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/IR/CallSite.h" 19 #include "llvm/IR/Instruction.h" 20 #include "llvm/IR/IntrinsicInst.h" 21 #include "llvm/Support/DataTypes.h" 22 23 namespace llvm { 24 template <typename T> class ArrayRef; 25 class APInt; 26 class AddOperator; 27 class AssumptionCache; 28 class DataLayout; 29 class DominatorTree; 30 class GEPOperator; 31 class Instruction; 32 struct KnownBits; 33 class Loop; 34 class LoopInfo; 35 class OptimizationRemarkEmitter; 36 class MDNode; 37 class StringRef; 38 class TargetLibraryInfo; 39 class Value; 40 41 namespace Intrinsic { 42 enum ID : unsigned; 43 } 44 45 /// Determine which bits of V are known to be either zero or one and return 46 /// them in the KnownZero/KnownOne bit sets. 47 /// 48 /// This function is defined on values with integer type, values with pointer 49 /// type, and vectors of integers. In the case 50 /// where V is a vector, the known zero and known one values are the 51 /// same width as the vector element, and the bit is set only if it is true 52 /// for all of the elements in the vector. 53 void computeKnownBits(const Value *V, KnownBits &Known, 54 const DataLayout &DL, unsigned Depth = 0, 55 AssumptionCache *AC = nullptr, 56 const Instruction *CxtI = nullptr, 57 const DominatorTree *DT = nullptr, 58 OptimizationRemarkEmitter *ORE = nullptr); 59 /// Returns the known bits rather than passing by reference. 60 KnownBits computeKnownBits(const Value *V, const DataLayout &DL, 61 unsigned Depth = 0, AssumptionCache *AC = nullptr, 62 const Instruction *CxtI = nullptr, 63 const DominatorTree *DT = nullptr, 64 OptimizationRemarkEmitter *ORE = nullptr); 65 /// Compute known bits from the range metadata. 66 /// \p KnownZero the set of bits that are known to be zero 67 /// \p KnownOne the set of bits that are known to be one 68 void computeKnownBitsFromRangeMetadata(const MDNode &Ranges, 69 KnownBits &Known); 70 /// Return true if LHS and RHS have no common bits set. 71 bool haveNoCommonBitsSet(const Value *LHS, const Value *RHS, 72 const DataLayout &DL, 73 AssumptionCache *AC = nullptr, 74 const Instruction *CxtI = nullptr, 75 const DominatorTree *DT = nullptr); 76 77 /// Return true if the given value is known to have exactly one bit set when 78 /// defined. For vectors return true if every element is known to be a power 79 /// of two when defined. Supports values with integer or pointer type and 80 /// vectors of integers. If 'OrZero' is set, then return true if the given 81 /// value is either a power of two or zero. 82 bool isKnownToBeAPowerOfTwo(const Value *V, const DataLayout &DL, 83 bool OrZero = false, unsigned Depth = 0, 84 AssumptionCache *AC = nullptr, 85 const Instruction *CxtI = nullptr, 86 const DominatorTree *DT = nullptr); 87 88 bool isOnlyUsedInZeroEqualityComparison(const Instruction *CxtI); 89 90 /// Return true if the given value is known to be non-zero when defined. For 91 /// vectors, return true if every element is known to be non-zero when 92 /// defined. For pointers, if the context instruction and dominator tree are 93 /// specified, perform context-sensitive analysis and return true if the 94 /// pointer couldn't possibly be null at the specified instruction. 95 /// Supports values with integer or pointer type and vectors of integers. 96 bool isKnownNonZero(const Value *V, const DataLayout &DL, unsigned Depth = 0, 97 AssumptionCache *AC = nullptr, 98 const Instruction *CxtI = nullptr, 99 const DominatorTree *DT = nullptr); 100 101 /// Returns true if the give value is known to be non-negative. 102 bool isKnownNonNegative(const Value *V, const DataLayout &DL, 103 unsigned Depth = 0, 104 AssumptionCache *AC = nullptr, 105 const Instruction *CxtI = nullptr, 106 const DominatorTree *DT = nullptr); 107 108 /// Returns true if the given value is known be positive (i.e. non-negative 109 /// and non-zero). 110 bool isKnownPositive(const Value *V, const DataLayout &DL, unsigned Depth = 0, 111 AssumptionCache *AC = nullptr, 112 const Instruction *CxtI = nullptr, 113 const DominatorTree *DT = nullptr); 114 115 /// Returns true if the given value is known be negative (i.e. non-positive 116 /// and non-zero). 117 bool isKnownNegative(const Value *V, const DataLayout &DL, unsigned Depth = 0, 118 AssumptionCache *AC = nullptr, 119 const Instruction *CxtI = nullptr, 120 const DominatorTree *DT = nullptr); 121 122 /// Return true if the given values are known to be non-equal when defined. 123 /// Supports scalar integer types only. 124 bool isKnownNonEqual(const Value *V1, const Value *V2, const DataLayout &DL, 125 AssumptionCache *AC = nullptr, 126 const Instruction *CxtI = nullptr, 127 const DominatorTree *DT = nullptr); 128 129 /// Return true if 'V & Mask' is known to be zero. We use this predicate to 130 /// simplify operations downstream. Mask is known to be zero for bits that V 131 /// cannot have. 132 /// 133 /// This function is defined on values with integer type, values with pointer 134 /// type, and vectors of integers. In the case 135 /// where V is a vector, the mask, known zero, and known one values are the 136 /// same width as the vector element, and the bit is set only if it is true 137 /// for all of the elements in the vector. 138 bool MaskedValueIsZero(const Value *V, const APInt &Mask, 139 const DataLayout &DL, 140 unsigned Depth = 0, AssumptionCache *AC = nullptr, 141 const Instruction *CxtI = nullptr, 142 const DominatorTree *DT = nullptr); 143 144 /// Return the number of times the sign bit of the register is replicated into 145 /// the other bits. We know that at least 1 bit is always equal to the sign 146 /// bit (itself), but other cases can give us information. For example, 147 /// immediately after an "ashr X, 2", we know that the top 3 bits are all 148 /// equal to each other, so we return 3. For vectors, return the number of 149 /// sign bits for the vector element with the mininum number of known sign 150 /// bits. 151 unsigned ComputeNumSignBits(const Value *Op, const DataLayout &DL, 152 unsigned Depth = 0, AssumptionCache *AC = nullptr, 153 const Instruction *CxtI = nullptr, 154 const DominatorTree *DT = nullptr); 155 156 /// This function computes the integer multiple of Base that equals V. If 157 /// successful, it returns true and returns the multiple in Multiple. If 158 /// unsuccessful, it returns false. Also, if V can be simplified to an 159 /// integer, then the simplified V is returned in Val. Look through sext only 160 /// if LookThroughSExt=true. 161 bool ComputeMultiple(Value *V, unsigned Base, Value *&Multiple, 162 bool LookThroughSExt = false, 163 unsigned Depth = 0); 164 165 /// Map a call instruction to an intrinsic ID. Libcalls which have equivalent 166 /// intrinsics are treated as-if they were intrinsics. 167 Intrinsic::ID getIntrinsicForCallSite(ImmutableCallSite ICS, 168 const TargetLibraryInfo *TLI); 169 170 /// Return true if we can prove that the specified FP value is never equal to 171 /// -0.0. 172 bool CannotBeNegativeZero(const Value *V, const TargetLibraryInfo *TLI, 173 unsigned Depth = 0); 174 175 /// Return true if we can prove that the specified FP value is either NaN or 176 /// never less than -0.0. 177 /// 178 /// NaN --> true 179 /// +0 --> true 180 /// -0 --> true 181 /// x > +0 --> true 182 /// x < -0 --> false 183 /// 184 bool CannotBeOrderedLessThanZero(const Value *V, const TargetLibraryInfo *TLI); 185 186 /// Return true if we can prove that the specified FP value's sign bit is 0. 187 /// 188 /// NaN --> true/false (depending on the NaN's sign bit) 189 /// +0 --> true 190 /// -0 --> false 191 /// x > +0 --> true 192 /// x < -0 --> false 193 /// 194 bool SignBitMustBeZero(const Value *V, const TargetLibraryInfo *TLI); 195 196 /// If the specified value can be set by repeating the same byte in memory, 197 /// return the i8 value that it is represented with. This is true for all i8 198 /// values obviously, but is also true for i32 0, i32 -1, i16 0xF0F0, double 199 /// 0.0 etc. If the value can't be handled with a repeated byte store (e.g. 200 /// i16 0x1234), return null. 201 Value *isBytewiseValue(Value *V); 202 203 /// Given an aggregrate and an sequence of indices, see if the scalar value 204 /// indexed is already around as a register, for example if it were inserted 205 /// directly into the aggregrate. 206 /// 207 /// If InsertBefore is not null, this function will duplicate (modified) 208 /// insertvalues when a part of a nested struct is extracted. 209 Value *FindInsertedValue(Value *V, 210 ArrayRef<unsigned> idx_range, 211 Instruction *InsertBefore = nullptr); 212 213 /// Analyze the specified pointer to see if it can be expressed as a base 214 /// pointer plus a constant offset. Return the base and offset to the caller. 215 Value *GetPointerBaseWithConstantOffset(Value *Ptr, int64_t &Offset, 216 const DataLayout &DL); 217 static inline const Value * 218 GetPointerBaseWithConstantOffset(const Value *Ptr, int64_t &Offset, 219 const DataLayout &DL) { 220 return GetPointerBaseWithConstantOffset(const_cast<Value *>(Ptr), Offset, 221 DL); 222 } 223 224 /// Returns true if the GEP is based on a pointer to a string (array of 225 // \p CharSize integers) and is indexing into this string. 226 bool isGEPBasedOnPointerToString(const GEPOperator *GEP, 227 unsigned CharSize = 8); 228 229 /// Represents offset+length into a ConstantDataArray. 230 struct ConstantDataArraySlice { 231 /// ConstantDataArray pointer. nullptr indicates a zeroinitializer (a valid 232 /// initializer, it just doesn't fit the ConstantDataArray interface). 233 const ConstantDataArray *Array; 234 /// Slice starts at this Offset. 235 uint64_t Offset; 236 /// Length of the slice. 237 uint64_t Length; 238 239 /// Moves the Offset and adjusts Length accordingly. 240 void move(uint64_t Delta) { 241 assert(Delta < Length); 242 Offset += Delta; 243 Length -= Delta; 244 } 245 /// Convenience accessor for elements in the slice. 246 uint64_t operator[](unsigned I) const { 247 return Array==nullptr ? 0 : Array->getElementAsInteger(I + Offset); 248 } 249 }; 250 251 /// Returns true if the value \p V is a pointer into a ContantDataArray. 252 /// If successful \p Index will point to a ConstantDataArray info object 253 /// with an appropriate offset. 254 bool getConstantDataArrayInfo(const Value *V, ConstantDataArraySlice &Slice, 255 unsigned ElementSize, uint64_t Offset = 0); 256 257 /// This function computes the length of a null-terminated C string pointed to 258 /// by V. If successful, it returns true and returns the string in Str. If 259 /// unsuccessful, it returns false. This does not include the trailing null 260 /// character by default. If TrimAtNul is set to false, then this returns any 261 /// trailing null characters as well as any other characters that come after 262 /// it. 263 bool getConstantStringInfo(const Value *V, StringRef &Str, 264 uint64_t Offset = 0, bool TrimAtNul = true); 265 266 /// If we can compute the length of the string pointed to by the specified 267 /// pointer, return 'len+1'. If we can't, return 0. 268 uint64_t GetStringLength(const Value *V, unsigned CharSize = 8); 269 270 /// This method strips off any GEP address adjustments and pointer casts from 271 /// the specified value, returning the original object being addressed. Note 272 /// that the returned value has pointer type if the specified value does. If 273 /// the MaxLookup value is non-zero, it limits the number of instructions to 274 /// be stripped off. 275 Value *GetUnderlyingObject(Value *V, const DataLayout &DL, 276 unsigned MaxLookup = 6); 277 static inline const Value *GetUnderlyingObject(const Value *V, 278 const DataLayout &DL, 279 unsigned MaxLookup = 6) { 280 return GetUnderlyingObject(const_cast<Value *>(V), DL, MaxLookup); 281 } 282 283 /// \brief This method is similar to GetUnderlyingObject except that it can 284 /// look through phi and select instructions and return multiple objects. 285 /// 286 /// If LoopInfo is passed, loop phis are further analyzed. If a pointer 287 /// accesses different objects in each iteration, we don't look through the 288 /// phi node. E.g. consider this loop nest: 289 /// 290 /// int **A; 291 /// for (i) 292 /// for (j) { 293 /// A[i][j] = A[i-1][j] * B[j] 294 /// } 295 /// 296 /// This is transformed by Load-PRE to stash away A[i] for the next iteration 297 /// of the outer loop: 298 /// 299 /// Curr = A[0]; // Prev_0 300 /// for (i: 1..N) { 301 /// Prev = Curr; // Prev = PHI (Prev_0, Curr) 302 /// Curr = A[i]; 303 /// for (j: 0..N) { 304 /// Curr[j] = Prev[j] * B[j] 305 /// } 306 /// } 307 /// 308 /// Since A[i] and A[i-1] are independent pointers, getUnderlyingObjects 309 /// should not assume that Curr and Prev share the same underlying object thus 310 /// it shouldn't look through the phi above. 311 void GetUnderlyingObjects(Value *V, SmallVectorImpl<Value *> &Objects, 312 const DataLayout &DL, LoopInfo *LI = nullptr, 313 unsigned MaxLookup = 6); 314 315 /// Return true if the only users of this pointer are lifetime markers. 316 bool onlyUsedByLifetimeMarkers(const Value *V); 317 318 /// Return true if the instruction does not have any effects besides 319 /// calculating the result and does not have undefined behavior. 320 /// 321 /// This method never returns true for an instruction that returns true for 322 /// mayHaveSideEffects; however, this method also does some other checks in 323 /// addition. It checks for undefined behavior, like dividing by zero or 324 /// loading from an invalid pointer (but not for undefined results, like a 325 /// shift with a shift amount larger than the width of the result). It checks 326 /// for malloc and alloca because speculatively executing them might cause a 327 /// memory leak. It also returns false for instructions related to control 328 /// flow, specifically terminators and PHI nodes. 329 /// 330 /// If the CtxI is specified this method performs context-sensitive analysis 331 /// and returns true if it is safe to execute the instruction immediately 332 /// before the CtxI. 333 /// 334 /// If the CtxI is NOT specified this method only looks at the instruction 335 /// itself and its operands, so if this method returns true, it is safe to 336 /// move the instruction as long as the correct dominance relationships for 337 /// the operands and users hold. 338 /// 339 /// This method can return true for instructions that read memory; 340 /// for such instructions, moving them may change the resulting value. 341 bool isSafeToSpeculativelyExecute(const Value *V, 342 const Instruction *CtxI = nullptr, 343 const DominatorTree *DT = nullptr); 344 345 /// Returns true if the result or effects of the given instructions \p I 346 /// depend on or influence global memory. 347 /// Memory dependence arises for example if the instruction reads from 348 /// memory or may produce effects or undefined behaviour. Memory dependent 349 /// instructions generally cannot be reorderd with respect to other memory 350 /// dependent instructions or moved into non-dominated basic blocks. 351 /// Instructions which just compute a value based on the values of their 352 /// operands are not memory dependent. 353 bool mayBeMemoryDependent(const Instruction &I); 354 355 /// Return true if this pointer couldn't possibly be null by its definition. 356 /// This returns true for allocas, non-extern-weak globals, and byval 357 /// arguments. 358 bool isKnownNonNull(const Value *V); 359 360 /// Return true if this pointer couldn't possibly be null. If the context 361 /// instruction and dominator tree are specified, perform context-sensitive 362 /// analysis and return true if the pointer couldn't possibly be null at the 363 /// specified instruction. 364 bool isKnownNonNullAt(const Value *V, 365 const Instruction *CtxI = nullptr, 366 const DominatorTree *DT = nullptr); 367 368 /// Return true if it is valid to use the assumptions provided by an 369 /// assume intrinsic, I, at the point in the control-flow identified by the 370 /// context instruction, CxtI. 371 bool isValidAssumeForContext(const Instruction *I, const Instruction *CxtI, 372 const DominatorTree *DT = nullptr); 373 374 enum class OverflowResult { AlwaysOverflows, MayOverflow, NeverOverflows }; 375 OverflowResult computeOverflowForUnsignedMul(const Value *LHS, 376 const Value *RHS, 377 const DataLayout &DL, 378 AssumptionCache *AC, 379 const Instruction *CxtI, 380 const DominatorTree *DT); 381 OverflowResult computeOverflowForUnsignedAdd(const Value *LHS, 382 const Value *RHS, 383 const DataLayout &DL, 384 AssumptionCache *AC, 385 const Instruction *CxtI, 386 const DominatorTree *DT); 387 OverflowResult computeOverflowForSignedAdd(const Value *LHS, const Value *RHS, 388 const DataLayout &DL, 389 AssumptionCache *AC = nullptr, 390 const Instruction *CxtI = nullptr, 391 const DominatorTree *DT = nullptr); 392 /// This version also leverages the sign bit of Add if known. 393 OverflowResult computeOverflowForSignedAdd(const AddOperator *Add, 394 const DataLayout &DL, 395 AssumptionCache *AC = nullptr, 396 const Instruction *CxtI = nullptr, 397 const DominatorTree *DT = nullptr); 398 399 /// Returns true if the arithmetic part of the \p II 's result is 400 /// used only along the paths control dependent on the computation 401 /// not overflowing, \p II being an <op>.with.overflow intrinsic. 402 bool isOverflowIntrinsicNoWrap(const IntrinsicInst *II, 403 const DominatorTree &DT); 404 405 /// Return true if this function can prove that the instruction I will 406 /// always transfer execution to one of its successors (including the next 407 /// instruction that follows within a basic block). E.g. this is not 408 /// guaranteed for function calls that could loop infinitely. 409 /// 410 /// In other words, this function returns false for instructions that may 411 /// transfer execution or fail to transfer execution in a way that is not 412 /// captured in the CFG nor in the sequence of instructions within a basic 413 /// block. 414 /// 415 /// Undefined behavior is assumed not to happen, so e.g. division is 416 /// guaranteed to transfer execution to the following instruction even 417 /// though division by zero might cause undefined behavior. 418 bool isGuaranteedToTransferExecutionToSuccessor(const Instruction *I); 419 420 /// Return true if this function can prove that the instruction I 421 /// is executed for every iteration of the loop L. 422 /// 423 /// Note that this currently only considers the loop header. 424 bool isGuaranteedToExecuteForEveryIteration(const Instruction *I, 425 const Loop *L); 426 427 /// Return true if this function can prove that I is guaranteed to yield 428 /// full-poison (all bits poison) if at least one of its operands are 429 /// full-poison (all bits poison). 430 /// 431 /// The exact rules for how poison propagates through instructions have 432 /// not been settled as of 2015-07-10, so this function is conservative 433 /// and only considers poison to be propagated in uncontroversial 434 /// cases. There is no attempt to track values that may be only partially 435 /// poison. 436 bool propagatesFullPoison(const Instruction *I); 437 438 /// Return either nullptr or an operand of I such that I will trigger 439 /// undefined behavior if I is executed and that operand has a full-poison 440 /// value (all bits poison). 441 const Value *getGuaranteedNonFullPoisonOp(const Instruction *I); 442 443 /// Return true if this function can prove that if PoisonI is executed 444 /// and yields a full-poison value (all bits poison), then that will 445 /// trigger undefined behavior. 446 /// 447 /// Note that this currently only considers the basic block that is 448 /// the parent of I. 449 bool programUndefinedIfFullPoison(const Instruction *PoisonI); 450 451 /// \brief Specific patterns of select instructions we can match. 452 enum SelectPatternFlavor { 453 SPF_UNKNOWN = 0, 454 SPF_SMIN, /// Signed minimum 455 SPF_UMIN, /// Unsigned minimum 456 SPF_SMAX, /// Signed maximum 457 SPF_UMAX, /// Unsigned maximum 458 SPF_FMINNUM, /// Floating point minnum 459 SPF_FMAXNUM, /// Floating point maxnum 460 SPF_ABS, /// Absolute value 461 SPF_NABS /// Negated absolute value 462 }; 463 /// \brief Behavior when a floating point min/max is given one NaN and one 464 /// non-NaN as input. 465 enum SelectPatternNaNBehavior { 466 SPNB_NA = 0, /// NaN behavior not applicable. 467 SPNB_RETURNS_NAN, /// Given one NaN input, returns the NaN. 468 SPNB_RETURNS_OTHER, /// Given one NaN input, returns the non-NaN. 469 SPNB_RETURNS_ANY /// Given one NaN input, can return either (or 470 /// it has been determined that no operands can 471 /// be NaN). 472 }; 473 struct SelectPatternResult { 474 SelectPatternFlavor Flavor; 475 SelectPatternNaNBehavior NaNBehavior; /// Only applicable if Flavor is 476 /// SPF_FMINNUM or SPF_FMAXNUM. 477 bool Ordered; /// When implementing this min/max pattern as 478 /// fcmp; select, does the fcmp have to be 479 /// ordered? 480 481 /// \brief Return true if \p SPF is a min or a max pattern. 482 static bool isMinOrMax(SelectPatternFlavor SPF) { 483 return !(SPF == SPF_UNKNOWN || SPF == SPF_ABS || SPF == SPF_NABS); 484 } 485 }; 486 /// Pattern match integer [SU]MIN, [SU]MAX and ABS idioms, returning the kind 487 /// and providing the out parameter results if we successfully match. 488 /// 489 /// If CastOp is not nullptr, also match MIN/MAX idioms where the type does 490 /// not match that of the original select. If this is the case, the cast 491 /// operation (one of Trunc,SExt,Zext) that must be done to transform the 492 /// type of LHS and RHS into the type of V is returned in CastOp. 493 /// 494 /// For example: 495 /// %1 = icmp slt i32 %a, i32 4 496 /// %2 = sext i32 %a to i64 497 /// %3 = select i1 %1, i64 %2, i64 4 498 /// 499 /// -> LHS = %a, RHS = i32 4, *CastOp = Instruction::SExt 500 /// 501 SelectPatternResult matchSelectPattern(Value *V, Value *&LHS, Value *&RHS, 502 Instruction::CastOps *CastOp = nullptr); 503 static inline SelectPatternResult 504 matchSelectPattern(const Value *V, const Value *&LHS, const Value *&RHS, 505 Instruction::CastOps *CastOp = nullptr) { 506 Value *L = const_cast<Value*>(LHS); 507 Value *R = const_cast<Value*>(RHS); 508 auto Result = matchSelectPattern(const_cast<Value*>(V), L, R); 509 LHS = L; 510 RHS = R; 511 return Result; 512 } 513 514 /// Return true if RHS is known to be implied true by LHS. Return false if 515 /// RHS is known to be implied false by LHS. Otherwise, return None if no 516 /// implication can be made. 517 /// A & B must be i1 (boolean) values or a vector of such values. Note that 518 /// the truth table for implication is the same as <=u on i1 values (but not 519 /// <=s!). The truth table for both is: 520 /// | T | F (B) 521 /// T | T | F 522 /// F | T | T 523 /// (A) 524 Optional<bool> isImpliedCondition(const Value *LHS, const Value *RHS, 525 const DataLayout &DL, 526 bool InvertAPred = false, 527 unsigned Depth = 0, 528 AssumptionCache *AC = nullptr, 529 const Instruction *CxtI = nullptr, 530 const DominatorTree *DT = nullptr); 531 } // end namespace llvm 532 533 #endif 534
