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 DataLayout; 26 class StringRef; 27 class MDNode; 28 class AssumptionCache; 29 class DominatorTree; 30 class TargetLibraryInfo; 31 32 /// Determine which bits of V are known to be either zero or one and return 33 /// them in the KnownZero/KnownOne bit sets. 34 /// 35 /// This function is defined on values with integer type, values with pointer 36 /// type (but only if TD is non-null), and vectors of integers. In the case 37 /// where V is a vector, the known zero and known one values are the 38 /// same width as the vector element, and the bit is set only if it is true 39 /// for all of the elements in the vector. 40 void computeKnownBits(Value *V, APInt &KnownZero, APInt &KnownOne, 41 const DataLayout *TD = nullptr, unsigned Depth = 0, 42 AssumptionCache *AC = nullptr, 43 const Instruction *CxtI = nullptr, 44 const DominatorTree *DT = nullptr); 45 /// Compute known bits from the range metadata. 46 /// \p KnownZero the set of bits that are known to be zero 47 void computeKnownBitsFromRangeMetadata(const MDNode &Ranges, 48 APInt &KnownZero); 49 50 /// ComputeSignBit - Determine whether the sign bit is known to be zero or 51 /// one. Convenience wrapper around computeKnownBits. 52 void ComputeSignBit(Value *V, bool &KnownZero, bool &KnownOne, 53 const DataLayout *TD = nullptr, unsigned Depth = 0, 54 AssumptionCache *AC = nullptr, 55 const Instruction *CxtI = nullptr, 56 const DominatorTree *DT = nullptr); 57 58 /// isKnownToBeAPowerOfTwo - Return true if the given value is known to have 59 /// exactly one bit set when defined. For vectors return true if every 60 /// element is known to be a power of two when defined. Supports values with 61 /// integer or pointer type and vectors of integers. If 'OrZero' is set then 62 /// returns true if the given value is either a power of two or zero. 63 bool isKnownToBeAPowerOfTwo(Value *V, bool OrZero = false, unsigned Depth = 0, 64 AssumptionCache *AC = nullptr, 65 const Instruction *CxtI = nullptr, 66 const DominatorTree *DT = nullptr); 67 68 /// isKnownNonZero - Return true if the given value is known to be non-zero 69 /// when defined. For vectors return true if every element is known to be 70 /// non-zero when defined. Supports values with integer or pointer type and 71 /// vectors of integers. 72 bool isKnownNonZero(Value *V, const DataLayout *TD = nullptr, 73 unsigned Depth = 0, AssumptionCache *AC = nullptr, 74 const Instruction *CxtI = nullptr, 75 const DominatorTree *DT = nullptr); 76 77 /// MaskedValueIsZero - Return true if 'V & Mask' is known to be zero. We use 78 /// this predicate to simplify operations downstream. Mask is known to be 79 /// zero for bits that V cannot have. 80 /// 81 /// This function is defined on values with integer type, values with pointer 82 /// type (but only if TD is non-null), and vectors of integers. In the case 83 /// where V is a vector, the mask, known zero, and known one values are the 84 /// same width as the vector element, and the bit is set only if it is true 85 /// for all of the elements in the vector. 86 bool MaskedValueIsZero(Value *V, const APInt &Mask, 87 const DataLayout *TD = nullptr, unsigned Depth = 0, 88 AssumptionCache *AC = nullptr, 89 const Instruction *CxtI = nullptr, 90 const DominatorTree *DT = nullptr); 91 92 /// ComputeNumSignBits - Return the number of times the sign bit of the 93 /// register is replicated into the other bits. We know that at least 1 bit 94 /// is always equal to the sign bit (itself), but other cases can give us 95 /// information. For example, immediately after an "ashr X, 2", we know that 96 /// the top 3 bits are all equal to each other, so we return 3. 97 /// 98 /// 'Op' must have a scalar integer type. 99 /// 100 unsigned ComputeNumSignBits(Value *Op, const DataLayout *TD = nullptr, 101 unsigned Depth = 0, AssumptionCache *AC = nullptr, 102 const Instruction *CxtI = nullptr, 103 const DominatorTree *DT = nullptr); 104 105 /// ComputeMultiple - This function computes the integer multiple of Base that 106 /// equals V. If successful, it returns true and returns the multiple in 107 /// Multiple. If unsuccessful, it returns false. Also, if V can be 108 /// simplified to an integer, then the simplified V is returned in Val. Look 109 /// through sext only if LookThroughSExt=true. 110 bool ComputeMultiple(Value *V, unsigned Base, Value *&Multiple, 111 bool LookThroughSExt = false, 112 unsigned Depth = 0); 113 114 /// CannotBeNegativeZero - Return true if we can prove that the specified FP 115 /// value is never equal to -0.0. 116 /// 117 bool CannotBeNegativeZero(const Value *V, unsigned Depth = 0); 118 119 /// isBytewiseValue - If the specified value can be set by repeating the same 120 /// byte in memory, return the i8 value that it is represented with. This is 121 /// true for all i8 values obviously, but is also true for i32 0, i32 -1, 122 /// i16 0xF0F0, double 0.0 etc. If the value can't be handled with a repeated 123 /// byte store (e.g. i16 0x1234), return null. 124 Value *isBytewiseValue(Value *V); 125 126 /// FindInsertedValue - Given an aggregrate and an sequence of indices, see if 127 /// the scalar value indexed is already around as a register, for example if 128 /// it were inserted directly into the aggregrate. 129 /// 130 /// If InsertBefore is not null, this function will duplicate (modified) 131 /// insertvalues when a part of a nested struct is extracted. 132 Value *FindInsertedValue(Value *V, 133 ArrayRef<unsigned> idx_range, 134 Instruction *InsertBefore = nullptr); 135 136 /// GetPointerBaseWithConstantOffset - Analyze the specified pointer to see if 137 /// it can be expressed as a base pointer plus a constant offset. Return the 138 /// base and offset to the caller. 139 Value *GetPointerBaseWithConstantOffset(Value *Ptr, int64_t &Offset, 140 const DataLayout *TD); 141 static inline const Value * GetPointerBaseWithConstantOffset(const Value * Ptr,int64_t & Offset,const DataLayout * TD)142 GetPointerBaseWithConstantOffset(const Value *Ptr, int64_t &Offset, 143 const DataLayout *TD) { 144 return GetPointerBaseWithConstantOffset(const_cast<Value*>(Ptr), Offset,TD); 145 } 146 147 /// getConstantStringInfo - This function computes the length of a 148 /// null-terminated C string pointed to by V. If successful, it returns true 149 /// and returns the string in Str. If unsuccessful, it returns false. This 150 /// does not include the trailing nul character by default. If TrimAtNul is 151 /// set to false, then this returns any trailing nul characters as well as any 152 /// other characters that come after it. 153 bool getConstantStringInfo(const Value *V, StringRef &Str, 154 uint64_t Offset = 0, bool TrimAtNul = true); 155 156 /// GetStringLength - If we can compute the length of the string pointed to by 157 /// the specified pointer, return 'len+1'. If we can't, return 0. 158 uint64_t GetStringLength(Value *V); 159 160 /// GetUnderlyingObject - This method strips off any GEP address adjustments 161 /// and pointer casts from the specified value, returning the original object 162 /// being addressed. Note that the returned value has pointer type if the 163 /// specified value does. If the MaxLookup value is non-zero, it limits the 164 /// number of instructions to be stripped off. 165 Value *GetUnderlyingObject(Value *V, const DataLayout *TD = nullptr, 166 unsigned MaxLookup = 6); 167 static inline const Value * 168 GetUnderlyingObject(const Value *V, const DataLayout *TD = nullptr, 169 unsigned MaxLookup = 6) { 170 return GetUnderlyingObject(const_cast<Value *>(V), TD, MaxLookup); 171 } 172 173 /// GetUnderlyingObjects - This method is similar to GetUnderlyingObject 174 /// except that it can look through phi and select instructions and return 175 /// multiple objects. 176 void GetUnderlyingObjects(Value *V, 177 SmallVectorImpl<Value *> &Objects, 178 const DataLayout *TD = nullptr, 179 unsigned MaxLookup = 6); 180 181 /// onlyUsedByLifetimeMarkers - Return true if the only users of this pointer 182 /// are lifetime markers. 183 bool onlyUsedByLifetimeMarkers(const Value *V); 184 185 /// isSafeToSpeculativelyExecute - Return true if the instruction does not 186 /// have any effects besides calculating the result and does not have 187 /// undefined behavior. 188 /// 189 /// This method never returns true for an instruction that returns true for 190 /// mayHaveSideEffects; however, this method also does some other checks in 191 /// addition. It checks for undefined behavior, like dividing by zero or 192 /// loading from an invalid pointer (but not for undefined results, like a 193 /// shift with a shift amount larger than the width of the result). It checks 194 /// for malloc and alloca because speculatively executing them might cause a 195 /// memory leak. It also returns false for instructions related to control 196 /// flow, specifically terminators and PHI nodes. 197 /// 198 /// This method only looks at the instruction itself and its operands, so if 199 /// this method returns true, it is safe to move the instruction as long as 200 /// the correct dominance relationships for the operands and users hold. 201 /// However, this method can return true for instructions that read memory; 202 /// for such instructions, moving them may change the resulting value. 203 bool isSafeToSpeculativelyExecute(const Value *V, 204 const DataLayout *TD = nullptr); 205 206 /// isKnownNonNull - Return true if this pointer couldn't possibly be null by 207 /// its definition. This returns true for allocas, non-extern-weak globals 208 /// and byval arguments. 209 bool isKnownNonNull(const Value *V, const TargetLibraryInfo *TLI = nullptr); 210 211 /// Return true if it is valid to use the assumptions provided by an 212 /// assume intrinsic, I, at the point in the control-flow identified by the 213 /// context instruction, CxtI. 214 bool isValidAssumeForContext(const Instruction *I, const Instruction *CxtI, 215 const DataLayout *DL = nullptr, 216 const DominatorTree *DT = nullptr); 217 218 enum class OverflowResult { AlwaysOverflows, MayOverflow, NeverOverflows }; 219 OverflowResult computeOverflowForUnsignedMul(Value *LHS, Value *RHS, 220 const DataLayout *DL, 221 AssumptionCache *AC, 222 const Instruction *CxtI, 223 const DominatorTree *DT); 224 OverflowResult computeOverflowForUnsignedAdd(Value *LHS, Value *RHS, 225 const DataLayout *DL, 226 AssumptionCache *AC, 227 const Instruction *CxtI, 228 const DominatorTree *DT); 229 } // end namespace llvm 230 231 #endif 232