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diff --git a/js/src/wasm/WasmInstanceData.h b/js/src/wasm/WasmInstanceData.h
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+++ b/js/src/wasm/WasmInstanceData.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ *
+ * Copyright 2021 Mozilla Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef wasm_instance_data_h
+#define wasm_instance_data_h
+
+#include <stdint.h>
+
+#include "NamespaceImports.h"
+
+#include "gc/Allocator.h"
+#include "gc/Pretenuring.h"
+#include "js/Utility.h"
+#include "wasm/WasmInstance.h"
+#include "wasm/WasmTypeDecls.h"
+
+namespace js {
+namespace wasm {
+
+// ExportArg holds the unboxed operands to the wasm entry trampoline which can
+// be called through an ExportFuncPtr.
+
+struct ExportArg {
+  uint64_t lo;
+  uint64_t hi;
+};
+
+using ExportFuncPtr = int32_t (*)(ExportArg*, Instance*);
+
+// TypeDefInstanceData describes the runtime information associated with a
+// module's type definition. This is accessed directly from JIT code and the
+// Instance.
+
+struct TypeDefInstanceData {
+  TypeDefInstanceData()
+      : typeDef(nullptr),
+        superTypeVector(nullptr),
+        shape(nullptr),
+        clasp(nullptr),
+        allocKind(gc::AllocKind::LIMIT) {}
+
+  // The canonicalized pointer to this type definition. This is kept alive by
+  // the type context associated with the instance.
+  const wasm::TypeDef* typeDef;
+
+  // The supertype vector for this type definition.  This is also kept alive
+  // by the type context associated with the instance.
+  //
+  const wasm::SuperTypeVector* superTypeVector;
+
+  // The remaining fields are only meaningful for, and used by, structs and
+  // arrays.
+  GCPtr<Shape*> shape;
+  const JSClass* clasp;
+  // The allocation site for GC types. This is used for pre-tenuring.
+  gc::AllocSite allocSite;
+  gc::AllocKind allocKind;
+};
+
+// FuncImportInstanceData describes the region of wasm global memory allocated
+// in the instance's thread-local storage for a function import. This is
+// accessed directly from JIT code and mutated by Instance as exits become
+// optimized and deoptimized.
+
+struct FuncImportInstanceData {
+  // The code to call at an import site: a wasm callee, a thunk into C++, or a
+  // thunk into JIT code.
+  void* code;
+
+  // The callee's Instance pointer, which must be loaded to InstanceReg
+  // (along with any pinned registers) before calling 'code'.
+  Instance* instance;
+
+  // The callee function's realm.
+  JS::Realm* realm;
+
+  // A GC pointer which keeps the callee alive and is used to recover import
+  // values for lazy table initialization.
+  GCPtr<JSObject*> callable;
+  static_assert(sizeof(GCPtr<JSObject*>) == sizeof(void*), "for JIT access");
+};
+
+// TableInstanceData describes the region of wasm global memory allocated in the
+// instance's thread-local storage which is accessed directly from JIT code
+// to bounds-check and index the table.
+
+struct TableInstanceData {
+  // Length of the table in number of elements (not bytes).
+  uint32_t length;
+
+  // Pointer to the array of elements (which can have various representations).
+  // For tables of anyref this is null.
+  // For tables of functions, this is a pointer to the array of code pointers.
+  void* elements;
+};
+
+// TagInstanceData describes the instance state associated with a tag.
+
+struct TagInstanceData {
+  GCPtr<WasmTagObject*> object;
+};
+
+// Table element for TableRepr::Func which carries both the code pointer and
+// a instance pointer (and thus anything reachable through the instance).
+
+struct FunctionTableElem {
+  // The code to call when calling this element. The table ABI is the system
+  // ABI with the additional ABI requirements that:
+  //  - InstanceReg and any pinned registers have been loaded appropriately
+  //  - if this is a heterogeneous table that requires a signature check,
+  //    WasmTableCallSigReg holds the signature id.
+  void* code;
+
+  // The pointer to the callee's instance's Instance. This must be loaded into
+  // InstanceReg before calling 'code'.
+  Instance* instance;
+};
+
+}  // namespace wasm
+}  // namespace js
+
+#endif  // wasm_instance_data_h