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diff --git a/js/src/wasm/WasmShareable.h b/js/src/wasm/WasmShareable.h
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+++ b/js/src/wasm/WasmShareable.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_shareable_h
+#define wasm_shareable_h
+
+#include "mozilla/RefPtr.h"
+#include "js/RefCounted.h"
+#include "wasm/WasmTypeDecls.h"
+
+namespace js {
+namespace wasm {
+
+using mozilla::MallocSizeOf;
+
+// This reusable base class factors out the logic for a resource that is shared
+// by multiple instances/modules but should only be counted once when computing
+// about:memory stats.
+
+template <class T>
+using SeenSet = HashSet<const T*, DefaultHasher<const T*>, SystemAllocPolicy>;
+
+template <class T>
+struct ShareableBase : AtomicRefCounted<T> {
+  using SeenSet = wasm::SeenSet<T>;
+
+  size_t sizeOfIncludingThisIfNotSeen(MallocSizeOf mallocSizeOf,
+                                      SeenSet* seen) const {
+    const T* self = static_cast<const T*>(this);
+    typename SeenSet::AddPtr p = seen->lookupForAdd(self);
+    if (p) {
+      return 0;
+    }
+    bool ok = seen->add(p, self);
+    (void)ok;  // oh well
+    return mallocSizeOf(self) + self->sizeOfExcludingThis(mallocSizeOf);
+  }
+};
+
+// ShareableBytes is a reference-counted Vector of bytes.
+
+struct ShareableBytes : ShareableBase<ShareableBytes> {
+  // Vector is 'final', so instead make Vector a member and add boilerplate.
+  Bytes bytes;
+
+  ShareableBytes() = default;
+  explicit ShareableBytes(Bytes&& bytes) : bytes(std::move(bytes)) {}
+  size_t sizeOfExcludingThis(MallocSizeOf mallocSizeOf) const {
+    return bytes.sizeOfExcludingThis(mallocSizeOf);
+  }
+  const uint8_t* begin() const { return bytes.begin(); }
+  const uint8_t* end() const { return bytes.end(); }
+  size_t length() const { return bytes.length(); }
+  bool append(const uint8_t* start, size_t len) {
+    return bytes.append(start, len);
+  }
+};
+
+using MutableBytes = RefPtr<ShareableBytes>;
+using SharedBytes = RefPtr<const ShareableBytes>;
+
+}  // namespace wasm
+}  // namespace js
+
+#endif  // wasm_shareable_h