1 //===- llvm/Analysis/ValueTracking.h - Walk computations --------*- C++ -*-===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file contains routines that help analyze properties that chains of
13 //===----------------------------------------------------------------------===//
15 #ifndef LLVM_ANALYSIS_VALUETRACKING_H
16 #define LLVM_ANALYSIS_VALUETRACKING_H
18 #include "llvm/ADT/ArrayRef.h"
19 #include "llvm/IR/Instruction.h"
20 #include "llvm/Support/DataTypes.h"
25 class AssumptionCache;
33 class TargetLibraryInfo;
36 /// Determine which bits of V are known to be either zero or one and return
37 /// them in the KnownZero/KnownOne bit sets.
39 /// This function is defined on values with integer type, values with pointer
40 /// type, and vectors of integers. In the case
41 /// where V is a vector, the known zero and known one values are the
42 /// same width as the vector element, and the bit is set only if it is true
43 /// for all of the elements in the vector.
44 void computeKnownBits(Value *V, APInt &KnownZero, APInt &KnownOne,
45 const DataLayout &DL, unsigned Depth = 0,
46 AssumptionCache *AC = nullptr,
47 const Instruction *CxtI = nullptr,
48 const DominatorTree *DT = nullptr);
49 /// Compute known bits from the range metadata.
50 /// \p KnownZero the set of bits that are known to be zero
51 void computeKnownBitsFromRangeMetadata(const MDNode &Ranges,
53 /// Return true if LHS and RHS have no common bits set.
54 bool haveNoCommonBitsSet(Value *LHS, Value *RHS, const DataLayout &DL,
55 AssumptionCache *AC = nullptr,
56 const Instruction *CxtI = nullptr,
57 const DominatorTree *DT = nullptr);
59 /// ComputeSignBit - Determine whether the sign bit is known to be zero or
60 /// one. Convenience wrapper around computeKnownBits.
61 void ComputeSignBit(Value *V, bool &KnownZero, bool &KnownOne,
62 const DataLayout &DL, unsigned Depth = 0,
63 AssumptionCache *AC = nullptr,
64 const Instruction *CxtI = nullptr,
65 const DominatorTree *DT = nullptr);
67 /// isKnownToBeAPowerOfTwo - Return true if the given value is known to have
68 /// exactly one bit set when defined. For vectors return true if every
69 /// element is known to be a power of two when defined. Supports values with
70 /// integer or pointer type and vectors of integers. If 'OrZero' is set then
71 /// return true if the given value is either a power of two or zero.
72 bool isKnownToBeAPowerOfTwo(Value *V, const DataLayout &DL,
73 bool OrZero = false, unsigned Depth = 0,
74 AssumptionCache *AC = nullptr,
75 const Instruction *CxtI = nullptr,
76 const DominatorTree *DT = nullptr);
78 /// isKnownNonZero - Return true if the given value is known to be non-zero
79 /// when defined. For vectors return true if every element is known to be
80 /// non-zero when defined. Supports values with integer or pointer type and
81 /// vectors of integers.
82 bool isKnownNonZero(Value *V, const DataLayout &DL, unsigned Depth = 0,
83 AssumptionCache *AC = nullptr,
84 const Instruction *CxtI = nullptr,
85 const DominatorTree *DT = nullptr);
87 /// Returns true if the give value is known to be non-negative.
88 bool isKnownNonNegative(Value *V, const DataLayout &DL, unsigned Depth = 0,
89 AssumptionCache *AC = nullptr,
90 const Instruction *CxtI = nullptr,
91 const DominatorTree *DT = nullptr);
93 /// isKnownNonEqual - Return true if the given values are known to be
94 /// non-equal when defined. Supports scalar integer types only.
95 bool isKnownNonEqual(Value *V1, Value *V2, const DataLayout &DL,
96 AssumptionCache *AC = nullptr,
97 const Instruction *CxtI = nullptr,
98 const DominatorTree *DT = nullptr);
100 /// MaskedValueIsZero - Return true if 'V & Mask' is known to be zero. We use
101 /// this predicate to simplify operations downstream. Mask is known to be
102 /// zero for bits that V cannot have.
104 /// This function is defined on values with integer type, values with pointer
105 /// type, and vectors of integers. In the case
106 /// where V is a vector, the mask, known zero, and known one values are the
107 /// same width as the vector element, and the bit is set only if it is true
108 /// for all of the elements in the vector.
109 bool MaskedValueIsZero(Value *V, const APInt &Mask, const DataLayout &DL,
110 unsigned Depth = 0, AssumptionCache *AC = nullptr,
111 const Instruction *CxtI = nullptr,
112 const DominatorTree *DT = nullptr);
114 /// ComputeNumSignBits - Return the number of times the sign bit of the
115 /// register is replicated into the other bits. We know that at least 1 bit
116 /// is always equal to the sign bit (itself), but other cases can give us
117 /// information. For example, immediately after an "ashr X, 2", we know that
118 /// the top 3 bits are all equal to each other, so we return 3.
120 /// 'Op' must have a scalar integer type.
122 unsigned ComputeNumSignBits(Value *Op, const DataLayout &DL,
123 unsigned Depth = 0, AssumptionCache *AC = nullptr,
124 const Instruction *CxtI = nullptr,
125 const DominatorTree *DT = nullptr);
127 /// ComputeMultiple - This function computes the integer multiple of Base that
128 /// equals V. If successful, it returns true and returns the multiple in
129 /// Multiple. If unsuccessful, it returns false. Also, if V can be
130 /// simplified to an integer, then the simplified V is returned in Val. Look
131 /// through sext only if LookThroughSExt=true.
132 bool ComputeMultiple(Value *V, unsigned Base, Value *&Multiple,
133 bool LookThroughSExt = false,
136 /// CannotBeNegativeZero - Return true if we can prove that the specified FP
137 /// value is never equal to -0.0.
139 bool CannotBeNegativeZero(const Value *V, unsigned Depth = 0);
141 /// CannotBeOrderedLessThanZero - Return true if we can prove that the
142 /// specified FP value is either a NaN or never less than 0.0.
144 bool CannotBeOrderedLessThanZero(const Value *V, unsigned Depth = 0);
146 /// isBytewiseValue - If the specified value can be set by repeating the same
147 /// byte in memory, return the i8 value that it is represented with. This is
148 /// true for all i8 values obviously, but is also true for i32 0, i32 -1,
149 /// i16 0xF0F0, double 0.0 etc. If the value can't be handled with a repeated
150 /// byte store (e.g. i16 0x1234), return null.
151 Value *isBytewiseValue(Value *V);
153 /// FindInsertedValue - Given an aggregrate and an sequence of indices, see if
154 /// the scalar value indexed is already around as a register, for example if
155 /// it were inserted directly into the aggregrate.
157 /// If InsertBefore is not null, this function will duplicate (modified)
158 /// insertvalues when a part of a nested struct is extracted.
159 Value *FindInsertedValue(Value *V,
160 ArrayRef<unsigned> idx_range,
161 Instruction *InsertBefore = nullptr);
163 /// GetPointerBaseWithConstantOffset - Analyze the specified pointer to see if
164 /// it can be expressed as a base pointer plus a constant offset. Return the
165 /// base and offset to the caller.
166 Value *GetPointerBaseWithConstantOffset(Value *Ptr, int64_t &Offset,
167 const DataLayout &DL);
168 static inline const Value *
169 GetPointerBaseWithConstantOffset(const Value *Ptr, int64_t &Offset,
170 const DataLayout &DL) {
171 return GetPointerBaseWithConstantOffset(const_cast<Value *>(Ptr), Offset,
175 /// getConstantStringInfo - This function computes the length of a
176 /// null-terminated C string pointed to by V. If successful, it returns true
177 /// and returns the string in Str. If unsuccessful, it returns false. This
178 /// does not include the trailing nul character by default. If TrimAtNul is
179 /// set to false, then this returns any trailing nul characters as well as any
180 /// other characters that come after it.
181 bool getConstantStringInfo(const Value *V, StringRef &Str,
182 uint64_t Offset = 0, bool TrimAtNul = true);
184 /// GetStringLength - If we can compute the length of the string pointed to by
185 /// the specified pointer, return 'len+1'. If we can't, return 0.
186 uint64_t GetStringLength(Value *V);
188 /// GetUnderlyingObject - This method strips off any GEP address adjustments
189 /// and pointer casts from the specified value, returning the original object
190 /// being addressed. Note that the returned value has pointer type if the
191 /// specified value does. If the MaxLookup value is non-zero, it limits the
192 /// number of instructions to be stripped off.
193 Value *GetUnderlyingObject(Value *V, const DataLayout &DL,
194 unsigned MaxLookup = 6);
195 static inline const Value *GetUnderlyingObject(const Value *V,
196 const DataLayout &DL,
197 unsigned MaxLookup = 6) {
198 return GetUnderlyingObject(const_cast<Value *>(V), DL, MaxLookup);
201 /// \brief This method is similar to GetUnderlyingObject except that it can
202 /// look through phi and select instructions and return multiple objects.
204 /// If LoopInfo is passed, loop phis are further analyzed. If a pointer
205 /// accesses different objects in each iteration, we don't look through the
206 /// phi node. E.g. consider this loop nest:
211 /// A[i][j] = A[i-1][j] * B[j]
214 /// This is transformed by Load-PRE to stash away A[i] for the next iteration
215 /// of the outer loop:
217 /// Curr = A[0]; // Prev_0
219 /// Prev = Curr; // Prev = PHI (Prev_0, Curr)
222 /// Curr[j] = Prev[j] * B[j]
226 /// Since A[i] and A[i-1] are independent pointers, getUnderlyingObjects
227 /// should not assume that Curr and Prev share the same underlying object thus
228 /// it shouldn't look through the phi above.
229 void GetUnderlyingObjects(Value *V, SmallVectorImpl<Value *> &Objects,
230 const DataLayout &DL, LoopInfo *LI = nullptr,
231 unsigned MaxLookup = 6);
233 /// onlyUsedByLifetimeMarkers - Return true if the only users of this pointer
234 /// are lifetime markers.
235 bool onlyUsedByLifetimeMarkers(const Value *V);
237 /// isDereferenceablePointer - Return true if this is always a dereferenceable
238 /// pointer. If the context instruction is specified perform context-sensitive
239 /// analysis and return true if the pointer is dereferenceable at the
240 /// specified instruction.
241 bool isDereferenceablePointer(const Value *V, const DataLayout &DL,
242 const Instruction *CtxI = nullptr,
243 const DominatorTree *DT = nullptr,
244 const TargetLibraryInfo *TLI = nullptr);
246 /// Returns true if V is always a dereferenceable pointer with alignment
247 /// greater or equal than requested. If the context instruction is specified
248 /// performs context-sensitive analysis and returns true if the pointer is
249 /// dereferenceable at the specified instruction.
250 bool isDereferenceableAndAlignedPointer(const Value *V, unsigned Align,
251 const DataLayout &DL,
252 const Instruction *CtxI = nullptr,
253 const DominatorTree *DT = nullptr,
254 const TargetLibraryInfo *TLI = nullptr);
256 /// isSafeToSpeculativelyExecute - Return true if the instruction does not
257 /// have any effects besides calculating the result and does not have
258 /// undefined behavior.
260 /// This method never returns true for an instruction that returns true for
261 /// mayHaveSideEffects; however, this method also does some other checks in
262 /// addition. It checks for undefined behavior, like dividing by zero or
263 /// loading from an invalid pointer (but not for undefined results, like a
264 /// shift with a shift amount larger than the width of the result). It checks
265 /// for malloc and alloca because speculatively executing them might cause a
266 /// memory leak. It also returns false for instructions related to control
267 /// flow, specifically terminators and PHI nodes.
269 /// If the CtxI is specified this method performs context-sensitive analysis
270 /// and returns true if it is safe to execute the instruction immediately
273 /// If the CtxI is NOT specified this method only looks at the instruction
274 /// itself and its operands, so if this method returns true, it is safe to
275 /// move the instruction as long as the correct dominance relationships for
276 /// the operands and users hold.
278 /// This method can return true for instructions that read memory;
279 /// for such instructions, moving them may change the resulting value.
280 bool isSafeToSpeculativelyExecute(const Value *V,
281 const Instruction *CtxI = nullptr,
282 const DominatorTree *DT = nullptr,
283 const TargetLibraryInfo *TLI = nullptr);
285 /// Returns true if the result or effects of the given instructions \p I
286 /// depend on or influence global memory.
287 /// Memory dependence arises for example if the the instruction reads from
288 /// memory or may produce effects or undefined behaviour. Memory dependent
289 /// instructions generally cannot be reorderd with respect to other memory
290 /// dependent instructions or moved into non-dominated basic blocks.
291 /// Instructions which just compute a value based on the values of their
292 /// operands are not memory dependent.
293 bool mayBeMemoryDependent(const Instruction &I);
295 /// isKnownNonNull - Return true if this pointer couldn't possibly be null by
296 /// its definition. This returns true for allocas, non-extern-weak globals
297 /// and byval arguments.
298 bool isKnownNonNull(const Value *V, const TargetLibraryInfo *TLI = nullptr);
300 /// isKnownNonNullAt - Return true if this pointer couldn't possibly be null.
301 /// If the context instruction is specified perform context-sensitive analysis
302 /// and return true if the pointer couldn't possibly be null at the specified
304 bool isKnownNonNullAt(const Value *V,
305 const Instruction *CtxI = nullptr,
306 const DominatorTree *DT = nullptr,
307 const TargetLibraryInfo *TLI = nullptr);
309 /// Return true if it is valid to use the assumptions provided by an
310 /// assume intrinsic, I, at the point in the control-flow identified by the
311 /// context instruction, CxtI.
312 bool isValidAssumeForContext(const Instruction *I, const Instruction *CxtI,
313 const DominatorTree *DT = nullptr);
315 enum class OverflowResult { AlwaysOverflows, MayOverflow, NeverOverflows };
316 OverflowResult computeOverflowForUnsignedMul(Value *LHS, Value *RHS,
317 const DataLayout &DL,
319 const Instruction *CxtI,
320 const DominatorTree *DT);
321 OverflowResult computeOverflowForUnsignedAdd(Value *LHS, Value *RHS,
322 const DataLayout &DL,
324 const Instruction *CxtI,
325 const DominatorTree *DT);
326 OverflowResult computeOverflowForSignedAdd(Value *LHS, Value *RHS,
327 const DataLayout &DL,
328 AssumptionCache *AC = nullptr,
329 const Instruction *CxtI = nullptr,
330 const DominatorTree *DT = nullptr);
331 /// This version also leverages the sign bit of Add if known.
332 OverflowResult computeOverflowForSignedAdd(AddOperator *Add,
333 const DataLayout &DL,
334 AssumptionCache *AC = nullptr,
335 const Instruction *CxtI = nullptr,
336 const DominatorTree *DT = nullptr);
338 /// Return true if this function can prove that the instruction I will
339 /// always transfer execution to one of its successors (including the next
340 /// instruction that follows within a basic block). E.g. this is not
341 /// guaranteed for function calls that could loop infinitely.
343 /// In other words, this function returns false for instructions that may
344 /// transfer execution or fail to transfer execution in a way that is not
345 /// captured in the CFG nor in the sequence of instructions within a basic
348 /// Undefined behavior is assumed not to happen, so e.g. division is
349 /// guaranteed to transfer execution to the following instruction even
350 /// though division by zero might cause undefined behavior.
351 bool isGuaranteedToTransferExecutionToSuccessor(const Instruction *I);
353 /// Return true if this function can prove that the instruction I
354 /// is executed for every iteration of the loop L.
356 /// Note that this currently only considers the loop header.
357 bool isGuaranteedToExecuteForEveryIteration(const Instruction *I,
360 /// Return true if this function can prove that I is guaranteed to yield
361 /// full-poison (all bits poison) if at least one of its operands are
362 /// full-poison (all bits poison).
364 /// The exact rules for how poison propagates through instructions have
365 /// not been settled as of 2015-07-10, so this function is conservative
366 /// and only considers poison to be propagated in uncontroversial
367 /// cases. There is no attempt to track values that may be only partially
369 bool propagatesFullPoison(const Instruction *I);
371 /// Return either nullptr or an operand of I such that I will trigger
372 /// undefined behavior if I is executed and that operand has a full-poison
373 /// value (all bits poison).
374 const Value *getGuaranteedNonFullPoisonOp(const Instruction *I);
376 /// Return true if this function can prove that if PoisonI is executed
377 /// and yields a full-poison value (all bits poison), then that will
378 /// trigger undefined behavior.
380 /// Note that this currently only considers the basic block that is
382 bool isKnownNotFullPoison(const Instruction *PoisonI);
384 /// \brief Specific patterns of select instructions we can match.
385 enum SelectPatternFlavor {
387 SPF_SMIN, /// Signed minimum
388 SPF_UMIN, /// Unsigned minimum
389 SPF_SMAX, /// Signed maximum
390 SPF_UMAX, /// Unsigned maximum
391 SPF_FMINNUM, /// Floating point minnum
392 SPF_FMAXNUM, /// Floating point maxnum
393 SPF_ABS, /// Absolute value
394 SPF_NABS /// Negated absolute value
396 /// \brief Behavior when a floating point min/max is given one NaN and one
397 /// non-NaN as input.
398 enum SelectPatternNaNBehavior {
399 SPNB_NA = 0, /// NaN behavior not applicable.
400 SPNB_RETURNS_NAN, /// Given one NaN input, returns the NaN.
401 SPNB_RETURNS_OTHER, /// Given one NaN input, returns the non-NaN.
402 SPNB_RETURNS_ANY /// Given one NaN input, can return either (or
403 /// it has been determined that no operands can
406 struct SelectPatternResult {
407 SelectPatternFlavor Flavor;
408 SelectPatternNaNBehavior NaNBehavior; /// Only applicable if Flavor is
409 /// SPF_FMINNUM or SPF_FMAXNUM.
410 bool Ordered; /// When implementing this min/max pattern as
411 /// fcmp; select, does the fcmp have to be
414 /// Pattern match integer [SU]MIN, [SU]MAX and ABS idioms, returning the kind
415 /// and providing the out parameter results if we successfully match.
417 /// If CastOp is not nullptr, also match MIN/MAX idioms where the type does
418 /// not match that of the original select. If this is the case, the cast
419 /// operation (one of Trunc,SExt,Zext) that must be done to transform the
420 /// type of LHS and RHS into the type of V is returned in CastOp.
423 /// %1 = icmp slt i32 %a, i32 4
424 /// %2 = sext i32 %a to i64
425 /// %3 = select i1 %1, i64 %2, i64 4
427 /// -> LHS = %a, RHS = i32 4, *CastOp = Instruction::SExt
429 SelectPatternResult matchSelectPattern(Value *V, Value *&LHS, Value *&RHS,
430 Instruction::CastOps *CastOp = nullptr);
432 } // end namespace llvm