CForEmitter.h - mozsearch

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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_CForEmitter_h

#define frontend_CForEmitter_h

#include "mozilla/Attributes.h"  // MOZ_STACK_CLASS

#include "mozilla/Maybe.h"       // mozilla::Maybe

#include <stdint.h>  // uint32_t

#include "frontend/BytecodeControlStructures.h"  // LoopControl

#include "frontend/TDZCheckCache.h"              // TDZCheckCache

namespace js {

namespace frontend {

struct BytecodeEmitter;

class EmitterScope;

// Class for emitting bytecode for c-style for block.

//

// Usage: (check for the return value is omitted for simplicity)

//

//   `for (init; cond; update) body`

//     CForEmitter cfor(this, headLexicalEmitterScopeForLet or nullptr);

//     cfor.emitInit(Some(offset_of_init));

//     emit(init); // without pushing value

//     cfor.emitCond(Some(offset_of_cond));

//     emit(cond);

//     cfor.emitBody(CForEmitter::Cond::Present);

//     emit(body);

//     cfor.emitUpdate(CForEmitter::Update::Present, Some(offset_of_update)));

//     emit(update);

//     cfor.emitEnd(offset_of_for);

//

//   `for (;;) body`

//     CForEmitter cfor(this, nullptr);

//     cfor.emitInit(Nothing());

//     cfor.emitCond(Nothing());

//     cfor.emitBody(CForEmitter::Cond::Missing);

//     emit(body);

//     cfor.emitUpdate(CForEmitter::Update::Missing, Nothing());

//     cfor.emitEnd(offset_of_for);

//

class MOZ_STACK_CLASS CForEmitter {

  // Basic structure of the bytecode (not complete).

//

  // If `cond` is not empty:

  //     {init}

  //   loop:

  //     JSOp::LoopHead

  //     {cond}

  //     JSOp::JumpIfFalse break

  //     {body}

  //   continue:

  //     {update}

  //     JSOp::Goto loop

  //   break:

//

  // If `cond` is empty:

  //     {init}

  //   loop:

  //     JSOp::LoopHead

  //     {body}

  //   continue:

  //     {update}

  //     JSOp::Goto loop

  //   break:

//

 public:

  enum class Cond { Missing, Present };

  enum class Update { Missing, Present };

 private:

  BytecodeEmitter* bce_;

  // Whether the c-style for loop has `cond` and `update`.

  Cond cond_ = Cond::Missing;

  Update update_ = Update::Missing;

  mozilla::Maybe<LoopControl> loopInfo_;

  // The lexical scope to be freshened for each iteration.

  // See the comment in `emitCond` for more details.

//

  // ### Scope freshening

//

  // Each iteration of a `for (let V...)` loop creates a fresh loop variable

  // binding for V, even if the loop is a C-style `for(;;)` loop:

//

  //     var funcs = [];

  //     for (let i = 0; i < 2; i++)

  //         funcs.push(function() { return i; });

  //     assertEq(funcs[0](), 0);  // the two closures capture...

  //     assertEq(funcs[1](), 1);  // ...two different `i` bindings

//

  // This is implemented by "freshening" the implicit block -- changing the

  // scope chain to a fresh clone of the instantaneous block object -- each

  // iteration, just before evaluating the "update" in for(;;) loops.

//

  // ECMAScript doesn't freshen in `for (const ...;;)`.  Lack of freshening

  // isn't directly observable in-language because `const`s can't be mutated,

  // but it *can* be observed in the Debugger API.

  const EmitterScope* headLexicalEmitterScopeForLet_;

  mozilla::Maybe<TDZCheckCache> tdzCache_;

#ifdef DEBUG

  // The state of this emitter.

//

  // +-------+ emitInit +------+ emitCond +------+ emitBody +------+

  // | Start |--------->| Init |--------->| Cond |--------->| Body |-+

  // +-------+          +------+          +------+          +------+ |

  //                                                                 |

  //                           +-------------------------------------+

  //                           |

  //                           | emitUpdate +--------+ emitEnd +-----+

  //                           +----------->| Update |-------->| End |

  //                                        +--------+         +-----+

  enum class State {

    // The initial state.

    Start,

    // After calling emitInit.

    Init,

    // After calling emitCond.

    Cond,

    // After calling emitBody.

    Body,

    // After calling emitUpdate.

    Update,

    // After calling emitEnd.

End

};

  State state_ = State::Start;

#endif

 public:

  CForEmitter(BytecodeEmitter* bce,

              const EmitterScope* headLexicalEmitterScopeForLet);

  // Parameters are the offset in the source code for each character below:

//

  //   for ( x = 10 ; x < 20 ; x ++ ) { f(x); }

  //   ^     ^        ^        ^

  //   |     |        |        |

  //   |     |        |        updatePos

  //   |     |        |

  //   |     |        condPos

  //   |     |

  //   |     initPos

  //   |

  //   forPos

//

  // Can be Nothing() if not available.

  [[nodiscard]] bool emitInit(const mozilla::Maybe<uint32_t>& initPos);

  [[nodiscard]] bool emitCond(const mozilla::Maybe<uint32_t>& condPos);

  [[nodiscard]] bool emitBody(Cond cond);

  [[nodiscard]] bool emitUpdate(Update update,

                                const mozilla::Maybe<uint32_t>& updatePos);

  [[nodiscard]] bool emitEnd(uint32_t forPos);

};

} /* namespace frontend */

} /* namespace js */

#endif /* frontend_CForEmitter_h */