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diff --git a/lib/AST/Interp/Descriptor.h b/lib/AST/Interp/Descriptor.h
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+//===--- Descriptor.h - Types for the constexpr VM --------------*- 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
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines descriptors which characterise allocations.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_DESCRIPTOR_H
+#define LLVM_CLANG_AST_INTERP_DESCRIPTOR_H
+
+#include "clang/AST/Decl.h"
+#include "clang/AST/Expr.h"
+
+namespace clang {
+namespace interp {
+class Block;
+class Record;
+struct Descriptor;
+enum PrimType : unsigned;
+
+using DeclTy = llvm::PointerUnion<const Decl *, const Expr *>;
+
+/// Invoked whenever a block is created. The constructor method fills in the
+/// inline descriptors of all fields and array elements. It also initializes
+/// all the fields which contain non-trivial types.
+using BlockCtorFn = void (*)(Block *Storage, char *FieldPtr, bool IsConst,
+                             bool IsMutable, bool IsActive,
+                             Descriptor *FieldDesc);
+
+/// Invoked when a block is destroyed. Invokes the destructors of all
+/// non-trivial nested fields of arrays and records.
+using BlockDtorFn = void (*)(Block *Storage, char *FieldPtr,
+                             Descriptor *FieldDesc);
+
+/// Invoked when a block with pointers referencing it goes out of scope. Such
+/// blocks are persisted: the move function copies all inline descriptors and
+/// non-trivial fields, as existing pointers might need to reference those
+/// descriptors. Data is not copied since it cannot be legally read.
+using BlockMoveFn = void (*)(Block *Storage, char *SrcFieldPtr,
+                             char *DstFieldPtr, Descriptor *FieldDesc);
+
+/// Object size as used by the interpreter.
+using InterpSize = unsigned;
+
+/// Describes a memory block created by an allocation site.
+struct Descriptor {
+private:
+  /// Original declaration, used to emit the error message.
+  const DeclTy Source;
+  /// Size of an element, in host bytes.
+  const InterpSize ElemSize;
+  /// Size of the storage, in host bytes.
+  const InterpSize Size;
+  /// Size of the allocation (storage + metadata), in host bytes.
+  const InterpSize AllocSize;
+
+  /// Value to denote arrays of unknown size.
+  static constexpr unsigned UnknownSizeMark = (unsigned)-1;
+
+public:
+  /// Token to denote structures of unknown size.
+  struct UnknownSize {};
+
+  /// Pointer to the record, if block contains records.
+  Record *const ElemRecord = nullptr;
+  /// Descriptor of the array element.
+  Descriptor *const ElemDesc = nullptr;
+  /// Flag indicating if the block is mutable.
+  const bool IsConst = false;
+  /// Flag indicating if a field is mutable.
+  const bool IsMutable = false;
+  /// Flag indicating if the block is a temporary.
+  const bool IsTemporary = false;
+  /// Flag indicating if the block is an array.
+  const bool IsArray = false;
+
+  /// Storage management methods.
+  const BlockCtorFn CtorFn = nullptr;
+  const BlockDtorFn DtorFn = nullptr;
+  const BlockMoveFn MoveFn = nullptr;
+
+  /// Allocates a descriptor for a primitive.
+  Descriptor(const DeclTy &D, PrimType Type, bool IsConst, bool IsTemporary,
+             bool IsMutable);
+
+  /// Allocates a descriptor for an array of primitives.
+  Descriptor(const DeclTy &D, PrimType Type, size_t NumElems, bool IsConst,
+             bool IsTemporary, bool IsMutable);
+
+  /// Allocates a descriptor for an array of primitives of unknown size.
+  Descriptor(const DeclTy &D, PrimType Type, bool IsTemporary, UnknownSize);
+
+  /// Allocates a descriptor for an array of composites.
+  Descriptor(const DeclTy &D, Descriptor *Elem, unsigned NumElems, bool IsConst,
+             bool IsTemporary, bool IsMutable);
+
+  /// Allocates a descriptor for an array of composites of unknown size.
+  Descriptor(const DeclTy &D, Descriptor *Elem, bool IsTemporary, UnknownSize);
+
+  /// Allocates a descriptor for a record.
+  Descriptor(const DeclTy &D, Record *R, bool IsConst, bool IsTemporary,
+             bool IsMutable);
+
+  QualType getType() const;
+  SourceLocation getLocation() const;
+
+  const Decl *asDecl() const { return Source.dyn_cast<const Decl *>(); }
+  const Expr *asExpr() const { return Source.dyn_cast<const Expr *>(); }
+
+  const ValueDecl *asValueDecl() const {
+    return dyn_cast_or_null<ValueDecl>(asDecl());
+  }
+
+  const FieldDecl *asFieldDecl() const {
+    return dyn_cast_or_null<FieldDecl>(asDecl());
+  }
+
+  const RecordDecl *asRecordDecl() const {
+    return dyn_cast_or_null<RecordDecl>(asDecl());
+  }
+
+  /// Returns the size of the object without metadata.
+  unsigned getSize() const {
+    assert(!isUnknownSizeArray() && "Array of unknown size");
+    return Size;
+  }
+
+  /// Returns the allocated size, including metadata.
+  unsigned getAllocSize() const { return AllocSize; }
+  /// returns the size of an element when the structure is viewed as an array.
+  unsigned getElemSize()  const { return ElemSize; }
+
+  /// Returns the number of elements stored in the block.
+  unsigned getNumElems() const {
+    return Size == UnknownSizeMark ? 0 : (getSize() / getElemSize());
+  }
+
+  /// Checks if the descriptor is of an array of primitives.
+  bool isPrimitiveArray() const { return IsArray && !ElemDesc; }
+  /// Checks if the descriptor is of an array of zero size.
+  bool isZeroSizeArray() const { return Size == 0; }
+  /// Checks if the descriptor is of an array of unknown size.
+  bool isUnknownSizeArray() const { return Size == UnknownSizeMark; }
+
+  /// Checks if the descriptor is of a primitive.
+  bool isPrimitive() const { return !IsArray && !ElemRecord; }
+
+  /// Checks if the descriptor is of an array.
+  bool isArray() const { return IsArray; }
+};
+
+/// Inline descriptor embedded in structures and arrays.
+///
+/// Such descriptors precede all composite array elements and structure fields.
+/// If the base of a pointer is not zero, the base points to the end of this
+/// structure. The offset field is used to traverse the pointer chain up
+/// to the root structure which allocated the object.
+struct InlineDescriptor {
+  /// Offset inside the structure/array.
+  unsigned Offset;
+
+  /// Flag indicating if the storage is constant or not.
+  /// Relevant for primitive fields.
+  unsigned IsConst : 1;
+  /// For primitive fields, it indicates if the field was initialized.
+  /// Primitive fields in static storage are always initialized.
+  /// Arrays are always initialized, even though their elements might not be.
+  /// Base classes are initialized after the constructor is invoked.
+  unsigned IsInitialized : 1;
+  /// Flag indicating if the field is an embedded base class.
+  unsigned IsBase : 1;
+  /// Flag indicating if the field is the active member of a union.
+  unsigned IsActive : 1;
+  /// Flag indicating if the field is mutable (if in a record).
+  unsigned IsMutable : 1;
+
+  Descriptor *Desc;
+};
+
+/// Bitfield tracking the initialisation status of elements of primitive arrays.
+/// A pointer to this is embedded at the end of all primitive arrays.
+/// If the map was not yet created and nothing was initialied, the pointer to
+/// this structure is 0. If the object was fully initialized, the pointer is -1.
+struct InitMap {
+private:
+  /// Type packing bits.
+  using T = uint64_t;
+  /// Bits stored in a single field.
+  static constexpr uint64_t PER_FIELD = sizeof(T) * CHAR_BIT;
+
+  /// Initializes the map with no fields set.
+  InitMap(unsigned N);
+
+  /// Returns a pointer to storage.
+  T *data();
+
+public:
+  /// Initializes an element. Returns true when object if fully initialized.
+  bool initialize(unsigned I);
+
+  /// Checks if an element was initialized.
+  bool isInitialized(unsigned I);
+
+  /// Allocates a map holding N elements.
+  static InitMap *allocate(unsigned N);
+
+private:
+  /// Number of fields initialized.
+  unsigned UninitFields;
+};
+
+} // namespace interp
+} // namespace clang
+
+#endif