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diff --git a/js/src/wasm/WasmInitExpr.h b/js/src/wasm/WasmInitExpr.h
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+++ b/js/src/wasm/WasmInitExpr.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_initexpr_h
+#define wasm_initexpr_h
+
+#include "wasm/WasmConstants.h"
+#include "wasm/WasmSerialize.h"
+#include "wasm/WasmTypeDecls.h"
+#include "wasm/WasmValType.h"
+#include "wasm/WasmValue.h"
+
+namespace js {
+namespace wasm {
+
+class Decoder;
+struct ModuleEnvironment;
+
+// Validates a constant expression. Returns an optional literal value if the
+// final value was from a simple instruction such as i32.const.
+[[nodiscard]] bool DecodeConstantExpression(Decoder& d, ModuleEnvironment* env,
+                                            ValType expected,
+                                            Maybe<LitVal>* literal);
+
+enum class InitExprKind {
+  None,
+  Literal,
+  Variable,
+};
+
+// A InitExpr describes a WebAssembly constant expression, such as those used to
+// initialize a global, specify memory offsets in data segments, or populate
+// element segments. Such expressions are created during decoding and actually
+// executed on module instantiation.
+//
+// An InitExpr can only contain constant instructions, which are defined here in
+// the spec:
+//
+// https://webassembly.github.io/spec/core/valid/instructions.html#constant-expressions
+
+class InitExpr {
+  InitExprKind kind_;
+  // The bytecode for this constant expression if this is not a literal.
+  Bytes bytecode_;
+  // The value if this is a literal.
+  LitVal literal_;
+  // The value type of this constant expression in either case.
+  ValType type_;
+
+ public:
+  InitExpr() : kind_(InitExprKind::None) {}
+
+  explicit InitExpr(LitVal literal)
+      : kind_(InitExprKind::Literal),
+        literal_(literal),
+        type_(literal.type()) {}
+
+  // Decode and validate a constant expression given at the current
+  // position of the decoder. Upon failure, the decoder contains the failure
+  // message or else the failure was an OOM.
+  static bool decodeAndValidate(Decoder& d, ModuleEnvironment* env,
+                                ValType expected, InitExpr* expr);
+
+  // Decode and evaluate a constant expression at the current position of the
+  // decoder. Does not validate the expression first, since all InitExprs are
+  // required to have been validated. Failure conditions are the same as
+  // InitExpr::evaluate, i.e. failing only on OOM.
+  [[nodiscard]] static bool decodeAndEvaluate(
+      JSContext* cx, Handle<WasmInstanceObject*> instanceObj, Decoder& d,
+      ValType expectedType, MutableHandleVal result);
+
+  // Evaluate the constant expresssion with the given context. This may only
+  // fail due to an OOM, as all InitExpr's are required to have been validated.
+  bool evaluate(JSContext* cx, Handle<WasmInstanceObject*> instanceObj,
+                MutableHandleVal result) const;
+
+  bool isLiteral() const { return kind_ == InitExprKind::Literal; }
+
+  // Gets the result of this expression if it was determined to be a literal.
+  LitVal literal() const {
+    MOZ_ASSERT(isLiteral());
+    return literal_;
+  }
+
+  // Get the type of the resulting value of this expression.
+  ValType type() const { return type_; }
+
+  // Allow moving, but not implicit copying
+  InitExpr(const InitExpr&) = delete;
+  InitExpr& operator=(const InitExpr&) = delete;
+  InitExpr(InitExpr&&) = default;
+  InitExpr& operator=(InitExpr&&) = default;
+
+  // Allow explicit cloning
+  [[nodiscard]] bool clone(const InitExpr& src);
+
+  size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) const;
+  WASM_DECLARE_FRIEND_SERIALIZE(InitExpr);
+};
+
+}  // namespace wasm
+}  // namespace js
+
+#endif  // wasm_initexpr_h