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 21 namespace 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, APInt &KnownZero, APInt &KnownOne, 40 const TargetData *TD = 0, unsigned Depth = 0); 41 void computeMaskedBitsLoad(const MDNode &Ranges, APInt &KnownZero); 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. This 131 /// does not include the trailing nul character by default. If TrimAtNul is 132 /// set to false, then this returns any trailing nul characters as well as any 133 /// other characters that come after it. 134 bool getConstantStringInfo(const Value *V, StringRef &Str, 135 uint64_t Offset = 0, bool TrimAtNul = true); 136 137 /// GetStringLength - If we can compute the length of the string pointed to by 138 /// the specified pointer, return 'len+1'. If we can't, return 0. 139 uint64_t GetStringLength(Value *V); 140 141 /// GetUnderlyingObject - This method strips off any GEP address adjustments 142 /// and pointer casts from the specified value, returning the original object 143 /// being addressed. Note that the returned value has pointer type if the 144 /// specified value does. If the MaxLookup value is non-zero, it limits the 145 /// number of instructions to be stripped off. 146 Value *GetUnderlyingObject(Value *V, const TargetData *TD = 0, 147 unsigned MaxLookup = 6); 148 static inline const Value * 149 GetUnderlyingObject(const Value *V, const TargetData *TD = 0, 150 unsigned MaxLookup = 6) { 151 return GetUnderlyingObject(const_cast<Value *>(V), TD, MaxLookup); 152 } 153 154 /// onlyUsedByLifetimeMarkers - Return true if the only users of this pointer 155 /// are lifetime markers. 156 bool onlyUsedByLifetimeMarkers(const Value *V); 157 158 /// isSafeToSpeculativelyExecute - Return true if the instruction does not 159 /// have any effects besides calculating the result and does not have 160 /// undefined behavior. 161 /// 162 /// This method never returns true for an instruction that returns true for 163 /// mayHaveSideEffects; however, this method also does some other checks in 164 /// addition. It checks for undefined behavior, like dividing by zero or 165 /// loading from an invalid pointer (but not for undefined results, like a 166 /// shift with a shift amount larger than the width of the result). It checks 167 /// for malloc and alloca because speculatively executing them might cause a 168 /// memory leak. It also returns false for instructions related to control 169 /// flow, specifically terminators and PHI nodes. 170 /// 171 /// This method only looks at the instruction itself and its operands, so if 172 /// this method returns true, it is safe to move the instruction as long as 173 /// the correct dominance relationships for the operands and users hold. 174 /// However, this method can return true for instructions that read memory; 175 /// for such instructions, moving them may change the resulting value. 176 bool isSafeToSpeculativelyExecute(const Value *V, 177 const TargetData *TD = 0); 178 179 } // end namespace llvm 180 181 #endif 182
