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 /// MaskedValueIsZero - Return true if 'V & Mask' is known to be zero. We use
94 /// this predicate to simplify operations downstream. Mask is known to be
95 /// zero for bits that V cannot have.
97 /// This function is defined on values with integer type, values with pointer
98 /// type, and vectors of integers. In the case
99 /// where V is a vector, the mask, known zero, and known one values are the
100 /// same width as the vector element, and the bit is set only if it is true
101 /// for all of the elements in the vector.
102 bool MaskedValueIsZero(Value *V, const APInt &Mask, const DataLayout &DL,
103 unsigned Depth = 0, AssumptionCache *AC = nullptr,
104 const Instruction *CxtI = nullptr,
105 const DominatorTree *DT = nullptr);
107 /// ComputeNumSignBits - Return the number of times the sign bit of the
108 /// register is replicated into the other bits. We know that at least 1 bit
109 /// is always equal to the sign bit (itself), but other cases can give us
110 /// information. For example, immediately after an "ashr X, 2", we know that
111 /// the top 3 bits are all equal to each other, so we return 3.
113 /// 'Op' must have a scalar integer type.
115 unsigned ComputeNumSignBits(Value *Op, const DataLayout &DL,
116 unsigned Depth = 0, AssumptionCache *AC = nullptr,
117 const Instruction *CxtI = nullptr,
118 const DominatorTree *DT = nullptr);
120 /// ComputeMultiple - This function computes the integer multiple of Base that
121 /// equals V. If successful, it returns true and returns the multiple in
122 /// Multiple. If unsuccessful, it returns false. Also, if V can be
123 /// simplified to an integer, then the simplified V is returned in Val. Look
124 /// through sext only if LookThroughSExt=true.
125 bool ComputeMultiple(Value *V, unsigned Base, Value *&Multiple,
126 bool LookThroughSExt = false,
129 /// CannotBeNegativeZero - Return true if we can prove that the specified FP
130 /// value is never equal to -0.0.
132 bool CannotBeNegativeZero(const Value *V, unsigned Depth = 0);
134 /// CannotBeOrderedLessThanZero - Return true if we can prove that the
135 /// specified FP value is either a NaN or never less than 0.0.
137 bool CannotBeOrderedLessThanZero(const Value *V, unsigned Depth = 0);
139 /// isBytewiseValue - If the specified value can be set by repeating the same
140 /// byte in memory, return the i8 value that it is represented with. This is
141 /// true for all i8 values obviously, but is also true for i32 0, i32 -1,
142 /// i16 0xF0F0, double 0.0 etc. If the value can't be handled with a repeated
143 /// byte store (e.g. i16 0x1234), return null.
144 Value *isBytewiseValue(Value *V);
146 /// FindInsertedValue - Given an aggregrate and an sequence of indices, see if
147 /// the scalar value indexed is already around as a register, for example if
148 /// it were inserted directly into the aggregrate.
150 /// If InsertBefore is not null, this function will duplicate (modified)
151 /// insertvalues when a part of a nested struct is extracted.
152 Value *FindInsertedValue(Value *V,
153 ArrayRef<unsigned> idx_range,
154 Instruction *InsertBefore = nullptr);
156 /// GetPointerBaseWithConstantOffset - Analyze the specified pointer to see if
157 /// it can be expressed as a base pointer plus a constant offset. Return the
158 /// base and offset to the caller.
159 Value *GetPointerBaseWithConstantOffset(Value *Ptr, int64_t &Offset,
160 const DataLayout &DL);
161 static inline const Value *
162 GetPointerBaseWithConstantOffset(const Value *Ptr, int64_t &Offset,
163 const DataLayout &DL) {
164 return GetPointerBaseWithConstantOffset(const_cast<Value *>(Ptr), Offset,
168 /// getConstantStringInfo - This function computes the length of a
169 /// null-terminated C string pointed to by V. If successful, it returns true
170 /// and returns the string in Str. If unsuccessful, it returns false. This
171 /// does not include the trailing nul character by default. If TrimAtNul is
172 /// set to false, then this returns any trailing nul characters as well as any
173 /// other characters that come after it.
174 bool getConstantStringInfo(const Value *V, StringRef &Str,
175 uint64_t Offset = 0, bool TrimAtNul = true);
177 /// GetStringLength - If we can compute the length of the string pointed to by
178 /// the specified pointer, return 'len+1'. If we can't, return 0.
179 uint64_t GetStringLength(Value *V);
181 /// GetUnderlyingObject - This method strips off any GEP address adjustments
182 /// and pointer casts from the specified value, returning the original object
183 /// being addressed. Note that the returned value has pointer type if the
184 /// specified value does. If the MaxLookup value is non-zero, it limits the
185 /// number of instructions to be stripped off.
186 Value *GetUnderlyingObject(Value *V, const DataLayout &DL,
187 unsigned MaxLookup = 6);
188 static inline const Value *GetUnderlyingObject(const Value *V,
189 const DataLayout &DL,
190 unsigned MaxLookup = 6) {
191 return GetUnderlyingObject(const_cast<Value *>(V), DL, MaxLookup);
194 /// \brief This method is similar to GetUnderlyingObject except that it can
195 /// look through phi and select instructions and return multiple objects.
197 /// If LoopInfo is passed, loop phis are further analyzed. If a pointer
198 /// accesses different objects in each iteration, we don't look through the
199 /// phi node. E.g. consider this loop nest:
204 /// A[i][j] = A[i-1][j] * B[j]
207 /// This is transformed by Load-PRE to stash away A[i] for the next iteration
208 /// of the outer loop:
210 /// Curr = A[0]; // Prev_0
212 /// Prev = Curr; // Prev = PHI (Prev_0, Curr)
215 /// Curr[j] = Prev[j] * B[j]
219 /// Since A[i] and A[i-1] are independent pointers, getUnderlyingObjects
220 /// should not assume that Curr and Prev share the same underlying object thus
221 /// it shouldn't look through the phi above.
222 void GetUnderlyingObjects(Value *V, SmallVectorImpl<Value *> &Objects,
223 const DataLayout &DL, LoopInfo *LI = nullptr,
224 unsigned MaxLookup = 6);
226 /// onlyUsedByLifetimeMarkers - Return true if the only users of this pointer
227 /// are lifetime markers.
228 bool onlyUsedByLifetimeMarkers(const Value *V);
230 /// isDereferenceablePointer - Return true if this is always a dereferenceable
231 /// pointer. If the context instruction is specified perform context-sensitive
232 /// analysis and return true if the pointer is dereferenceable at the
233 /// specified instruction.
234 bool isDereferenceablePointer(const Value *V, const DataLayout &DL,
235 const Instruction *CtxI = nullptr,
236 const DominatorTree *DT = nullptr,
237 const TargetLibraryInfo *TLI = nullptr);
239 /// Returns true if V is always a dereferenceable pointer with alignment
240 /// greater or equal than requested. If the context instruction is specified
241 /// performs context-sensitive analysis and returns true if the pointer is
242 /// dereferenceable at the specified instruction.
243 bool isDereferenceableAndAlignedPointer(const Value *V, unsigned Align,
244 const DataLayout &DL,
245 const Instruction *CtxI = nullptr,
246 const DominatorTree *DT = nullptr,
247 const TargetLibraryInfo *TLI = nullptr);
249 /// isSafeToSpeculativelyExecute - Return true if the instruction does not
250 /// have any effects besides calculating the result and does not have
251 /// undefined behavior.
253 /// This method never returns true for an instruction that returns true for
254 /// mayHaveSideEffects; however, this method also does some other checks in
255 /// addition. It checks for undefined behavior, like dividing by zero or
256 /// loading from an invalid pointer (but not for undefined results, like a
257 /// shift with a shift amount larger than the width of the result). It checks
258 /// for malloc and alloca because speculatively executing them might cause a
259 /// memory leak. It also returns false for instructions related to control
260 /// flow, specifically terminators and PHI nodes.
262 /// If the CtxI is specified this method performs context-sensitive analysis
263 /// and returns true if it is safe to execute the instruction immediately
266 /// If the CtxI is NOT specified this method only looks at the instruction
267 /// itself and its operands, so if this method returns true, it is safe to
268 /// move the instruction as long as the correct dominance relationships for
269 /// the operands and users hold.
271 /// This method can return true for instructions that read memory;
272 /// for such instructions, moving them may change the resulting value.
273 bool isSafeToSpeculativelyExecute(const Value *V,
274 const Instruction *CtxI = nullptr,
275 const DominatorTree *DT = nullptr,
276 const TargetLibraryInfo *TLI = nullptr);
278 /// Returns true if the result or effects of the given instructions \p I
279 /// depend on or influence global memory.
280 /// Memory dependence arises for example if the the instruction reads from
281 /// memory or may produce effects or undefined behaviour. Memory dependent
282 /// instructions generally cannot be reorderd with respect to other memory
283 /// dependent instructions or moved into non-dominated basic blocks.
284 /// Instructions which just compute a value based on the values of their
285 /// operands are not memory dependent.
286 bool mayBeMemoryDependent(const Instruction &I);
288 /// isKnownNonNull - Return true if this pointer couldn't possibly be null by
289 /// its definition. This returns true for allocas, non-extern-weak globals
290 /// and byval arguments.
291 bool isKnownNonNull(const Value *V, const TargetLibraryInfo *TLI = nullptr);
293 /// isKnownNonNullAt - Return true if this pointer couldn't possibly be null.
294 /// If the context instruction is specified perform context-sensitive analysis
295 /// and return true if the pointer couldn't possibly be null at the specified
297 bool isKnownNonNullAt(const Value *V,
298 const Instruction *CtxI = nullptr,
299 const DominatorTree *DT = nullptr,
300 const TargetLibraryInfo *TLI = nullptr);
302 /// Return true if it is valid to use the assumptions provided by an
303 /// assume intrinsic, I, at the point in the control-flow identified by the
304 /// context instruction, CxtI.
305 bool isValidAssumeForContext(const Instruction *I, const Instruction *CxtI,
306 const DominatorTree *DT = nullptr);
308 enum class OverflowResult { AlwaysOverflows, MayOverflow, NeverOverflows };
309 OverflowResult computeOverflowForUnsignedMul(Value *LHS, Value *RHS,
310 const DataLayout &DL,
312 const Instruction *CxtI,
313 const DominatorTree *DT);
314 OverflowResult computeOverflowForUnsignedAdd(Value *LHS, Value *RHS,
315 const DataLayout &DL,
317 const Instruction *CxtI,
318 const DominatorTree *DT);
319 OverflowResult computeOverflowForSignedAdd(Value *LHS, Value *RHS,
320 const DataLayout &DL,
321 AssumptionCache *AC = nullptr,
322 const Instruction *CxtI = nullptr,
323 const DominatorTree *DT = nullptr);
324 /// This version also leverages the sign bit of Add if known.
325 OverflowResult computeOverflowForSignedAdd(AddOperator *Add,
326 const DataLayout &DL,
327 AssumptionCache *AC = nullptr,
328 const Instruction *CxtI = nullptr,
329 const DominatorTree *DT = nullptr);
331 /// Return true if this function can prove that the instruction I will
332 /// always transfer execution to one of its successors (including the next
333 /// instruction that follows within a basic block). E.g. this is not
334 /// guaranteed for function calls that could loop infinitely.
336 /// In other words, this function returns false for instructions that may
337 /// transfer execution or fail to transfer execution in a way that is not
338 /// captured in the CFG nor in the sequence of instructions within a basic
341 /// Undefined behavior is assumed not to happen, so e.g. division is
342 /// guaranteed to transfer execution to the following instruction even
343 /// though division by zero might cause undefined behavior.
344 bool isGuaranteedToTransferExecutionToSuccessor(const Instruction *I);
346 /// Return true if this function can prove that the instruction I
347 /// is executed for every iteration of the loop L.
349 /// Note that this currently only considers the loop header.
350 bool isGuaranteedToExecuteForEveryIteration(const Instruction *I,
353 /// Return true if this function can prove that I is guaranteed to yield
354 /// full-poison (all bits poison) if at least one of its operands are
355 /// full-poison (all bits poison).
357 /// The exact rules for how poison propagates through instructions have
358 /// not been settled as of 2015-07-10, so this function is conservative
359 /// and only considers poison to be propagated in uncontroversial
360 /// cases. There is no attempt to track values that may be only partially
362 bool propagatesFullPoison(const Instruction *I);
364 /// Return either nullptr or an operand of I such that I will trigger
365 /// undefined behavior if I is executed and that operand has a full-poison
366 /// value (all bits poison).
367 const Value *getGuaranteedNonFullPoisonOp(const Instruction *I);
369 /// Return true if this function can prove that if PoisonI is executed
370 /// and yields a full-poison value (all bits poison), then that will
371 /// trigger undefined behavior.
373 /// Note that this currently only considers the basic block that is
375 bool isKnownNotFullPoison(const Instruction *PoisonI);
377 /// \brief Specific patterns of select instructions we can match.
378 enum SelectPatternFlavor {
380 SPF_SMIN, /// Signed minimum
381 SPF_UMIN, /// Unsigned minimum
382 SPF_SMAX, /// Signed maximum
383 SPF_UMAX, /// Unsigned maximum
384 SPF_FMINNUM, /// Floating point minnum
385 SPF_FMAXNUM, /// Floating point maxnum
386 SPF_ABS, /// Absolute value
387 SPF_NABS /// Negated absolute value
389 /// \brief Behavior when a floating point min/max is given one NaN and one
390 /// non-NaN as input.
391 enum SelectPatternNaNBehavior {
392 SPNB_NA = 0, /// NaN behavior not applicable.
393 SPNB_RETURNS_NAN, /// Given one NaN input, returns the NaN.
394 SPNB_RETURNS_OTHER, /// Given one NaN input, returns the non-NaN.
395 SPNB_RETURNS_ANY /// Given one NaN input, can return either (or
396 /// it has been determined that no operands can
399 struct SelectPatternResult {
400 SelectPatternFlavor Flavor;
401 SelectPatternNaNBehavior NaNBehavior; /// Only applicable if Flavor is
402 /// SPF_FMINNUM or SPF_FMAXNUM.
403 bool Ordered; /// When implementing this min/max pattern as
404 /// fcmp; select, does the fcmp have to be
407 /// Pattern match integer [SU]MIN, [SU]MAX and ABS idioms, returning the kind
408 /// and providing the out parameter results if we successfully match.
410 /// If CastOp is not nullptr, also match MIN/MAX idioms where the type does
411 /// not match that of the original select. If this is the case, the cast
412 /// operation (one of Trunc,SExt,Zext) that must be done to transform the
413 /// type of LHS and RHS into the type of V is returned in CastOp.
416 /// %1 = icmp slt i32 %a, i32 4
417 /// %2 = sext i32 %a to i64
418 /// %3 = select i1 %1, i64 %2, i64 4
420 /// -> LHS = %a, RHS = i32 4, *CastOp = Instruction::SExt
422 SelectPatternResult matchSelectPattern(Value *V, Value *&LHS, Value *&RHS,
423 Instruction::CastOps *CastOp = nullptr);
425 } // end namespace llvm