text: //===- FunctionComparator.h - Function Comparator ---------------*- C++ -*-===//
  //
  //                     The LLVM Compiler Infrastructure
  //
  // This file is distributed under the University of Illinois Open Source
  // License. See LICENSE.TXT for details.
  //
  //===----------------------------------------------------------------------===//
  //
  // This file defines the FunctionComparator and GlobalNumberState classes which
  // are used by the MergeFunctions pass for comparing functions.
  //
  //===----------------------------------------------------------------------===//
  
  #ifndef LLVM_TRANSFORMS_UTILS_FUNCTIONCOMPARATOR_H
  #define LLVM_TRANSFORMS_UTILS_FUNCTIONCOMPARATOR_H
  
  #include "llvm/ADT/DenseMap.h"
  #include "llvm/ADT/StringRef.h"
  #include "llvm/IR/Attributes.h"
  #include "llvm/IR/Instructions.h"
  #include "llvm/IR/Operator.h"
  #include "llvm/IR/ValueMap.h"
  #include "llvm/Support/AtomicOrdering.h"
  #include "llvm/Support/Casting.h"
  #include <cstdint>
  #include <tuple>
  
  namespace llvm {
  
  class APFloat;
  class APInt;
  class BasicBlock;
  class Constant;
  class Function;
  class GlobalValue;
  class InlineAsm;
  class Instruction;
  class MDNode;
  class Type;
  class Value;
  
  /// GlobalNumberState assigns an integer to each global value in the program,
  /// which is used by the comparison routine to order references to globals. This
  /// state must be preserved throughout the pass, because Functions and other
  /// globals need to maintain their relative order. Globals are assigned a number
  /// when they are first visited. This order is deterministic, and so the
  /// assigned numbers are as well. When two functions are merged, neither number
  /// is updated. If the symbols are weak, this would be incorrect. If they are
  /// strong, then one will be replaced at all references to the other, and so
  /// direct callsites will now see one or the other symbol, and no update is
  /// necessary. Note that if we were guaranteed unique names, we could just
  /// compare those, but this would not work for stripped bitcodes or for those
  /// few symbols without a name.
  class GlobalNumberState {
    struct Config : ValueMapConfig<GlobalValue *> {
      enum { FollowRAUW = false };
    };
  
    // Each GlobalValue is mapped to an identifier. The Config ensures when RAUW
    // occurs, the mapping does not change. Tracking changes is unnecessary, and
    // also problematic for weak symbols (which may be overwritten).
    using ValueNumberMap = ValueMap<GlobalValue *, uint64_t, Config>;
    ValueNumberMap GlobalNumbers;
  
    // The next unused serial number to assign to a global.
    uint64_t NextNumber = 0;
  
  public:
    GlobalNumberState() = default;
  
    uint64_t getNumber(GlobalValue* Global) {
      ValueNumberMap::iterator MapIter;
      bool Inserted;
      std::tie(MapIter, Inserted) = GlobalNumbers.insert({Global, NextNumber});
      if (Inserted)
        NextNumber++;
      return MapIter->second;
    }
  
    void erase(GlobalValue *Global) {
      GlobalNumbers.erase(Global);
    }
  
    void clear() {
      GlobalNumbers.clear();
    }
  };
  
  /// FunctionComparator - Compares two functions to determine whether or not
  /// they will generate machine code with the same behaviour. DataLayout is
  /// used if available. The comparator always fails conservatively (erring on the
  /// side of claiming that two functions are different).
  class FunctionComparator {
  public:
    FunctionComparator(const Function *F1, const Function *F2,
                       GlobalNumberState* GN)
        : FnL(F1), FnR(F2), GlobalNumbers(GN) {}
  
    /// Test whether the two functions have equivalent behaviour.
    int compare();
  
    /// Hash a function. Equivalent functions will have the same hash, and unequal
    /// functions will have different hashes with high probability.
    using FunctionHash = uint64_t;
    static FunctionHash functionHash(Function &);
  
  protected:
    /// Start the comparison.
    void beginCompare() {
      sn_mapL.clear();
      sn_mapR.clear();
    }
  
    /// Compares the signature and other general attributes of the two functions.
    int compareSignature() const;
  
    /// Test whether two basic blocks have equivalent behaviour.
    int cmpBasicBlocks(const BasicBlock *BBL, const BasicBlock *BBR) const;
  
    /// Constants comparison.
    /// Its analog to lexicographical comparison between hypothetical numbers
    /// of next format:
    /// <bitcastability-trait><raw-bit-contents>
    ///
    /// 1. Bitcastability.
    /// Check whether L's type could be losslessly bitcasted to R's type.
    /// On this stage method, in case when lossless bitcast is not possible
    /// method returns -1 or 1, thus also defining which type is greater in
    /// context of bitcastability.
    /// Stage 0: If types are equal in terms of cmpTypes, then we can go straight
    ///          to the contents comparison.
    ///          If types differ, remember types comparison result and check
    ///          whether we still can bitcast types.
    /// Stage 1: Types that satisfies isFirstClassType conditions are always
    ///          greater then others.
    /// Stage 2: Vector is greater then non-vector.
    ///          If both types are vectors, then vector with greater bitwidth is
    ///          greater.
    ///          If both types are vectors with the same bitwidth, then types
    ///          are bitcastable, and we can skip other stages, and go to contents
    ///          comparison.
    /// Stage 3: Pointer types are greater than non-pointers. If both types are
    ///          pointers of the same address space - go to contents comparison.
    ///          Different address spaces: pointer with greater address space is
    ///          greater.
    /// Stage 4: Types are neither vectors, nor pointers. And they differ.
    ///          We don't know how to bitcast them. So, we better don't do it,
    ///          and return types comparison result (so it determines the
    ///          relationship among constants we don't know how to bitcast).
    ///
    /// Just for clearance, let's see how the set of constants could look
    /// on single dimension axis:
    ///
    /// [NFCT], [FCT, "others"], [FCT, pointers], [FCT, vectors]
    /// Where: NFCT - Not a FirstClassType
    ///        FCT - FirstClassTyp:
    ///
    /// 2. Compare raw contents.
    /// It ignores types on this stage and only compares bits from L and R.
    /// Returns 0, if L and R has equivalent contents.
    /// -1 or 1 if values are different.
    /// Pretty trivial:
    /// 2.1. If contents are numbers, compare numbers.
    ///    Ints with greater bitwidth are greater. Ints with same bitwidths
    ///    compared by their contents.
    /// 2.2. "And so on". Just to avoid discrepancies with comments
    /// perhaps it would be better to read the implementation itself.
    /// 3. And again about overall picture. Let's look back at how the ordered set
    /// of constants will look like:
    /// [NFCT], [FCT, "others"], [FCT, pointers], [FCT, vectors]
    ///
    /// Now look, what could be inside [FCT, "others"], for example:
    /// [FCT, "others"] =
    /// [
    ///   [double 0.1], [double 1.23],
    ///   [i32 1], [i32 2],
    ///   { double 1.0 },       ; StructTyID, NumElements = 1
    ///   { i32 1 },            ; StructTyID, NumElements = 1
    ///   { double 1, i32 1 },  ; StructTyID, NumElements = 2
    ///   { i32 1, double 1 }   ; StructTyID, NumElements = 2
    /// ]
    ///
    /// Let's explain the order. Float numbers will be less than integers, just
    /// because of cmpType terms: FloatTyID < IntegerTyID.
    /// Floats (with same fltSemantics) are sorted according to their value.
    /// Then you can see integers, and they are, like a floats,
    /// could be easy sorted among each others.
    /// The structures. Structures are grouped at the tail, again because of their
    /// TypeID: StructTyID > IntegerTyID > FloatTyID.
    /// Structures with greater number of elements are greater. Structures with
    /// greater elements going first are greater.
    /// The same logic with vectors, arrays and other possible complex types.
    ///
    /// Bitcastable constants.
    /// Let's assume, that some constant, belongs to some group of
    /// "so-called-equal" values with different types, and at the same time
    /// belongs to another group of constants with equal types
    /// and "really" equal values.
    ///
    /// Now, prove that this is impossible:
    ///
    /// If constant A with type TyA is bitcastable to B with type TyB, then:
    /// 1. All constants with equal types to TyA, are bitcastable to B. Since
    ///    those should be vectors (if TyA is vector), pointers
    ///    (if TyA is pointer), or else (if TyA equal to TyB), those types should
    ///    be equal to TyB.
    /// 2. All constants with non-equal, but bitcastable types to TyA, are
    ///    bitcastable to B.
    ///    Once again, just because we allow it to vectors and pointers only.
    ///    This statement could be expanded as below:
    /// 2.1. All vectors with equal bitwidth to vector A, has equal bitwidth to
    ///      vector B, and thus bitcastable to B as well.
    /// 2.2. All pointers of the same address space, no matter what they point to,
    ///      bitcastable. So if C is pointer, it could be bitcasted to A and to B.
    /// So any constant equal or bitcastable to A is equal or bitcastable to B.
    /// QED.
    ///
    /// In another words, for pointers and vectors, we ignore top-level type and
    /// look at their particular properties (bit-width for vectors, and
    /// address space for pointers).
    /// If these properties are equal - compare their contents.
    int cmpConstants(const Constant *L, const Constant *R) const;
  
    /// Compares two global values by number. Uses the GlobalNumbersState to
    /// identify the same gobals across function calls.
    int cmpGlobalValues(GlobalValue *L, GlobalValue *R) const;
  
    /// Assign or look up previously assigned numbers for the two values, and
    /// return whether the numbers are equal. Numbers are assigned in the order
    /// visited.
    /// Comparison order:
    /// Stage 0: Value that is function itself is always greater then others.
    ///          If left and right values are references to their functions, then
    ///          they are equal.
    /// Stage 1: Constants are greater than non-constants.
    ///          If both left and right are constants, then the result of
    ///          cmpConstants is used as cmpValues result.
    /// Stage 2: InlineAsm instances are greater than others. If both left and
    ///          right are InlineAsm instances, InlineAsm* pointers casted to
    ///          integers and compared as numbers.
    /// Stage 3: For all other cases we compare order we meet these values in
    ///          their functions. If right value was met first during scanning,
    ///          then left value is greater.
    ///          In another words, we compare serial numbers, for more details
    ///          see comments for sn_mapL and sn_mapR.
    int cmpValues(const Value *L, const Value *R) const;
  
    /// Compare two Instructions for equivalence, similar to
    /// Instruction::isSameOperationAs.
    ///
    /// Stages are listed in "most significant stage first" order:
    /// On each stage below, we do comparison between some left and right
    /// operation parts. If parts are non-equal, we assign parts comparison
    /// result to the operation comparison result and exit from method.
    /// Otherwise we proceed to the next stage.
    /// Stages:
    /// 1. Operations opcodes. Compared as numbers.
    /// 2. Number of operands.
    /// 3. Operation types. Compared with cmpType method.
    /// 4. Compare operation subclass optional data as stream of bytes:
    /// just convert it to integers and call cmpNumbers.
    /// 5. Compare in operation operand types with cmpType in
    /// most significant operand first order.
    /// 6. Last stage. Check operations for some specific attributes.
    /// For example, for Load it would be:
    /// 6.1.Load: volatile (as boolean flag)
    /// 6.2.Load: alignment (as integer numbers)
    /// 6.3.Load: ordering (as underlying enum class value)
    /// 6.4.Load: synch-scope (as integer numbers)
    /// 6.5.Load: range metadata (as integer ranges)
    /// On this stage its better to see the code, since its not more than 10-15
    /// strings for particular instruction, and could change sometimes.
    ///
    /// Sets \p needToCmpOperands to true if the operands of the instructions
    /// still must be compared afterwards. In this case it's already guaranteed
    /// that both instructions have the same number of operands.
    int cmpOperations(const Instruction *L, const Instruction *R,
                      bool &needToCmpOperands) const;
  
    /// cmpType - compares two types,
    /// defines total ordering among the types set.
    ///
    /// Return values:
    /// 0 if types are equal,
    /// -1 if Left is less than Right,
    /// +1 if Left is greater than Right.
    ///
    /// Description:
    /// Comparison is broken onto stages. Like in lexicographical comparison
    /// stage coming first has higher priority.
    /// On each explanation stage keep in mind total ordering properties.
    ///
    /// 0. Before comparison we coerce pointer types of 0 address space to
    /// integer.
    /// We also don't bother with same type at left and right, so
    /// just return 0 in this case.
    ///
    /// 1. If types are of different kind (different type IDs).
    ///    Return result of type IDs comparison, treating them as numbers.
    /// 2. If types are integers, check that they have the same width. If they
    /// are vectors, check that they have the same count and subtype.
    /// 3. Types have the same ID, so check whether they are one of:
    /// * Void
    /// * Float
    /// * Double
    /// * X86_FP80
    /// * FP128
    /// * PPC_FP128
    /// * Label
    /// * Metadata
    /// We can treat these types as equal whenever their IDs are same.
    /// 4. If Left and Right are pointers, return result of address space
    /// comparison (numbers comparison). We can treat pointer types of same
    /// address space as equal.
    /// 5. If types are complex.
    /// Then both Left and Right are to be expanded and their element types will
    /// be checked with the same way. If we get Res != 0 on some stage, return it.
    /// Otherwise return 0.
    /// 6. For all other cases put llvm_unreachable.
    int cmpTypes(Type *TyL, Type *TyR) const;
  
    int cmpNumbers(uint64_t L, uint64_t R) const;
    int cmpAPInts(const APInt &L, const APInt &R) const;
    int cmpAPFloats(const APFloat &L, const APFloat &R) const;
    int cmpMem(StringRef L, StringRef R) const;
  
    // The two functions undergoing comparison.
    const Function *FnL, *FnR;
  
  private:
    int cmpOrderings(AtomicOrdering L, AtomicOrdering R) const;
    int cmpInlineAsm(const InlineAsm *L, const InlineAsm *R) const;
    int cmpAttrs(const AttributeList L, const AttributeList R) const;
    int cmpRangeMetadata(const MDNode *L, const MDNode *R) const;
    int cmpOperandBundlesSchema(const Instruction *L, const Instruction *R) const;
  
    /// Compare two GEPs for equivalent pointer arithmetic.
    /// Parts to be compared for each comparison stage,
    /// most significant stage first:
    /// 1. Address space. As numbers.
    /// 2. Constant offset, (using GEPOperator::accumulateConstantOffset method).
    /// 3. Pointer operand type (using cmpType method).
    /// 4. Number of operands.
    /// 5. Compare operands, using cmpValues method.
    int cmpGEPs(const GEPOperator *GEPL, const GEPOperator *GEPR) const;
    int cmpGEPs(const GetElementPtrInst *GEPL,
                const GetElementPtrInst *GEPR) const {
      return cmpGEPs(cast<GEPOperator>(GEPL), cast<GEPOperator>(GEPR));
    }
  
    /// Assign serial numbers to values from left function, and values from
    /// right function.
    /// Explanation:
    /// Being comparing functions we need to compare values we meet at left and
    /// right sides.
    /// Its easy to sort things out for external values. It just should be
    /// the same value at left and right.
    /// But for local values (those were introduced inside function body)
    /// we have to ensure they were introduced at exactly the same place,
    /// and plays the same role.
    /// Let's assign serial number to each value when we meet it first time.
    /// Values that were met at same place will be with same serial numbers.
    /// In this case it would be good to explain few points about values assigned
    /// to BBs and other ways of implementation (see below).
    ///
    /// 1. Safety of BB reordering.
    /// It's safe to change the order of BasicBlocks in function.
    /// Relationship with other functions and serial numbering will not be
    /// changed in this case.
    /// As follows from FunctionComparator::compare(), we do CFG walk: we start
    /// from the entry, and then take each terminator. So it doesn't matter how in
    /// fact BBs are ordered in function. And since cmpValues are called during
    /// this walk, the numbering depends only on how BBs located inside the CFG.
    /// So the answer is - yes. We will get the same numbering.
    ///
    /// 2. Impossibility to use dominance properties of values.
    /// If we compare two instruction operands: first is usage of local
    /// variable AL from function FL, and second is usage of local variable AR
    /// from FR, we could compare their origins and check whether they are
    /// defined at the same place.
    /// But, we are still not able to compare operands of PHI nodes, since those
    /// could be operands from further BBs we didn't scan yet.
    /// So it's impossible to use dominance properties in general.
    mutable DenseMap<const Value*, int> sn_mapL, sn_mapR;
  
    // The global state we will use
    GlobalNumberState* GlobalNumbers;
  };
  
  } // end namespace llvm
  
  #endif // LLVM_TRANSFORMS_UTILS_FUNCTIONCOMPARATOR_H
  