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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/ConstantRange.h"
20 #include "llvm/IR/Instruction.h"
21 #include "llvm/IR/IntrinsicInst.h"
22 #include "llvm/Support/DataTypes.h"
23 
24 namespace llvm {
25 template <typename T> class ArrayRef;
26   class APInt;
27   class AddOperator;
28   class AssumptionCache;
29   class DataLayout;
30   class DominatorTree;
31   class GEPOperator;
32   class Instruction;
33   class Loop;
34   class LoopInfo;
35   class MDNode;
36   class StringRef;
37   class TargetLibraryInfo;
38   class Value;
39 
40   namespace Intrinsic {
41   enum ID : unsigned;
42   }
43 
44   /// Determine which bits of V are known to be either zero or one and return
45   /// them in the KnownZero/KnownOne bit sets.
46   ///
47   /// This function is defined on values with integer type, values with pointer
48   /// type, and vectors of integers.  In the case
49   /// where V is a vector, the known zero and known one values are the
50   /// same width as the vector element, and the bit is set only if it is true
51   /// for all of the elements in the vector.
52   void computeKnownBits(Value *V, APInt &KnownZero, APInt &KnownOne,
53                         const DataLayout &DL, unsigned Depth = 0,
54                         AssumptionCache *AC = nullptr,
55                         const Instruction *CxtI = nullptr,
56                         const DominatorTree *DT = nullptr);
57   /// Compute known bits from the range metadata.
58   /// \p KnownZero the set of bits that are known to be zero
59   /// \p KnownOne the set of bits that are known to be one
60   void computeKnownBitsFromRangeMetadata(const MDNode &Ranges,
61                                          APInt &KnownZero, APInt &KnownOne);
62   /// Return true if LHS and RHS have no common bits set.
63   bool haveNoCommonBitsSet(Value *LHS, Value *RHS, const DataLayout &DL,
64                            AssumptionCache *AC = nullptr,
65                            const Instruction *CxtI = nullptr,
66                            const DominatorTree *DT = nullptr);
67 
68   /// Determine whether the sign bit is known to be zero or one. Convenience
69   /// wrapper around computeKnownBits.
70   void ComputeSignBit(Value *V, bool &KnownZero, bool &KnownOne,
71                       const DataLayout &DL, unsigned Depth = 0,
72                       AssumptionCache *AC = nullptr,
73                       const Instruction *CxtI = nullptr,
74                       const DominatorTree *DT = nullptr);
75 
76   /// Return true if the given value is known to have exactly one bit set when
77   /// defined. For vectors return true if every element is known to be a power
78   /// of two when defined. Supports values with integer or pointer type and
79   /// vectors of integers. If 'OrZero' is set, then return true if the given
80   /// value is either a power of two or zero.
81   bool isKnownToBeAPowerOfTwo(Value *V, const DataLayout &DL,
82                               bool OrZero = false, unsigned Depth = 0,
83                               AssumptionCache *AC = nullptr,
84                               const Instruction *CxtI = nullptr,
85                               const DominatorTree *DT = nullptr);
86 
87   /// Return true if the given value is known to be non-zero when defined. For
88   /// vectors, return true if every element is known to be non-zero when
89   /// defined. Supports values with integer or pointer type and vectors of
90   /// integers.
91   bool isKnownNonZero(Value *V, const DataLayout &DL, unsigned Depth = 0,
92                       AssumptionCache *AC = nullptr,
93                       const Instruction *CxtI = nullptr,
94                       const DominatorTree *DT = nullptr);
95 
96   /// Returns true if the give value is known to be non-negative.
97   bool isKnownNonNegative(Value *V, const DataLayout &DL, unsigned Depth = 0,
98                           AssumptionCache *AC = nullptr,
99                           const Instruction *CxtI = nullptr,
100                           const DominatorTree *DT = nullptr);
101 
102   /// Returns true if the given value is known be positive (i.e. non-negative
103   /// and non-zero).
104   bool isKnownPositive(Value *V, const DataLayout &DL, unsigned Depth = 0,
105                        AssumptionCache *AC = nullptr,
106                        const Instruction *CxtI = nullptr,
107                        const DominatorTree *DT = nullptr);
108 
109   /// Returns true if the given value is known be negative (i.e. non-positive
110   /// and non-zero).
111   bool isKnownNegative(Value *V, const DataLayout &DL, unsigned Depth = 0,
112                        AssumptionCache *AC = nullptr,
113                        const Instruction *CxtI = nullptr,
114                        const DominatorTree *DT = nullptr);
115 
116   /// Return true if the given values are known to be non-equal when defined.
117   /// Supports scalar integer types only.
118   bool isKnownNonEqual(Value *V1, Value *V2, const DataLayout &DL,
119                       AssumptionCache *AC = nullptr,
120                       const Instruction *CxtI = nullptr,
121                       const DominatorTree *DT = nullptr);
122 
123   /// Return true if 'V & Mask' is known to be zero. We use this predicate to
124   /// simplify operations downstream. Mask is known to be zero for bits that V
125   /// cannot have.
126   ///
127   /// This function is defined on values with integer type, values with pointer
128   /// type, and vectors of integers.  In the case
129   /// where V is a vector, the mask, known zero, and known one values are the
130   /// same width as the vector element, and the bit is set only if it is true
131   /// for all of the elements in the vector.
132   bool MaskedValueIsZero(Value *V, const APInt &Mask, const DataLayout &DL,
133                          unsigned Depth = 0, AssumptionCache *AC = nullptr,
134                          const Instruction *CxtI = nullptr,
135                          const DominatorTree *DT = nullptr);
136 
137   /// Return the number of times the sign bit of the register is replicated into
138   /// the other bits. We know that at least 1 bit is always equal to the sign
139   /// bit (itself), but other cases can give us information. For example,
140   /// immediately after an "ashr X, 2", we know that the top 3 bits are all
141   /// equal to each other, so we return 3. For vectors, return the number of
142   /// sign bits for the vector element with the mininum number of known sign
143   /// bits.
144   unsigned ComputeNumSignBits(Value *Op, const DataLayout &DL,
145                               unsigned Depth = 0, AssumptionCache *AC = nullptr,
146                               const Instruction *CxtI = nullptr,
147                               const DominatorTree *DT = nullptr);
148 
149   /// This function computes the integer multiple of Base that equals V. If
150   /// successful, it returns true and returns the multiple in Multiple. If
151   /// unsuccessful, it returns false. Also, if V can be simplified to an
152   /// integer, then the simplified V is returned in Val. Look through sext only
153   /// if LookThroughSExt=true.
154   bool ComputeMultiple(Value *V, unsigned Base, Value *&Multiple,
155                        bool LookThroughSExt = false,
156                        unsigned Depth = 0);
157 
158   /// Map a call instruction to an intrinsic ID.  Libcalls which have equivalent
159   /// intrinsics are treated as-if they were intrinsics.
160   Intrinsic::ID getIntrinsicForCallSite(ImmutableCallSite ICS,
161                                         const TargetLibraryInfo *TLI);
162 
163   /// Return true if we can prove that the specified FP value is never equal to
164   /// -0.0.
165   bool CannotBeNegativeZero(const Value *V, const TargetLibraryInfo *TLI,
166                             unsigned Depth = 0);
167 
168   /// Return true if we can prove that the specified FP value is either a NaN or
169   /// never less than 0.0.
170   bool CannotBeOrderedLessThanZero(const Value *V, const TargetLibraryInfo *TLI,
171                                    unsigned Depth = 0);
172 
173   /// If the specified value can be set by repeating the same byte in memory,
174   /// return the i8 value that it is represented with. This is true for all i8
175   /// values obviously, but is also true for i32 0, i32 -1, i16 0xF0F0, double
176   /// 0.0 etc. If the value can't be handled with a repeated byte store (e.g.
177   /// i16 0x1234), return null.
178   Value *isBytewiseValue(Value *V);
179 
180   /// Given an aggregrate and an sequence of indices, see if the scalar value
181   /// indexed is already around as a register, for example if it were inserted
182   /// directly into the aggregrate.
183   ///
184   /// If InsertBefore is not null, this function will duplicate (modified)
185   /// insertvalues when a part of a nested struct is extracted.
186   Value *FindInsertedValue(Value *V,
187                            ArrayRef<unsigned> idx_range,
188                            Instruction *InsertBefore = nullptr);
189 
190   /// Analyze the specified pointer to see if it can be expressed as a base
191   /// pointer plus a constant offset. Return the base and offset to the caller.
192   Value *GetPointerBaseWithConstantOffset(Value *Ptr, int64_t &Offset,
193                                           const DataLayout &DL);
194   static inline const Value *
GetPointerBaseWithConstantOffset(const Value * Ptr,int64_t & Offset,const DataLayout & DL)195   GetPointerBaseWithConstantOffset(const Value *Ptr, int64_t &Offset,
196                                    const DataLayout &DL) {
197     return GetPointerBaseWithConstantOffset(const_cast<Value *>(Ptr), Offset,
198                                             DL);
199   }
200 
201   /// Returns true if the GEP is based on a pointer to a string (array of i8),
202   /// and is indexing into this string.
203   bool isGEPBasedOnPointerToString(const GEPOperator *GEP);
204 
205   /// This function computes the length of a null-terminated C string pointed to
206   /// by V. If successful, it returns true and returns the string in Str. If
207   /// unsuccessful, it returns false. This does not include the trailing null
208   /// character by default. If TrimAtNul is set to false, then this returns any
209   /// trailing null characters as well as any other characters that come after
210   /// it.
211   bool getConstantStringInfo(const Value *V, StringRef &Str,
212                              uint64_t Offset = 0, bool TrimAtNul = true);
213 
214   /// If we can compute the length of the string pointed to by the specified
215   /// pointer, return 'len+1'.  If we can't, return 0.
216   uint64_t GetStringLength(Value *V);
217 
218   /// This method strips off any GEP address adjustments and pointer casts from
219   /// the specified value, returning the original object being addressed. Note
220   /// that the returned value has pointer type if the specified value does. If
221   /// the MaxLookup value is non-zero, it limits the number of instructions to
222   /// be stripped off.
223   Value *GetUnderlyingObject(Value *V, const DataLayout &DL,
224                              unsigned MaxLookup = 6);
225   static inline const Value *GetUnderlyingObject(const Value *V,
226                                                  const DataLayout &DL,
227                                                  unsigned MaxLookup = 6) {
228     return GetUnderlyingObject(const_cast<Value *>(V), DL, MaxLookup);
229   }
230 
231   /// \brief This method is similar to GetUnderlyingObject except that it can
232   /// look through phi and select instructions and return multiple objects.
233   ///
234   /// If LoopInfo is passed, loop phis are further analyzed.  If a pointer
235   /// accesses different objects in each iteration, we don't look through the
236   /// phi node. E.g. consider this loop nest:
237   ///
238   ///   int **A;
239   ///   for (i)
240   ///     for (j) {
241   ///        A[i][j] = A[i-1][j] * B[j]
242   ///     }
243   ///
244   /// This is transformed by Load-PRE to stash away A[i] for the next iteration
245   /// of the outer loop:
246   ///
247   ///   Curr = A[0];          // Prev_0
248   ///   for (i: 1..N) {
249   ///     Prev = Curr;        // Prev = PHI (Prev_0, Curr)
250   ///     Curr = A[i];
251   ///     for (j: 0..N) {
252   ///        Curr[j] = Prev[j] * B[j]
253   ///     }
254   ///   }
255   ///
256   /// Since A[i] and A[i-1] are independent pointers, getUnderlyingObjects
257   /// should not assume that Curr and Prev share the same underlying object thus
258   /// it shouldn't look through the phi above.
259   void GetUnderlyingObjects(Value *V, SmallVectorImpl<Value *> &Objects,
260                             const DataLayout &DL, LoopInfo *LI = nullptr,
261                             unsigned MaxLookup = 6);
262 
263   /// Return true if the only users of this pointer are lifetime markers.
264   bool onlyUsedByLifetimeMarkers(const Value *V);
265 
266   /// Return true if the instruction does not have any effects besides
267   /// calculating the result and does not have undefined behavior.
268   ///
269   /// This method never returns true for an instruction that returns true for
270   /// mayHaveSideEffects; however, this method also does some other checks in
271   /// addition. It checks for undefined behavior, like dividing by zero or
272   /// loading from an invalid pointer (but not for undefined results, like a
273   /// shift with a shift amount larger than the width of the result). It checks
274   /// for malloc and alloca because speculatively executing them might cause a
275   /// memory leak. It also returns false for instructions related to control
276   /// flow, specifically terminators and PHI nodes.
277   ///
278   /// If the CtxI is specified this method performs context-sensitive analysis
279   /// and returns true if it is safe to execute the instruction immediately
280   /// before the CtxI.
281   ///
282   /// If the CtxI is NOT specified this method only looks at the instruction
283   /// itself and its operands, so if this method returns true, it is safe to
284   /// move the instruction as long as the correct dominance relationships for
285   /// the operands and users hold.
286   ///
287   /// This method can return true for instructions that read memory;
288   /// for such instructions, moving them may change the resulting value.
289   bool isSafeToSpeculativelyExecute(const Value *V,
290                                     const Instruction *CtxI = nullptr,
291                                     const DominatorTree *DT = nullptr);
292 
293   /// Returns true if the result or effects of the given instructions \p I
294   /// depend on or influence global memory.
295   /// Memory dependence arises for example if the instruction reads from
296   /// memory or may produce effects or undefined behaviour. Memory dependent
297   /// instructions generally cannot be reorderd with respect to other memory
298   /// dependent instructions or moved into non-dominated basic blocks.
299   /// Instructions which just compute a value based on the values of their
300   /// operands are not memory dependent.
301   bool mayBeMemoryDependent(const Instruction &I);
302 
303   /// Return true if this pointer couldn't possibly be null by its definition.
304   /// This returns true for allocas, non-extern-weak globals, and byval
305   /// arguments.
306   bool isKnownNonNull(const Value *V);
307 
308   /// Return true if this pointer couldn't possibly be null. If the context
309   /// instruction is specified, perform context-sensitive analysis and return
310   /// true if the pointer couldn't possibly be null at the specified
311   /// instruction.
312   bool isKnownNonNullAt(const Value *V,
313                         const Instruction *CtxI = nullptr,
314                         const DominatorTree *DT  = nullptr);
315 
316   /// Return true if it is valid to use the assumptions provided by an
317   /// assume intrinsic, I, at the point in the control-flow identified by the
318   /// context instruction, CxtI.
319   bool isValidAssumeForContext(const Instruction *I, const Instruction *CxtI,
320                                const DominatorTree *DT = nullptr);
321 
322   enum class OverflowResult { AlwaysOverflows, MayOverflow, NeverOverflows };
323   OverflowResult computeOverflowForUnsignedMul(Value *LHS, Value *RHS,
324                                                const DataLayout &DL,
325                                                AssumptionCache *AC,
326                                                const Instruction *CxtI,
327                                                const DominatorTree *DT);
328   OverflowResult computeOverflowForUnsignedAdd(Value *LHS, Value *RHS,
329                                                const DataLayout &DL,
330                                                AssumptionCache *AC,
331                                                const Instruction *CxtI,
332                                                const DominatorTree *DT);
333   OverflowResult computeOverflowForSignedAdd(Value *LHS, Value *RHS,
334                                              const DataLayout &DL,
335                                              AssumptionCache *AC = nullptr,
336                                              const Instruction *CxtI = nullptr,
337                                              const DominatorTree *DT = nullptr);
338   /// This version also leverages the sign bit of Add if known.
339   OverflowResult computeOverflowForSignedAdd(AddOperator *Add,
340                                              const DataLayout &DL,
341                                              AssumptionCache *AC = nullptr,
342                                              const Instruction *CxtI = nullptr,
343                                              const DominatorTree *DT = nullptr);
344 
345   /// Returns true if the arithmetic part of the \p II 's result is
346   /// used only along the paths control dependent on the computation
347   /// not overflowing, \p II being an <op>.with.overflow intrinsic.
348   bool isOverflowIntrinsicNoWrap(IntrinsicInst *II, DominatorTree &DT);
349 
350   /// Return true if this function can prove that the instruction I will
351   /// always transfer execution to one of its successors (including the next
352   /// instruction that follows within a basic block). E.g. this is not
353   /// guaranteed for function calls that could loop infinitely.
354   ///
355   /// In other words, this function returns false for instructions that may
356   /// transfer execution or fail to transfer execution in a way that is not
357   /// captured in the CFG nor in the sequence of instructions within a basic
358   /// block.
359   ///
360   /// Undefined behavior is assumed not to happen, so e.g. division is
361   /// guaranteed to transfer execution to the following instruction even
362   /// though division by zero might cause undefined behavior.
363   bool isGuaranteedToTransferExecutionToSuccessor(const Instruction *I);
364 
365   /// Return true if this function can prove that the instruction I
366   /// is executed for every iteration of the loop L.
367   ///
368   /// Note that this currently only considers the loop header.
369   bool isGuaranteedToExecuteForEveryIteration(const Instruction *I,
370                                               const Loop *L);
371 
372   /// Return true if this function can prove that I is guaranteed to yield
373   /// full-poison (all bits poison) if at least one of its operands are
374   /// full-poison (all bits poison).
375   ///
376   /// The exact rules for how poison propagates through instructions have
377   /// not been settled as of 2015-07-10, so this function is conservative
378   /// and only considers poison to be propagated in uncontroversial
379   /// cases. There is no attempt to track values that may be only partially
380   /// poison.
381   bool propagatesFullPoison(const Instruction *I);
382 
383   /// Return either nullptr or an operand of I such that I will trigger
384   /// undefined behavior if I is executed and that operand has a full-poison
385   /// value (all bits poison).
386   const Value *getGuaranteedNonFullPoisonOp(const Instruction *I);
387 
388   /// Return true if this function can prove that if PoisonI is executed
389   /// and yields a full-poison value (all bits poison), then that will
390   /// trigger undefined behavior.
391   ///
392   /// Note that this currently only considers the basic block that is
393   /// the parent of I.
394   bool isKnownNotFullPoison(const Instruction *PoisonI);
395 
396   /// \brief Specific patterns of select instructions we can match.
397   enum SelectPatternFlavor {
398     SPF_UNKNOWN = 0,
399     SPF_SMIN,                   /// Signed minimum
400     SPF_UMIN,                   /// Unsigned minimum
401     SPF_SMAX,                   /// Signed maximum
402     SPF_UMAX,                   /// Unsigned maximum
403     SPF_FMINNUM,                /// Floating point minnum
404     SPF_FMAXNUM,                /// Floating point maxnum
405     SPF_ABS,                    /// Absolute value
406     SPF_NABS                    /// Negated absolute value
407   };
408   /// \brief Behavior when a floating point min/max is given one NaN and one
409   /// non-NaN as input.
410   enum SelectPatternNaNBehavior {
411     SPNB_NA = 0,                /// NaN behavior not applicable.
412     SPNB_RETURNS_NAN,           /// Given one NaN input, returns the NaN.
413     SPNB_RETURNS_OTHER,         /// Given one NaN input, returns the non-NaN.
414     SPNB_RETURNS_ANY            /// Given one NaN input, can return either (or
415                                 /// it has been determined that no operands can
416                                 /// be NaN).
417   };
418   struct SelectPatternResult {
419     SelectPatternFlavor Flavor;
420     SelectPatternNaNBehavior NaNBehavior; /// Only applicable if Flavor is
421                                           /// SPF_FMINNUM or SPF_FMAXNUM.
422     bool Ordered;               /// When implementing this min/max pattern as
423                                 /// fcmp; select, does the fcmp have to be
424                                 /// ordered?
425 
426     /// \brief Return true if \p SPF is a min or a max pattern.
isMinOrMaxSelectPatternResult427     static bool isMinOrMax(SelectPatternFlavor SPF) {
428       return !(SPF == SPF_UNKNOWN || SPF == SPF_ABS || SPF == SPF_NABS);
429     }
430   };
431   /// Pattern match integer [SU]MIN, [SU]MAX and ABS idioms, returning the kind
432   /// and providing the out parameter results if we successfully match.
433   ///
434   /// If CastOp is not nullptr, also match MIN/MAX idioms where the type does
435   /// not match that of the original select. If this is the case, the cast
436   /// operation (one of Trunc,SExt,Zext) that must be done to transform the
437   /// type of LHS and RHS into the type of V is returned in CastOp.
438   ///
439   /// For example:
440   ///   %1 = icmp slt i32 %a, i32 4
441   ///   %2 = sext i32 %a to i64
442   ///   %3 = select i1 %1, i64 %2, i64 4
443   ///
444   /// -> LHS = %a, RHS = i32 4, *CastOp = Instruction::SExt
445   ///
446   SelectPatternResult matchSelectPattern(Value *V, Value *&LHS, Value *&RHS,
447                                          Instruction::CastOps *CastOp = nullptr);
448 
449   /// Parse out a conservative ConstantRange from !range metadata.
450   ///
451   /// E.g. if RangeMD is !{i32 0, i32 10, i32 15, i32 20} then return [0, 20).
452   ConstantRange getConstantRangeFromMetadata(MDNode &RangeMD);
453 
454   /// Return true if RHS is known to be implied true by LHS.  Return false if
455   /// RHS is known to be implied false by LHS.  Otherwise, return None if no
456   /// implication can be made.
457   /// A & B must be i1 (boolean) values or a vector of such values. Note that
458   /// the truth table for implication is the same as <=u on i1 values (but not
459   /// <=s!).  The truth table for both is:
460   ///    | T | F (B)
461   ///  T | T | F
462   ///  F | T | T
463   /// (A)
464   Optional<bool> isImpliedCondition(
465       Value *LHS, Value *RHS, const DataLayout &DL, bool InvertAPred = false,
466       unsigned Depth = 0, AssumptionCache *AC = nullptr,
467       const Instruction *CxtI = nullptr, const DominatorTree *DT = nullptr);
468 } // end namespace llvm
469 
470 #endif
471