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/Support/DataTypes.h"
28 class AssumptionCache;
30 class TargetLibraryInfo;
33 /// Determine which bits of V are known to be either zero or one and return
34 /// them in the KnownZero/KnownOne bit sets.
36 /// This function is defined on values with integer type, values with pointer
37 /// type, and vectors of integers. In the case
38 /// where V is a vector, the known zero and known one values are the
39 /// same width as the vector element, and the bit is set only if it is true
40 /// for all of the elements in the vector.
41 void computeKnownBits(Value *V, APInt &KnownZero, APInt &KnownOne,
42 const DataLayout &DL, unsigned Depth = 0,
43 AssumptionCache *AC = nullptr,
44 const Instruction *CxtI = nullptr,
45 const DominatorTree *DT = nullptr);
46 /// Compute known bits from the range metadata.
47 /// \p KnownZero the set of bits that are known to be zero
48 void computeKnownBitsFromRangeMetadata(const MDNode &Ranges,
51 /// ComputeSignBit - Determine whether the sign bit is known to be zero or
52 /// one. Convenience wrapper around computeKnownBits.
53 void ComputeSignBit(Value *V, bool &KnownZero, bool &KnownOne,
54 const DataLayout &DL, unsigned Depth = 0,
55 AssumptionCache *AC = nullptr,
56 const Instruction *CxtI = nullptr,
57 const DominatorTree *DT = nullptr);
59 /// isKnownToBeAPowerOfTwo - Return true if the given value is known to have
60 /// exactly one bit set when defined. For vectors return true if every
61 /// element is known to be a power of two when defined. Supports values with
62 /// integer or pointer type and vectors of integers. If 'OrZero' is set then
63 /// returns true if the given value is either a power of two or zero.
64 bool isKnownToBeAPowerOfTwo(Value *V, const DataLayout &DL,
65 bool OrZero = false, unsigned Depth = 0,
66 AssumptionCache *AC = nullptr,
67 const Instruction *CxtI = nullptr,
68 const DominatorTree *DT = nullptr);
70 /// isKnownNonZero - Return true if the given value is known to be non-zero
71 /// when defined. For vectors return true if every element is known to be
72 /// non-zero when defined. Supports values with integer or pointer type and
73 /// vectors of integers.
74 bool isKnownNonZero(Value *V, const DataLayout &DL, unsigned Depth = 0,
75 AssumptionCache *AC = nullptr,
76 const Instruction *CxtI = nullptr,
77 const DominatorTree *DT = nullptr);
79 /// MaskedValueIsZero - Return true if 'V & Mask' is known to be zero. We use
80 /// this predicate to simplify operations downstream. Mask is known to be
81 /// zero for bits that V cannot have.
83 /// This function is defined on values with integer type, values with pointer
84 /// type, and vectors of integers. In the case
85 /// where V is a vector, the mask, known zero, and known one values are the
86 /// same width as the vector element, and the bit is set only if it is true
87 /// for all of the elements in the vector.
88 bool MaskedValueIsZero(Value *V, const APInt &Mask, const DataLayout &DL,
89 unsigned Depth = 0, AssumptionCache *AC = nullptr,
90 const Instruction *CxtI = nullptr,
91 const DominatorTree *DT = nullptr);
93 /// ComputeNumSignBits - Return the number of times the sign bit of the
94 /// register is replicated into the other bits. We know that at least 1 bit
95 /// is always equal to the sign bit (itself), but other cases can give us
96 /// information. For example, immediately after an "ashr X, 2", we know that
97 /// the top 3 bits are all equal to each other, so we return 3.
99 /// 'Op' must have a scalar integer type.
101 unsigned ComputeNumSignBits(Value *Op, const DataLayout &DL,
102 unsigned Depth = 0, AssumptionCache *AC = nullptr,
103 const Instruction *CxtI = nullptr,
104 const DominatorTree *DT = nullptr);
106 /// ComputeMultiple - This function computes the integer multiple of Base that
107 /// equals V. If successful, it returns true and returns the multiple in
108 /// Multiple. If unsuccessful, it returns false. Also, if V can be
109 /// simplified to an integer, then the simplified V is returned in Val. Look
110 /// through sext only if LookThroughSExt=true.
111 bool ComputeMultiple(Value *V, unsigned Base, Value *&Multiple,
112 bool LookThroughSExt = false,
115 /// CannotBeNegativeZero - Return true if we can prove that the specified FP
116 /// value is never equal to -0.0.
118 bool CannotBeNegativeZero(const Value *V, unsigned Depth = 0);
120 /// CannotBeOrderedLessThanZero - Return true if we can prove that the
121 /// specified FP value is either a NaN or never less than 0.0.
123 bool CannotBeOrderedLessThanZero(const Value *V, unsigned Depth = 0);
125 /// isBytewiseValue - If the specified value can be set by repeating the same
126 /// byte in memory, return the i8 value that it is represented with. This is
127 /// true for all i8 values obviously, but is also true for i32 0, i32 -1,
128 /// i16 0xF0F0, double 0.0 etc. If the value can't be handled with a repeated
129 /// byte store (e.g. i16 0x1234), return null.
130 Value *isBytewiseValue(Value *V);
132 /// FindInsertedValue - Given an aggregrate and an sequence of indices, see if
133 /// the scalar value indexed is already around as a register, for example if
134 /// it were inserted directly into the aggregrate.
136 /// If InsertBefore is not null, this function will duplicate (modified)
137 /// insertvalues when a part of a nested struct is extracted.
138 Value *FindInsertedValue(Value *V,
139 ArrayRef<unsigned> idx_range,
140 Instruction *InsertBefore = nullptr);
142 /// GetPointerBaseWithConstantOffset - Analyze the specified pointer to see if
143 /// it can be expressed as a base pointer plus a constant offset. Return the
144 /// base and offset to the caller.
145 Value *GetPointerBaseWithConstantOffset(Value *Ptr, int64_t &Offset,
146 const DataLayout &DL);
147 static inline const Value *
148 GetPointerBaseWithConstantOffset(const Value *Ptr, int64_t &Offset,
149 const DataLayout &DL) {
150 return GetPointerBaseWithConstantOffset(const_cast<Value *>(Ptr), Offset,
154 /// getConstantStringInfo - This function computes the length of a
155 /// null-terminated C string pointed to by V. If successful, it returns true
156 /// and returns the string in Str. If unsuccessful, it returns false. This
157 /// does not include the trailing nul character by default. If TrimAtNul is
158 /// set to false, then this returns any trailing nul characters as well as any
159 /// other characters that come after it.
160 bool getConstantStringInfo(const Value *V, StringRef &Str,
161 uint64_t Offset = 0, bool TrimAtNul = true);
163 /// GetStringLength - If we can compute the length of the string pointed to by
164 /// the specified pointer, return 'len+1'. If we can't, return 0.
165 uint64_t GetStringLength(Value *V);
167 /// GetUnderlyingObject - This method strips off any GEP address adjustments
168 /// and pointer casts from the specified value, returning the original object
169 /// being addressed. Note that the returned value has pointer type if the
170 /// specified value does. If the MaxLookup value is non-zero, it limits the
171 /// number of instructions to be stripped off.
172 Value *GetUnderlyingObject(Value *V, const DataLayout &DL,
173 unsigned MaxLookup = 6);
174 static inline const Value *GetUnderlyingObject(const Value *V,
175 const DataLayout &DL,
176 unsigned MaxLookup = 6) {
177 return GetUnderlyingObject(const_cast<Value *>(V), DL, MaxLookup);
180 /// \brief This method is similar to GetUnderlyingObject except that it can
181 /// look through phi and select instructions and return multiple objects.
183 /// If LoopInfo is passed, loop phis are further analyzed. If a pointer
184 /// accesses different objects in each iteration, we don't look through the
185 /// phi node. E.g. consider this loop nest:
190 /// A[i][j] = A[i-1][j] * B[j]
193 /// This is transformed by Load-PRE to stash away A[i] for the next iteration
194 /// of the outer loop:
196 /// Curr = A[0]; // Prev_0
198 /// Prev = Curr; // Prev = PHI (Prev_0, Curr)
201 /// Curr[j] = Prev[j] * B[j]
205 /// Since A[i] and A[i-1] are independent pointers, getUnderlyingObjects
206 /// should not assume that Curr and Prev share the same underlying object thus
207 /// it shouldn't look through the phi above.
208 void GetUnderlyingObjects(Value *V, SmallVectorImpl<Value *> &Objects,
209 const DataLayout &DL, LoopInfo *LI = nullptr,
210 unsigned MaxLookup = 6);
212 /// onlyUsedByLifetimeMarkers - Return true if the only users of this pointer
213 /// are lifetime markers.
214 bool onlyUsedByLifetimeMarkers(const Value *V);
216 /// isDereferenceablePointer - Return true if this is always a dereferenceable
219 /// Test if this value is always a pointer to allocated and suitably aligned
220 /// memory for a simple load or store.
221 bool isDereferenceablePointer(const Value *V, const DataLayout &DL);
223 /// isSafeToSpeculativelyExecute - Return true if the instruction does not
224 /// have any effects besides calculating the result and does not have
225 /// undefined behavior.
227 /// This method never returns true for an instruction that returns true for
228 /// mayHaveSideEffects; however, this method also does some other checks in
229 /// addition. It checks for undefined behavior, like dividing by zero or
230 /// loading from an invalid pointer (but not for undefined results, like a
231 /// shift with a shift amount larger than the width of the result). It checks
232 /// for malloc and alloca because speculatively executing them might cause a
233 /// memory leak. It also returns false for instructions related to control
234 /// flow, specifically terminators and PHI nodes.
236 /// This method only looks at the instruction itself and its operands, so if
237 /// this method returns true, it is safe to move the instruction as long as
238 /// the correct dominance relationships for the operands and users hold.
239 /// However, this method can return true for instructions that read memory;
240 /// for such instructions, moving them may change the resulting value.
241 bool isSafeToSpeculativelyExecute(const Value *V);
243 /// isKnownNonNull - Return true if this pointer couldn't possibly be null by
244 /// its definition. This returns true for allocas, non-extern-weak globals
245 /// and byval arguments.
246 bool isKnownNonNull(const Value *V, const TargetLibraryInfo *TLI = nullptr);
248 /// Return true if it is valid to use the assumptions provided by an
249 /// assume intrinsic, I, at the point in the control-flow identified by the
250 /// context instruction, CxtI.
251 bool isValidAssumeForContext(const Instruction *I, const Instruction *CxtI,
252 const DominatorTree *DT = nullptr);
254 enum class OverflowResult { AlwaysOverflows, MayOverflow, NeverOverflows };
255 OverflowResult computeOverflowForUnsignedMul(Value *LHS, Value *RHS,
256 const DataLayout &DL,
258 const Instruction *CxtI,
259 const DominatorTree *DT);
260 OverflowResult computeOverflowForUnsignedAdd(Value *LHS, Value *RHS,
261 const DataLayout &DL,
263 const Instruction *CxtI,
264 const DominatorTree *DT);
266 /// \brief Specific patterns of select instructions we can match.
267 enum SelectPatternFlavor {
269 SPF_SMIN, // Signed minimum
270 SPF_UMIN, // Unsigned minimum
271 SPF_SMAX, // Signed maximum
272 SPF_UMAX, // Unsigned maximum
273 SPF_ABS, // Absolute value
274 SPF_NABS // Negated absolute value
276 /// Pattern match integer [SU]MIN, [SU]MAX and ABS idioms, returning the kind
277 /// and providing the out parameter results if we successfully match.
278 SelectPatternFlavor matchSelectPattern(Value *V, Value *&LHS, Value *&RHS);
280 } // end namespace llvm