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+//===- Twine.h - Fast Temporary String Concatenation ------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ADT_TWINE_H
+#define LLVM_ADT_TWINE_H
+
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/ErrorHandling.h"
+#include <cassert>
+#include <cstdint>
+#include <string>
+
+namespace llvm {
+
+  class formatv_object_base;
+  class raw_ostream;
+
+  /// Twine - A lightweight data structure for efficiently representing the
+  /// concatenation of temporary values as strings.
+  ///
+  /// A Twine is a kind of rope, it represents a concatenated string using a
+  /// binary-tree, where the string is the preorder of the nodes. Since the
+  /// Twine can be efficiently rendered into a buffer when its result is used,
+  /// it avoids the cost of generating temporary values for intermediate string
+  /// results -- particularly in cases when the Twine result is never
+  /// required. By explicitly tracking the type of leaf nodes, we can also avoid
+  /// the creation of temporary strings for conversions operations (such as
+  /// appending an integer to a string).
+  ///
+  /// A Twine is not intended for use directly and should not be stored, its
+  /// implementation relies on the ability to store pointers to temporary stack
+  /// objects which may be deallocated at the end of a statement. Twines should
+  /// only be used accepted as const references in arguments, when an API wishes
+  /// to accept possibly-concatenated strings.
+  ///
+  /// Twines support a special 'null' value, which always concatenates to form
+  /// itself, and renders as an empty string. This can be returned from APIs to
+  /// effectively nullify any concatenations performed on the result.
+  ///
+  /// \b Implementation
+  ///
+  /// Given the nature of a Twine, it is not possible for the Twine's
+  /// concatenation method to construct interior nodes; the result must be
+  /// represented inside the returned value. For this reason a Twine object
+  /// actually holds two values, the left- and right-hand sides of a
+  /// concatenation. We also have nullary Twine objects, which are effectively
+  /// sentinel values that represent empty strings.
+  ///
+  /// Thus, a Twine can effectively have zero, one, or two children. The \see
+  /// isNullary(), \see isUnary(), and \see isBinary() predicates exist for
+  /// testing the number of children.
+  ///
+  /// We maintain a number of invariants on Twine objects (FIXME: Why):
+  ///  - Nullary twines are always represented with their Kind on the left-hand
+  ///    side, and the Empty kind on the right-hand side.
+  ///  - Unary twines are always represented with the value on the left-hand
+  ///    side, and the Empty kind on the right-hand side.
+  ///  - If a Twine has another Twine as a child, that child should always be
+  ///    binary (otherwise it could have been folded into the parent).
+  ///
+  /// These invariants are check by \see isValid().
+  ///
+  /// \b Efficiency Considerations
+  ///
+  /// The Twine is designed to yield efficient and small code for common
+  /// situations. For this reason, the concat() method is inlined so that
+  /// concatenations of leaf nodes can be optimized into stores directly into a
+  /// single stack allocated object.
+  ///
+  /// In practice, not all compilers can be trusted to optimize concat() fully,
+  /// so we provide two additional methods (and accompanying operator+
+  /// overloads) to guarantee that particularly important cases (cstring plus
+  /// StringRef) codegen as desired.
+  class Twine {
+    /// NodeKind - Represent the type of an argument.
+    enum NodeKind : unsigned char {
+      /// An empty string; the result of concatenating anything with it is also
+      /// empty.
+      NullKind,
+
+      /// The empty string.
+      EmptyKind,
+
+      /// A pointer to a Twine instance.
+      TwineKind,
+
+      /// A pointer to a C string instance.
+      CStringKind,
+
+      /// A pointer to an std::string instance.
+      StdStringKind,
+
+      /// A pointer to a StringRef instance.
+      StringRefKind,
+
+      /// A pointer to a SmallString instance.
+      SmallStringKind,
+
+      /// A pointer to a formatv_object_base instance.
+      FormatvObjectKind,
+
+      /// A char value, to render as a character.
+      CharKind,
+
+      /// An unsigned int value, to render as an unsigned decimal integer.
+      DecUIKind,
+
+      /// An int value, to render as a signed decimal integer.
+      DecIKind,
+
+      /// A pointer to an unsigned long value, to render as an unsigned decimal
+      /// integer.
+      DecULKind,
+
+      /// A pointer to a long value, to render as a signed decimal integer.
+      DecLKind,
+
+      /// A pointer to an unsigned long long value, to render as an unsigned
+      /// decimal integer.
+      DecULLKind,
+
+      /// A pointer to a long long value, to render as a signed decimal integer.
+      DecLLKind,
+
+      /// A pointer to a uint64_t value, to render as an unsigned hexadecimal
+      /// integer.
+      UHexKind
+    };
+
+    union Child
+    {
+      const Twine *twine;
+      const char *cString;
+      const std::string *stdString;
+      const StringRef *stringRef;
+      const SmallVectorImpl<char> *smallString;
+      const formatv_object_base *formatvObject;
+      char character;
+      unsigned int decUI;
+      int decI;
+      const unsigned long *decUL;
+      const long *decL;
+      const unsigned long long *decULL;
+      const long long *decLL;
+      const uint64_t *uHex;
+    };
+
+    /// LHS - The prefix in the concatenation, which may be uninitialized for
+    /// Null or Empty kinds.
+    Child LHS = {0};
+
+    /// RHS - The suffix in the concatenation, which may be uninitialized for
+    /// Null or Empty kinds.
+    Child RHS = {0};
+
+    /// LHSKind - The NodeKind of the left hand side, \see getLHSKind().
+    NodeKind LHSKind = EmptyKind;
+
+    /// RHSKind - The NodeKind of the right hand side, \see getRHSKind().
+    NodeKind RHSKind = EmptyKind;
+
+    /// Construct a nullary twine; the kind must be NullKind or EmptyKind.
+    explicit Twine(NodeKind Kind) : LHSKind(Kind) {
+      assert(isNullary() && "Invalid kind!");
+    }
+
+    /// Construct a binary twine.
+    explicit Twine(const Twine &LHS, const Twine &RHS)
+        : LHSKind(TwineKind), RHSKind(TwineKind) {
+      this->LHS.twine = &LHS;
+      this->RHS.twine = &RHS;
+      assert(isValid() && "Invalid twine!");
+    }
+
+    /// Construct a twine from explicit values.
+    explicit Twine(Child LHS, NodeKind LHSKind, Child RHS, NodeKind RHSKind)
+        : LHS(LHS), RHS(RHS), LHSKind(LHSKind), RHSKind(RHSKind) {
+      assert(isValid() && "Invalid twine!");
+    }
+
+    /// Check for the null twine.
+    bool isNull() const {
+      return getLHSKind() == NullKind;
+    }
+
+    /// Check for the empty twine.
+    bool isEmpty() const {
+      return getLHSKind() == EmptyKind;
+    }
+
+    /// Check if this is a nullary twine (null or empty).
+    bool isNullary() const {
+      return isNull() || isEmpty();
+    }
+
+    /// Check if this is a unary twine.
+    bool isUnary() const {
+      return getRHSKind() == EmptyKind && !isNullary();
+    }
+
+    /// Check if this is a binary twine.
+    bool isBinary() const {
+      return getLHSKind() != NullKind && getRHSKind() != EmptyKind;
+    }
+
+    /// Check if this is a valid twine (satisfying the invariants on
+    /// order and number of arguments).
+    bool isValid() const {
+      // Nullary twines always have Empty on the RHS.
+      if (isNullary() && getRHSKind() != EmptyKind)
+        return false;
+
+      // Null should never appear on the RHS.
+      if (getRHSKind() == NullKind)
+        return false;
+
+      // The RHS cannot be non-empty if the LHS is empty.
+      if (getRHSKind() != EmptyKind && getLHSKind() == EmptyKind)
+        return false;
+
+      // A twine child should always be binary.
+      if (getLHSKind() == TwineKind &&
+          !LHS.twine->isBinary())
+        return false;
+      if (getRHSKind() == TwineKind &&
+          !RHS.twine->isBinary())
+        return false;
+
+      return true;
+    }
+
+    /// Get the NodeKind of the left-hand side.
+    NodeKind getLHSKind() const { return LHSKind; }
+
+    /// Get the NodeKind of the right-hand side.
+    NodeKind getRHSKind() const { return RHSKind; }
+
+    /// Print one child from a twine.
+    void printOneChild(raw_ostream &OS, Child Ptr, NodeKind Kind) const;
+
+    /// Print the representation of one child from a twine.
+    void printOneChildRepr(raw_ostream &OS, Child Ptr,
+                           NodeKind Kind) const;
+
+  public:
+    /// @name Constructors
+    /// @{
+
+    /// Construct from an empty string.
+    /*implicit*/ Twine() {
+      assert(isValid() && "Invalid twine!");
+    }
+
+    Twine(const Twine &) = default;
+
+    /// Construct from a C string.
+    ///
+    /// We take care here to optimize "" into the empty twine -- this will be
+    /// optimized out for string constants. This allows Twine arguments have
+    /// default "" values, without introducing unnecessary string constants.
+    /*implicit*/ Twine(const char *Str) {
+      if (Str[0] != '\0') {
+        LHS.cString = Str;
+        LHSKind = CStringKind;
+      } else
+        LHSKind = EmptyKind;
+
+      assert(isValid() && "Invalid twine!");
+    }
+    /// Delete the implicit conversion from nullptr as Twine(const char *)
+    /// cannot take nullptr.
+    /*implicit*/ Twine(std::nullptr_t) = delete;
+
+    /// Construct from an std::string.
+    /*implicit*/ Twine(const std::string &Str) : LHSKind(StdStringKind) {
+      LHS.stdString = &Str;
+      assert(isValid() && "Invalid twine!");
+    }
+
+    /// Construct from a StringRef.
+    /*implicit*/ Twine(const StringRef &Str) : LHSKind(StringRefKind) {
+      LHS.stringRef = &Str;
+      assert(isValid() && "Invalid twine!");
+    }
+
+    /// Construct from a SmallString.
+    /*implicit*/ Twine(const SmallVectorImpl<char> &Str)
+        : LHSKind(SmallStringKind) {
+      LHS.smallString = &Str;
+      assert(isValid() && "Invalid twine!");
+    }
+
+    /// Construct from a formatv_object_base.
+    /*implicit*/ Twine(const formatv_object_base &Fmt)
+        : LHSKind(FormatvObjectKind) {
+      LHS.formatvObject = &Fmt;
+      assert(isValid() && "Invalid twine!");
+    }
+
+    /// Construct from a char.
+    explicit Twine(char Val) : LHSKind(CharKind) {
+      LHS.character = Val;
+    }
+
+    /// Construct from a signed char.
+    explicit Twine(signed char Val) : LHSKind(CharKind) {
+      LHS.character = static_cast<char>(Val);
+    }
+
+    /// Construct from an unsigned char.
+    explicit Twine(unsigned char Val) : LHSKind(CharKind) {
+      LHS.character = static_cast<char>(Val);
+    }
+
+    /// Construct a twine to print \p Val as an unsigned decimal integer.
+    explicit Twine(unsigned Val) : LHSKind(DecUIKind) {
+      LHS.decUI = Val;
+    }
+
+    /// Construct a twine to print \p Val as a signed decimal integer.
+    explicit Twine(int Val) : LHSKind(DecIKind) {
+      LHS.decI = Val;
+    }
+
+    /// Construct a twine to print \p Val as an unsigned decimal integer.
+    explicit Twine(const unsigned long &Val) : LHSKind(DecULKind) {
+      LHS.decUL = &Val;
+    }
+
+    /// Construct a twine to print \p Val as a signed decimal integer.
+    explicit Twine(const long &Val) : LHSKind(DecLKind) {
+      LHS.decL = &Val;
+    }
+
+    /// Construct a twine to print \p Val as an unsigned decimal integer.
+    explicit Twine(const unsigned long long &Val) : LHSKind(DecULLKind) {
+      LHS.decULL = &Val;
+    }
+
+    /// Construct a twine to print \p Val as a signed decimal integer.
+    explicit Twine(const long long &Val) : LHSKind(DecLLKind) {
+      LHS.decLL = &Val;
+    }
+
+    // FIXME: Unfortunately, to make sure this is as efficient as possible we
+    // need extra binary constructors from particular types. We can't rely on
+    // the compiler to be smart enough to fold operator+()/concat() down to the
+    // right thing. Yet.
+
+    /// Construct as the concatenation of a C string and a StringRef.
+    /*implicit*/ Twine(const char *LHS, const StringRef &RHS)
+        : LHSKind(CStringKind), RHSKind(StringRefKind) {
+      this->LHS.cString = LHS;
+      this->RHS.stringRef = &RHS;
+      assert(isValid() && "Invalid twine!");
+    }
+
+    /// Construct as the concatenation of a StringRef and a C string.
+    /*implicit*/ Twine(const StringRef &LHS, const char *RHS)
+        : LHSKind(StringRefKind), RHSKind(CStringKind) {
+      this->LHS.stringRef = &LHS;
+      this->RHS.cString = RHS;
+      assert(isValid() && "Invalid twine!");
+    }
+
+    /// Since the intended use of twines is as temporary objects, assignments
+    /// when concatenating might cause undefined behavior or stack corruptions
+    Twine &operator=(const Twine &) = delete;
+
+    /// Create a 'null' string, which is an empty string that always
+    /// concatenates to form another empty string.
+    static Twine createNull() {
+      return Twine(NullKind);
+    }
+
+    /// @}
+    /// @name Numeric Conversions
+    /// @{
+
+    // Construct a twine to print \p Val as an unsigned hexadecimal integer.
+    static Twine utohexstr(const uint64_t &Val) {
+      Child LHS, RHS;
+      LHS.uHex = &Val;
+      RHS.twine = nullptr;
+      return Twine(LHS, UHexKind, RHS, EmptyKind);
+    }
+
+    /// @}
+    /// @name Predicate Operations
+    /// @{
+
+    /// Check if this twine is trivially empty; a false return value does not
+    /// necessarily mean the twine is empty.
+    bool isTriviallyEmpty() const {
+      return isNullary();
+    }
+
+    /// Return true if this twine can be dynamically accessed as a single
+    /// StringRef value with getSingleStringRef().
+    bool isSingleStringRef() const {
+      if (getRHSKind() != EmptyKind) return false;
+
+      switch (getLHSKind()) {
+      case EmptyKind:
+      case CStringKind:
+      case StdStringKind:
+      case StringRefKind:
+      case SmallStringKind:
+        return true;
+      default:
+        return false;
+      }
+    }
+
+    /// @}
+    /// @name String Operations
+    /// @{
+
+    Twine concat(const Twine &Suffix) const;
+
+    /// @}
+    /// @name Output & Conversion.
+    /// @{
+
+    /// Return the twine contents as a std::string.
+    std::string str() const;
+
+    /// Append the concatenated string into the given SmallString or SmallVector.
+    void toVector(SmallVectorImpl<char> &Out) const;
+
+    /// This returns the twine as a single StringRef.  This method is only valid
+    /// if isSingleStringRef() is true.
+    StringRef getSingleStringRef() const {
+      assert(isSingleStringRef() &&"This cannot be had as a single stringref!");
+      switch (getLHSKind()) {
+      default: llvm_unreachable("Out of sync with isSingleStringRef");
+      case EmptyKind:      return StringRef();
+      case CStringKind:    return StringRef(LHS.cString);
+      case StdStringKind:  return StringRef(*LHS.stdString);
+      case StringRefKind:  return *LHS.stringRef;
+      case SmallStringKind:
+        return StringRef(LHS.smallString->data(), LHS.smallString->size());
+      }
+    }
+
+    /// This returns the twine as a single StringRef if it can be
+    /// represented as such. Otherwise the twine is written into the given
+    /// SmallVector and a StringRef to the SmallVector's data is returned.
+    StringRef toStringRef(SmallVectorImpl<char> &Out) const {
+      if (isSingleStringRef())
+        return getSingleStringRef();
+      toVector(Out);
+      return StringRef(Out.data(), Out.size());
+    }
+
+    /// This returns the twine as a single null terminated StringRef if it
+    /// can be represented as such. Otherwise the twine is written into the
+    /// given SmallVector and a StringRef to the SmallVector's data is returned.
+    ///
+    /// The returned StringRef's size does not include the null terminator.
+    StringRef toNullTerminatedStringRef(SmallVectorImpl<char> &Out) const;
+
+    /// Write the concatenated string represented by this twine to the
+    /// stream \p OS.
+    void print(raw_ostream &OS) const;
+
+    /// Dump the concatenated string represented by this twine to stderr.
+    void dump() const;
+
+    /// Write the representation of this twine to the stream \p OS.
+    void printRepr(raw_ostream &OS) const;
+
+    /// Dump the representation of this twine to stderr.
+    void dumpRepr() const;
+
+    /// @}
+  };
+
+  /// @name Twine Inline Implementations
+  /// @{
+
+  inline Twine Twine::concat(const Twine &Suffix) const {
+    // Concatenation with null is null.
+    if (isNull() || Suffix.isNull())
+      return Twine(NullKind);
+
+    // Concatenation with empty yields the other side.
+    if (isEmpty())
+      return Suffix;
+    if (Suffix.isEmpty())
+      return *this;
+
+    // Otherwise we need to create a new node, taking care to fold in unary
+    // twines.
+    Child NewLHS, NewRHS;
+    NewLHS.twine = this;
+    NewRHS.twine = &Suffix;
+    NodeKind NewLHSKind = TwineKind, NewRHSKind = TwineKind;
+    if (isUnary()) {
+      NewLHS = LHS;
+      NewLHSKind = getLHSKind();
+    }
+    if (Suffix.isUnary()) {
+      NewRHS = Suffix.LHS;
+      NewRHSKind = Suffix.getLHSKind();
+    }
+
+    return Twine(NewLHS, NewLHSKind, NewRHS, NewRHSKind);
+  }
+
+  inline Twine operator+(const Twine &LHS, const Twine &RHS) {
+    return LHS.concat(RHS);
+  }
+
+  /// Additional overload to guarantee simplified codegen; this is equivalent to
+  /// concat().
+
+  inline Twine operator+(const char *LHS, const StringRef &RHS) {
+    return Twine(LHS, RHS);
+  }
+
+  /// Additional overload to guarantee simplified codegen; this is equivalent to
+  /// concat().
+
+  inline Twine operator+(const StringRef &LHS, const char *RHS) {
+    return Twine(LHS, RHS);
+  }
+
+  inline raw_ostream &operator<<(raw_ostream &OS, const Twine &RHS) {
+    RHS.print(OS);
+    return OS;
+  }
+
+  /// @}
+
+} // end namespace llvm
+
+#endif // LLVM_ADT_TWINE_H