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/IR/ConstantRange.h" 20 #include "llvm/IR/Instruction.h" 21 #include "llvm/Support/DataTypes.h" 22 23 namespace llvm { 24 class APInt; 25 class AddOperator; 26 class AssumptionCache; 27 class DataLayout; 28 class DominatorTree; 29 class Instruction; 30 class Loop; 31 class LoopInfo; 32 class MDNode; 33 class StringRef; 34 class TargetLibraryInfo; 35 class Value; 36 37 /// Determine which bits of V are known to be either zero or one and return 38 /// them in the KnownZero/KnownOne bit sets. 39 /// 40 /// This function is defined on values with integer type, values with pointer 41 /// type, and vectors of integers. In the case 42 /// where V is a vector, the known zero and known one values are the 43 /// same width as the vector element, and the bit is set only if it is true 44 /// for all of the elements in the vector. 45 void computeKnownBits(Value *V, APInt &KnownZero, APInt &KnownOne, 46 const DataLayout &DL, unsigned Depth = 0, 47 AssumptionCache *AC = nullptr, 48 const Instruction *CxtI = nullptr, 49 const DominatorTree *DT = nullptr); 50 /// Compute known bits from the range metadata. 51 /// \p KnownZero the set of bits that are known to be zero 52 /// \p KnownOne the set of bits that are known to be one 53 void computeKnownBitsFromRangeMetadata(const MDNode &Ranges, 54 APInt &KnownZero, APInt &KnownOne); 55 /// Return true if LHS and RHS have no common bits set. 56 bool haveNoCommonBitsSet(Value *LHS, Value *RHS, const DataLayout &DL, 57 AssumptionCache *AC = nullptr, 58 const Instruction *CxtI = nullptr, 59 const DominatorTree *DT = nullptr); 60 61 /// ComputeSignBit - Determine whether the sign bit is known to be zero or 62 /// one. Convenience wrapper around computeKnownBits. 63 void ComputeSignBit(Value *V, bool &KnownZero, bool &KnownOne, 64 const DataLayout &DL, unsigned Depth = 0, 65 AssumptionCache *AC = nullptr, 66 const Instruction *CxtI = nullptr, 67 const DominatorTree *DT = nullptr); 68 69 /// isKnownToBeAPowerOfTwo - Return true if the given value is known to have 70 /// exactly one bit set when defined. For vectors return true if every 71 /// element is known to be a power of two when defined. Supports values with 72 /// integer or pointer type and vectors of integers. If 'OrZero' is set then 73 /// return true if the given value is either a power of two or zero. 74 bool isKnownToBeAPowerOfTwo(Value *V, const DataLayout &DL, 75 bool OrZero = false, unsigned Depth = 0, 76 AssumptionCache *AC = nullptr, 77 const Instruction *CxtI = nullptr, 78 const DominatorTree *DT = nullptr); 79 80 /// isKnownNonZero - Return true if the given value is known to be non-zero 81 /// when defined. For vectors return true if every element is known to be 82 /// non-zero when defined. Supports values with integer or pointer type and 83 /// vectors of integers. 84 bool isKnownNonZero(Value *V, const DataLayout &DL, unsigned Depth = 0, 85 AssumptionCache *AC = nullptr, 86 const Instruction *CxtI = nullptr, 87 const DominatorTree *DT = nullptr); 88 89 /// Returns true if the give value is known to be non-negative. 90 bool isKnownNonNegative(Value *V, const DataLayout &DL, unsigned Depth = 0, 91 AssumptionCache *AC = nullptr, 92 const Instruction *CxtI = nullptr, 93 const DominatorTree *DT = nullptr); 94 95 /// isKnownNonEqual - Return true if the given values are known to be 96 /// non-equal when defined. Supports scalar integer types only. 97 bool isKnownNonEqual(Value *V1, Value *V2, const DataLayout &DL, 98 AssumptionCache *AC = nullptr, 99 const Instruction *CxtI = nullptr, 100 const DominatorTree *DT = nullptr); 101 102 /// MaskedValueIsZero - Return true if 'V & Mask' is known to be zero. We use 103 /// this predicate to simplify operations downstream. Mask is known to be 104 /// zero for bits that V cannot have. 105 /// 106 /// This function is defined on values with integer type, values with pointer 107 /// type, and vectors of integers. In the case 108 /// where V is a vector, the mask, known zero, and known one values are the 109 /// same width as the vector element, and the bit is set only if it is true 110 /// for all of the elements in the vector. 111 bool MaskedValueIsZero(Value *V, const APInt &Mask, const DataLayout &DL, 112 unsigned Depth = 0, AssumptionCache *AC = nullptr, 113 const Instruction *CxtI = nullptr, 114 const DominatorTree *DT = nullptr); 115 116 /// ComputeNumSignBits - Return the number of times the sign bit of the 117 /// register is replicated into the other bits. We know that at least 1 bit 118 /// is always equal to the sign bit (itself), but other cases can give us 119 /// information. For example, immediately after an "ashr X, 2", we know that 120 /// the top 3 bits are all equal to each other, so we return 3. 121 /// 122 /// 'Op' must have a scalar integer type. 123 /// 124 unsigned ComputeNumSignBits(Value *Op, const DataLayout &DL, 125 unsigned Depth = 0, AssumptionCache *AC = nullptr, 126 const Instruction *CxtI = nullptr, 127 const DominatorTree *DT = nullptr); 128 129 /// ComputeMultiple - This function computes the integer multiple of Base that 130 /// equals V. If successful, it returns true and returns the multiple in 131 /// Multiple. If unsuccessful, it returns false. Also, if V can be 132 /// simplified to an integer, then the simplified V is returned in Val. Look 133 /// through sext only if LookThroughSExt=true. 134 bool ComputeMultiple(Value *V, unsigned Base, Value *&Multiple, 135 bool LookThroughSExt = false, 136 unsigned Depth = 0); 137 138 /// CannotBeNegativeZero - Return true if we can prove that the specified FP 139 /// value is never equal to -0.0. 140 /// 141 bool CannotBeNegativeZero(const Value *V, unsigned Depth = 0); 142 143 /// CannotBeOrderedLessThanZero - Return true if we can prove that the 144 /// specified FP value is either a NaN or never less than 0.0. 145 /// 146 bool CannotBeOrderedLessThanZero(const Value *V, unsigned Depth = 0); 147 148 /// isBytewiseValue - If the specified value can be set by repeating the same 149 /// byte in memory, return the i8 value that it is represented with. This is 150 /// true for all i8 values obviously, but is also true for i32 0, i32 -1, 151 /// i16 0xF0F0, double 0.0 etc. If the value can't be handled with a repeated 152 /// byte store (e.g. i16 0x1234), return null. 153 Value *isBytewiseValue(Value *V); 154 155 /// FindInsertedValue - Given an aggregrate and an sequence of indices, see if 156 /// the scalar value indexed is already around as a register, for example if 157 /// it were inserted directly into the aggregrate. 158 /// 159 /// If InsertBefore is not null, this function will duplicate (modified) 160 /// insertvalues when a part of a nested struct is extracted. 161 Value *FindInsertedValue(Value *V, 162 ArrayRef<unsigned> idx_range, 163 Instruction *InsertBefore = nullptr); 164 165 /// GetPointerBaseWithConstantOffset - Analyze the specified pointer to see if 166 /// it can be expressed as a base pointer plus a constant offset. Return the 167 /// base and offset to the caller. 168 Value *GetPointerBaseWithConstantOffset(Value *Ptr, int64_t &Offset, 169 const DataLayout &DL); 170 static inline const Value * 171 GetPointerBaseWithConstantOffset(const Value *Ptr, int64_t &Offset, 172 const DataLayout &DL) { 173 return GetPointerBaseWithConstantOffset(const_cast<Value *>(Ptr), Offset, 174 DL); 175 } 176 177 /// getConstantStringInfo - This function computes the length of a 178 /// null-terminated C string pointed to by V. If successful, it returns true 179 /// and returns the string in Str. If unsuccessful, it returns false. This 180 /// does not include the trailing nul character by default. If TrimAtNul is 181 /// set to false, then this returns any trailing nul characters as well as any 182 /// other characters that come after it. 183 bool getConstantStringInfo(const Value *V, StringRef &Str, 184 uint64_t Offset = 0, bool TrimAtNul = true); 185 186 /// GetStringLength - If we can compute the length of the string pointed to by 187 /// the specified pointer, return 'len+1'. If we can't, return 0. 188 uint64_t GetStringLength(Value *V); 189 190 /// GetUnderlyingObject - This method strips off any GEP address adjustments 191 /// and pointer casts from the specified value, returning the original object 192 /// being addressed. Note that the returned value has pointer type if the 193 /// specified value does. If the MaxLookup value is non-zero, it limits the 194 /// number of instructions to be stripped off. 195 Value *GetUnderlyingObject(Value *V, const DataLayout &DL, 196 unsigned MaxLookup = 6); 197 static inline const Value *GetUnderlyingObject(const Value *V, 198 const DataLayout &DL, 199 unsigned MaxLookup = 6) { 200 return GetUnderlyingObject(const_cast<Value *>(V), DL, MaxLookup); 201 } 202 203 /// \brief This method is similar to GetUnderlyingObject except that it can 204 /// look through phi and select instructions and return multiple objects. 205 /// 206 /// If LoopInfo is passed, loop phis are further analyzed. If a pointer 207 /// accesses different objects in each iteration, we don't look through the 208 /// phi node. E.g. consider this loop nest: 209 /// 210 /// int **A; 211 /// for (i) 212 /// for (j) { 213 /// A[i][j] = A[i-1][j] * B[j] 214 /// } 215 /// 216 /// This is transformed by Load-PRE to stash away A[i] for the next iteration 217 /// of the outer loop: 218 /// 219 /// Curr = A[0]; // Prev_0 220 /// for (i: 1..N) { 221 /// Prev = Curr; // Prev = PHI (Prev_0, Curr) 222 /// Curr = A[i]; 223 /// for (j: 0..N) { 224 /// Curr[j] = Prev[j] * B[j] 225 /// } 226 /// } 227 /// 228 /// Since A[i] and A[i-1] are independent pointers, getUnderlyingObjects 229 /// should not assume that Curr and Prev share the same underlying object thus 230 /// it shouldn't look through the phi above. 231 void GetUnderlyingObjects(Value *V, SmallVectorImpl<Value *> &Objects, 232 const DataLayout &DL, LoopInfo *LI = nullptr, 233 unsigned MaxLookup = 6); 234 235 /// onlyUsedByLifetimeMarkers - Return true if the only users of this pointer 236 /// are lifetime markers. 237 bool onlyUsedByLifetimeMarkers(const Value *V); 238 239 /// isDereferenceablePointer - Return true if this is always a dereferenceable 240 /// pointer. If the context instruction is specified perform context-sensitive 241 /// analysis and return true if the pointer is dereferenceable at the 242 /// specified instruction. 243 bool isDereferenceablePointer(const Value *V, const DataLayout &DL, 244 const Instruction *CtxI = nullptr, 245 const DominatorTree *DT = nullptr, 246 const TargetLibraryInfo *TLI = nullptr); 247 248 /// Returns true if V is always a dereferenceable pointer with alignment 249 /// greater or equal than requested. If the context instruction is specified 250 /// performs context-sensitive analysis and returns true if the pointer is 251 /// dereferenceable at the specified instruction. 252 bool isDereferenceableAndAlignedPointer(const Value *V, unsigned Align, 253 const DataLayout &DL, 254 const Instruction *CtxI = nullptr, 255 const DominatorTree *DT = nullptr, 256 const TargetLibraryInfo *TLI = nullptr); 257 258 /// isSafeToSpeculativelyExecute - Return true if the instruction does not 259 /// have any effects besides calculating the result and does not have 260 /// undefined behavior. 261 /// 262 /// This method never returns true for an instruction that returns true for 263 /// mayHaveSideEffects; however, this method also does some other checks in 264 /// addition. It checks for undefined behavior, like dividing by zero or 265 /// loading from an invalid pointer (but not for undefined results, like a 266 /// shift with a shift amount larger than the width of the result). It checks 267 /// for malloc and alloca because speculatively executing them might cause a 268 /// memory leak. It also returns false for instructions related to control 269 /// flow, specifically terminators and PHI nodes. 270 /// 271 /// If the CtxI is specified this method performs context-sensitive analysis 272 /// and returns true if it is safe to execute the instruction immediately 273 /// before the CtxI. 274 /// 275 /// If the CtxI is NOT specified this method only looks at the instruction 276 /// itself and its operands, so if this method returns true, it is safe to 277 /// move the instruction as long as the correct dominance relationships for 278 /// the operands and users hold. 279 /// 280 /// This method can return true for instructions that read memory; 281 /// for such instructions, moving them may change the resulting value. 282 bool isSafeToSpeculativelyExecute(const Value *V, 283 const Instruction *CtxI = nullptr, 284 const DominatorTree *DT = nullptr, 285 const TargetLibraryInfo *TLI = nullptr); 286 287 /// Returns true if the result or effects of the given instructions \p I 288 /// depend on or influence global memory. 289 /// Memory dependence arises for example if the instruction reads from 290 /// memory or may produce effects or undefined behaviour. Memory dependent 291 /// instructions generally cannot be reorderd with respect to other memory 292 /// dependent instructions or moved into non-dominated basic blocks. 293 /// Instructions which just compute a value based on the values of their 294 /// operands are not memory dependent. 295 bool mayBeMemoryDependent(const Instruction &I); 296 297 /// isKnownNonNull - Return true if this pointer couldn't possibly be null by 298 /// its definition. This returns true for allocas, non-extern-weak globals 299 /// and byval arguments. 300 bool isKnownNonNull(const Value *V, const TargetLibraryInfo *TLI = nullptr); 301 302 /// isKnownNonNullAt - Return true if this pointer couldn't possibly be null. 303 /// If the context instruction is specified perform context-sensitive analysis 304 /// and return true if the pointer couldn't possibly be null at the specified 305 /// instruction. 306 bool isKnownNonNullAt(const Value *V, 307 const Instruction *CtxI = nullptr, 308 const DominatorTree *DT = nullptr, 309 const TargetLibraryInfo *TLI = nullptr); 310 311 /// Return true if it is valid to use the assumptions provided by an 312 /// assume intrinsic, I, at the point in the control-flow identified by the 313 /// context instruction, CxtI. 314 bool isValidAssumeForContext(const Instruction *I, const Instruction *CxtI, 315 const DominatorTree *DT = nullptr); 316 317 enum class OverflowResult { AlwaysOverflows, MayOverflow, NeverOverflows }; 318 OverflowResult computeOverflowForUnsignedMul(Value *LHS, Value *RHS, 319 const DataLayout &DL, 320 AssumptionCache *AC, 321 const Instruction *CxtI, 322 const DominatorTree *DT); 323 OverflowResult computeOverflowForUnsignedAdd(Value *LHS, Value *RHS, 324 const DataLayout &DL, 325 AssumptionCache *AC, 326 const Instruction *CxtI, 327 const DominatorTree *DT); 328 OverflowResult computeOverflowForSignedAdd(Value *LHS, Value *RHS, 329 const DataLayout &DL, 330 AssumptionCache *AC = nullptr, 331 const Instruction *CxtI = nullptr, 332 const DominatorTree *DT = nullptr); 333 /// This version also leverages the sign bit of Add if known. 334 OverflowResult computeOverflowForSignedAdd(AddOperator *Add, 335 const DataLayout &DL, 336 AssumptionCache *AC = nullptr, 337 const Instruction *CxtI = nullptr, 338 const DominatorTree *DT = nullptr); 339 340 /// Return true if this function can prove that the instruction I will 341 /// always transfer execution to one of its successors (including the next 342 /// instruction that follows within a basic block). E.g. this is not 343 /// guaranteed for function calls that could loop infinitely. 344 /// 345 /// In other words, this function returns false for instructions that may 346 /// transfer execution or fail to transfer execution in a way that is not 347 /// captured in the CFG nor in the sequence of instructions within a basic 348 /// block. 349 /// 350 /// Undefined behavior is assumed not to happen, so e.g. division is 351 /// guaranteed to transfer execution to the following instruction even 352 /// though division by zero might cause undefined behavior. 353 bool isGuaranteedToTransferExecutionToSuccessor(const Instruction *I); 354 355 /// Return true if this function can prove that the instruction I 356 /// is executed for every iteration of the loop L. 357 /// 358 /// Note that this currently only considers the loop header. 359 bool isGuaranteedToExecuteForEveryIteration(const Instruction *I, 360 const Loop *L); 361 362 /// Return true if this function can prove that I is guaranteed to yield 363 /// full-poison (all bits poison) if at least one of its operands are 364 /// full-poison (all bits poison). 365 /// 366 /// The exact rules for how poison propagates through instructions have 367 /// not been settled as of 2015-07-10, so this function is conservative 368 /// and only considers poison to be propagated in uncontroversial 369 /// cases. There is no attempt to track values that may be only partially 370 /// poison. 371 bool propagatesFullPoison(const Instruction *I); 372 373 /// Return either nullptr or an operand of I such that I will trigger 374 /// undefined behavior if I is executed and that operand has a full-poison 375 /// value (all bits poison). 376 const Value *getGuaranteedNonFullPoisonOp(const Instruction *I); 377 378 /// Return true if this function can prove that if PoisonI is executed 379 /// and yields a full-poison value (all bits poison), then that will 380 /// trigger undefined behavior. 381 /// 382 /// Note that this currently only considers the basic block that is 383 /// the parent of I. 384 bool isKnownNotFullPoison(const Instruction *PoisonI); 385 386 /// \brief Specific patterns of select instructions we can match. 387 enum SelectPatternFlavor { 388 SPF_UNKNOWN = 0, 389 SPF_SMIN, /// Signed minimum 390 SPF_UMIN, /// Unsigned minimum 391 SPF_SMAX, /// Signed maximum 392 SPF_UMAX, /// Unsigned maximum 393 SPF_FMINNUM, /// Floating point minnum 394 SPF_FMAXNUM, /// Floating point maxnum 395 SPF_ABS, /// Absolute value 396 SPF_NABS /// Negated absolute value 397 }; 398 /// \brief Behavior when a floating point min/max is given one NaN and one 399 /// non-NaN as input. 400 enum SelectPatternNaNBehavior { 401 SPNB_NA = 0, /// NaN behavior not applicable. 402 SPNB_RETURNS_NAN, /// Given one NaN input, returns the NaN. 403 SPNB_RETURNS_OTHER, /// Given one NaN input, returns the non-NaN. 404 SPNB_RETURNS_ANY /// Given one NaN input, can return either (or 405 /// it has been determined that no operands can 406 /// be NaN). 407 }; 408 struct SelectPatternResult { 409 SelectPatternFlavor Flavor; 410 SelectPatternNaNBehavior NaNBehavior; /// Only applicable if Flavor is 411 /// SPF_FMINNUM or SPF_FMAXNUM. 412 bool Ordered; /// When implementing this min/max pattern as 413 /// fcmp; select, does the fcmp have to be 414 /// ordered? 415 416 /// \brief Return true if \p SPF is a min or a max pattern. 417 static bool isMinOrMax(SelectPatternFlavor SPF) { 418 return !(SPF == SPF_UNKNOWN || SPF == SPF_ABS || SPF == SPF_NABS); 419 } 420 }; 421 /// Pattern match integer [SU]MIN, [SU]MAX and ABS idioms, returning the kind 422 /// and providing the out parameter results if we successfully match. 423 /// 424 /// If CastOp is not nullptr, also match MIN/MAX idioms where the type does 425 /// not match that of the original select. If this is the case, the cast 426 /// operation (one of Trunc,SExt,Zext) that must be done to transform the 427 /// type of LHS and RHS into the type of V is returned in CastOp. 428 /// 429 /// For example: 430 /// %1 = icmp slt i32 %a, i32 4 431 /// %2 = sext i32 %a to i64 432 /// %3 = select i1 %1, i64 %2, i64 4 433 /// 434 /// -> LHS = %a, RHS = i32 4, *CastOp = Instruction::SExt 435 /// 436 SelectPatternResult matchSelectPattern(Value *V, Value *&LHS, Value *&RHS, 437 Instruction::CastOps *CastOp = nullptr); 438 439 /// Parse out a conservative ConstantRange from !range metadata. 440 /// 441 /// E.g. if RangeMD is !{i32 0, i32 10, i32 15, i32 20} then return [0, 20). 442 ConstantRange getConstantRangeFromMetadata(MDNode &RangeMD); 443 444 /// Return true if RHS is known to be implied by LHS. A & B must be i1 445 /// (boolean) values or a vector of such values. Note that the truth table for 446 /// implication is the same as <=u on i1 values (but not <=s!). The truth 447 /// table for both is: 448 /// | T | F (B) 449 /// T | T | F 450 /// F | T | T 451 /// (A) 452 bool isImpliedCondition(Value *LHS, Value *RHS, const DataLayout &DL, 453 unsigned Depth = 0, AssumptionCache *AC = nullptr, 454 const Instruction *CxtI = nullptr, 455 const DominatorTree *DT = nullptr); 456 } // end namespace llvm 457 458 #endif 459
