   /// This function is defined on values with integer type, values with pointer
65   /// type (but only if TD is non-null), and vectors of integers.  In the case
66   /// where V is a vector, the mask, known zero, and known one values are the
67   /// same width as the vector element, and the bit is set only if it is true
68   /// for all of the elements in the vector.
69   bool MaskedValueIsZero(Value *V, const APInt &Mask,
70                          const TargetData *TD = 0, unsigned Depth = 0);
71 
72 
73   /// ComputeNumSignBits - Return the number of times the sign bit of the
74   /// register is replicated into the other bits.  We know that at least 1 bit
75   /// is always equal to the sign bit (itself), but other cases can give us
76   /// information.  For example, immediately after an "ashr X, 2", we know that
77   /// the top 3 bits are all equal to each other, so we return 3.
78   ///
79   /// 'Op' must have a scalar integer type.
80   ///
81   unsigned ComputeNumSignBits(Value *Op, const TargetData *TD = 0,
82                               unsigned Depth = 0);
83 
84   /// ComputeMultiple - This function computes the integer multiple of Base that
85   /// equals V.  If successful, it returns true and returns the multiple in
86   /// Multiple.  If unsuccessful, it returns false.  Also, if V can be
87   /// simplified to an integer, then the simplified V is returned in Val.  Look
88   /// through sext only if LookThroughSExt=true.
89   bool ComputeMultiple(Value *V, unsigned Base, Value *&Multiple,
90                        bool LookThroughSExt = false,
91                        unsigned Depth = 0);
92 
93   /// CannotBeNegativeZero - Return true if we can prove that the specified FP
94   /// value is never equal to -0.0.
95   ///
96   bool CannotBeNegativeZero(const Value *V, unsigned Depth = 0);
97 
98   /// isBytewiseValue - If the specified value can be set by repeating the same
99   /// byte in memory, return the i8 value that it is represented with.  This is
100   /// true for all i8 values obviously, but is also true for i32 0, i32 -1,
101   /// i16 0xF0F0, double 0.0 etc.  If the value can't be handled with a repeated
102   /// byte store (e.g. i16 0x1234), return null.
103   Value *isBytewiseValue(Value *V);
104 
105   /// FindInsertedValue - Given an aggregrate and an sequence of indices, see if
106   /// the scalar value indexed is already around as a register, for example if
107   /// it were inserted directly into the aggregrate.
108   ///
109   /// If InsertBefore is not null, this function will duplicate (modified)
110   /// insertvalues when a part of a nested struct is extracted.
111   Value *FindInsertedValue(Value *V,
112                            ArrayRef<unsigned> idx_range,
113                            Instruction *InsertBefore = 0);
114 
115   /// GetPointerBaseWithConstantOffset - Analyze the specified pointer to see if
116   /// it can be expressed as a base pointer plus a constant offset.  Return the
117   /// base and offset to the caller.
118   Value *GetPointerBaseWithConstantOffset(Value *Ptr, int64_t &Offset,
119                                           const TargetData &TD);
120   static inline const Value *
GetPointerBaseWithConstantOffset(const Value * Ptr,int64_t & Offset,const TargetData & TD)121   GetPointerBaseWithConstantOffset(const Value *Ptr, int64_t &Offset,
122                                    const TargetData &TD) {
123     return GetPointerBaseWithConstantOffset(const_cast<Value*>(Ptr), Offset,TD);
124   }
125 
126   /// GetConstantStringInfo - This function computes the length of a
127   /// null-terminated C string pointed to by V.  If successful, it returns true
128   /// and returns the string in Str.  If unsuccessful, it returns false.  If
129   /// StopAtNul is set to true (the default), the returned string is truncated
130   /// by a nul character in the global.  If StopAtNul is false, the nul
131   /// character is included in the result string.
132   bool GetConstantStringInfo(const Value *V, std::string &Str,
133                              uint64_t Offset = 0,
134                              bool StopAtNul = true);
135 
136   /// GetStringLength - If we can compute the length of the string pointed to by
137   /// the specified pointer, return 'len+1'.  If we can't, return 0.
138   uint64_t GetStringLength(Value *V);
139 
140   /// GetUnderlyingObject - This method strips off any GEP address adjustments
141   /// and pointer casts from the specified value, returning the original object
142   /// being addressed.  Note that the returned value has pointer type if the
143   /// specified value does.  If the MaxLookup value is non-zero, it limits the
144   /// number of instructions to be stripped off.
145   Value *GetUnderlyingObject(Value *V, const TargetData *TD = 0,
146                              unsigned MaxLookup = 6);
147   static inline const Value *
148   GetUnderlyingObject(const Value *V, const TargetData *TD = 0,
149                       unsigned MaxLookup = 6) {
150     return GetUnderlyingObject(const_cast<Value *>(V), TD, MaxLookup);
151   }
152 
153   /// onlyUsedByLifetimeMarkers - Return true if the only users of this pointer
154   /// are lifetime markers.
155   bool onlyUsedByLifetimeMarkers(const Value *V);
156 
157 } // end namespace llvm
158 
159 #endif
160