js/src/frontend/TDZCheckCache.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
   2  * vim: set ts=8 sts=2 et sw=2 tw=80:
   3  * This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 #ifndef frontend_TDZCheckCache_h
   8 #define frontend_TDZCheckCache_h
   9
  10 #include "mozilla/Maybe.h"
  11
  12 #include "ds/Nestable.h"
  13 #include "frontend/NameCollections.h"
  14
  15 namespace js {
  16 namespace frontend {
  17
  18 struct BytecodeEmitter;
  19 class TaggedParserAtomIndex;
  20
  21 enum MaybeCheckTDZ { CheckTDZ = true, DontCheckTDZ = false };
  22
  23 using CheckTDZMap = RecyclableNameMap<MaybeCheckTDZ>;
  24
  25 // A cache that tracks Temporal Dead Zone (TDZ) checks, so that any use of a
  26 // lexical variable that's dominated by an earlier use, or by evaluation of its
  27 // declaration (which will initialize it, perhaps to |undefined|), doesn't have
  28 // to redundantly check that the lexical variable has been initialized
  29 //
  30 // Each basic block should have a TDZCheckCache in scope. Some NestableControl
  31 // subclasses contain a TDZCheckCache.
  32 //
  33 // When a scope containing lexical variables is entered, all such variables are
  34 // marked as CheckTDZ.  When a lexical variable is accessed, its entry is
  35 // checked.  If it's CheckTDZ, a JSOp::CheckLexical is emitted and then the
  36 // entry is marked DontCheckTDZ.  If it's DontCheckTDZ, no check is emitted
  37 // because a prior check would have already failed.  Finally, because
  38 // evaluating a lexical variable declaration initializes it (after any
  39 // initializer is evaluated), evaluating a lexical declaration marks its entry
  40 // as DontCheckTDZ.
  41 class TDZCheckCache : public Nestable<TDZCheckCache> {
  42   PooledMapPtr<CheckTDZMap> cache_;
  43
  44   [[nodiscard]] bool ensureCache(BytecodeEmitter* bce);
  45
  46  public:
  47   explicit TDZCheckCache(BytecodeEmitter* bce);
  48
  49   mozilla::Maybe<MaybeCheckTDZ> needsTDZCheck(BytecodeEmitter* bce,
  50                                               TaggedParserAtomIndex name);
  51   [[nodiscard]] bool noteTDZCheck(BytecodeEmitter* bce,
  52                                   TaggedParserAtomIndex name,
  53                                   MaybeCheckTDZ check);
  54 };
  55
  56 } /* namespace frontend */
  57 } /* namespace js */
  58
  59 #endif /* frontend_TDZCheckCache_h */