Bug 1852740: add tests for the `fetchpriority` attribute in Link headers. r=necko...

[gecko.git] / js / src / frontend / CForEmitter.h

/* -*- 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 */