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diff --git a/js/src/frontend/TDZCheckCache.h b/js/src/frontend/TDZCheckCache.h
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+++ b/js/src/frontend/TDZCheckCache.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 frontend_TDZCheckCache_h
+#define frontend_TDZCheckCache_h
+
+#include "mozilla/Attributes.h"
+#include "mozilla/Maybe.h"
+
+#include "ds/Nestable.h"
+#include "frontend/NameCollections.h"
+#include "js/TypeDecls.h"
+#include "vm/Stack.h"
+
+namespace js {
+namespace frontend {
+
+struct BytecodeEmitter;
+
+using CheckTDZMap = RecyclableNameMap<MaybeCheckTDZ>;
+
+// A cache that tracks Temporal Dead Zone (TDZ) checks, so that any use of a
+// lexical variable that's dominated by an earlier use, or by evaluation of its
+// declaration (which will initialize it, perhaps to |undefined|), doesn't have
+// to redundantly check that the lexical variable has been initialized
+//
+// Each basic block should have a TDZCheckCache in scope. Some NestableControl
+// subclasses contain a TDZCheckCache.
+//
+// When a scope containing lexical variables is entered, all such variables are
+// marked as CheckTDZ.  When a lexical variable is accessed, its entry is
+// checked.  If it's CheckTDZ, a JSOp::CheckLexical is emitted and then the
+// entry is marked DontCheckTDZ.  If it's DontCheckTDZ, no check is emitted
+// because a prior check would have already failed.  Finally, because
+// evaluating a lexical variable declaration initializes it (after any
+// initializer is evaluated), evaluating a lexical declaration marks its entry
+// as DontCheckTDZ.
+class TDZCheckCache : public Nestable<TDZCheckCache> {
+  PooledMapPtr<CheckTDZMap> cache_;
+
+  MOZ_MUST_USE bool ensureCache(BytecodeEmitter* bce);
+
+ public:
+  explicit TDZCheckCache(BytecodeEmitter* bce);
+
+  mozilla::Maybe<MaybeCheckTDZ> needsTDZCheck(BytecodeEmitter* bce,
+                                              const ParserAtom* name);
+  MOZ_MUST_USE bool noteTDZCheck(BytecodeEmitter* bce, const ParserAtom* name,
+                                 MaybeCheckTDZ check);
+};
+
+} /* namespace frontend */
+} /* namespace js */
+
+#endif /* frontend_TDZCheckCache_h */