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diff --git a/js/src/wasm/WasmSummarizeInsn.h b/js/src/wasm/WasmSummarizeInsn.h
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+++ b/js/src/wasm/WasmSummarizeInsn.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:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef wasm_WasmSummarizeInsn_h
+#define wasm_WasmSummarizeInsn_h
+
+#include "mozilla/Maybe.h"
+#include "wasm/WasmCodegenTypes.h"  // TrapMachineInsn
+
+namespace js {
+namespace wasm {
+
+#ifdef DEBUG
+
+// Inspect the machine instruction at `insn` and return a classification as to
+// what it is.  If the instruction can't be identified, return
+// `mozilla::Nothing`.  If the instruction is identified, the identification
+// must be correct -- in other words, if a `mozilla::Some(i)` is returned, `i`
+// must be the correct classification for the instruction.  Return
+// `mozilla::Nothing` in case of doubt.
+//
+// This function is only used by ModuleGenerator::finishCodeTier to audit wasm
+// trap sites.  So it doesn't need to handle the whole complexity of the
+// machine's instruction set.  It only needs to handle the tiny sub-dialect
+// used by the trappable instructions we actually generate.
+mozilla::Maybe<TrapMachineInsn> SummarizeTrapInstruction(const uint8_t* insn);
+
+#endif
+
+}  // namespace wasm
+}  // namespace js
+
+#endif /* wasm_WasmSummarizeInsn_h */