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diff --git a/js/src/frontend/ElemOpEmitter.h b/js/src/frontend/ElemOpEmitter.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_ElemOpEmitter_h
+#define frontend_ElemOpEmitter_h
+
+#include "mozilla/Attributes.h"
+
+namespace js {
+namespace frontend {
+
+struct BytecodeEmitter;
+enum class ValueUsage;
+
+// Class for emitting bytecode for element operation.
+//
+// Usage: (check for the return value is omitted for simplicity)
+//
+// `obj[key];`
+// ElemOpEmitter eoe(this,
+// ElemOpEmitter::Kind::Get,
+// ElemOpEmitter::ObjKind::Other);
+// eoe.prepareForObj();
+// emit(obj);
+// eoe.prepareForKey();
+// emit(key);
+// eoe.emitGet();
+//
+// `super[key];`
+// ElemOpEmitter eoe(this,
+// ElemOpEmitter::Kind::Get,
+// ElemOpEmitter::ObjKind::Super);
+// eoe.prepareForObj();
+// emit(this_for_super);
+// eoe.prepareForKey();
+// emit(key);
+// eoe.emitGet();
+//
+// `obj[key]();`
+// ElemOpEmitter eoe(this,
+// ElemOpEmitter::Kind::Call,
+// ElemOpEmitter::ObjKind::Other);
+// eoe.prepareForObj();
+// emit(obj);
+// eoe.prepareForKey();
+// emit(key);
+// eoe.emitGet();
+// emit_call_here();
+//
+// `new obj[key]();`
+// ElemOpEmitter eoe(this,
+// ElemOpEmitter::Kind::Call,
+// ElemOpEmitter::ObjKind::Other);
+// eoe.prepareForObj();
+// emit(obj);
+// eoe.prepareForKey();
+// emit(key);
+// eoe.emitGet();
+// emit_call_here();
+//
+// `delete obj[key];`
+// ElemOpEmitter eoe(this,
+// ElemOpEmitter::Kind::Delete,
+// ElemOpEmitter::ObjKind::Other);
+// eoe.prepareForObj();
+// emit(obj);
+// eoe.prepareForKey();
+// emit(key);
+// eoe.emitDelete();
+//
+// `delete super[key];`
+// ElemOpEmitter eoe(this,
+// ElemOpEmitter::Kind::Delete,
+// ElemOpEmitter::ObjKind::Super);
+// eoe.prepareForObj();
+// emit(this_for_super);
+// eoe.prepareForKey();
+// emit(key);
+// eoe.emitDelete();
+//
+// `obj[key]++;`
+// ElemOpEmitter eoe(this,
+// ElemOpEmitter::Kind::PostIncrement,
+// ElemOpEmitter::ObjKind::Other);
+// eoe.prepareForObj();
+// emit(obj);
+// eoe.prepareForKey();
+// emit(key);
+// eoe.emitIncDec();
+//
+// `obj[key] = value;`
+// ElemOpEmitter eoe(this,
+// ElemOpEmitter::Kind::SimpleAssignment,
+// ElemOpEmitter::ObjKind::Other);
+// eoe.prepareForObj();
+// emit(obj);
+// eoe.prepareForKey();
+// emit(key);
+// eoe.prepareForRhs();
+// emit(value);
+// eoe.emitAssignment();
+//
+// `obj[key] += value;`
+// ElemOpEmitter eoe(this,
+// ElemOpEmitter::Kind::CompoundAssignment,
+// ElemOpEmitter::ObjKind::Other);
+// eoe.prepareForObj();
+// emit(obj);
+// eoe.prepareForKey();
+// emit(key);
+// eoe.emitGet();
+// eoe.prepareForRhs();
+// emit(value);
+// emit_add_op_here();
+// eoe.emitAssignment();
+//
+class MOZ_STACK_CLASS ElemOpEmitter {
+ public:
+ enum class Kind {
+ Get,
+ Call,
+ Delete,
+ PostIncrement,
+ PreIncrement,
+ PostDecrement,
+ PreDecrement,
+ SimpleAssignment,
+ PropInit,
+ CompoundAssignment
+ };
+ enum class ObjKind { Super, Other };
+
+ private:
+ BytecodeEmitter* bce_;
+
+ Kind kind_;
+ ObjKind objKind_;
+
+#ifdef DEBUG
+ // The state of this emitter.
+ //
+ // skipObjAndKeyAndRhs
+ // +------------------------------------------------+
+ // | |
+ // +-------+ | prepareForObj +-----+ prepareForKey +-----+ |
+ // | Start |-+-------------->| Obj |-------------->| Key |-+ |
+ // +-------+ +-----+ +-----+ | |
+ // | |
+ // +-------------------------------------------------------+ |
+ // | |
+ // | [Get] |
+ // | [Call] |
+ // | emitGet +-----+ |
+ // +---------->| Get | |
+ // | +-----+ |
+ // | |
+ // | [Delete] |
+ // | emitDelete +--------+ |
+ // +------------->| Delete | |
+ // | +--------+ |
+ // | |
+ // | [PostIncrement] |
+ // | [PreIncrement] |
+ // | [PostDecrement] |
+ // | [PreDecrement] |
+ // | emitIncDec +--------+ |
+ // +------------->| IncDec | |
+ // | +--------+ |
+ // | +-------------------+
+ // | [SimpleAssignment] |
+ // | [PropInit] |
+ // | prepareForRhs v +-----+
+ // +--------------------->+-------------->+->| Rhs |-+
+ // | ^ +-----+ |
+ // | | |
+ // | | +-------------+
+ // | [CompoundAssignment] | |
+ // | emitGet +-----+ | | emitAssignment +------------+
+ // +---------->| Get |----+ +--------------->| Assignment |
+ // +-----+ +------------+
+ enum class State {
+ // The initial state.
+ Start,
+
+ // After calling prepareForObj.
+ Obj,
+
+ // After calling emitKey.
+ Key,
+
+ // After calling emitGet.
+ Get,
+
+ // After calling emitDelete.
+ Delete,
+
+ // After calling emitIncDec.
+ IncDec,
+
+ // After calling prepareForRhs or skipObjAndKeyAndRhs.
+ Rhs,
+
+ // After calling emitAssignment.
+ Assignment,
+ };
+ State state_ = State::Start;
+#endif
+
+ public:
+ ElemOpEmitter(BytecodeEmitter* bce, Kind kind, ObjKind objKind);
+
+ private:
+ [[nodiscard]] bool isCall() const { return kind_ == Kind::Call; }
+
+ [[nodiscard]] bool isSimpleAssignment() const {
+ return kind_ == Kind::SimpleAssignment;
+ }
+
+ [[nodiscard]] bool isPropInit() const { return kind_ == Kind::PropInit; }
+
+ [[nodiscard]] bool isDelete() const { return kind_ == Kind::Delete; }
+
+ [[nodiscard]] bool isCompoundAssignment() const {
+ return kind_ == Kind::CompoundAssignment;
+ }
+
+ [[nodiscard]] bool isIncDec() const {
+ return isPostIncDec() || isPreIncDec();
+ }
+
+ [[nodiscard]] bool isPostIncDec() const {
+ return kind_ == Kind::PostIncrement || kind_ == Kind::PostDecrement;
+ }
+
+ [[nodiscard]] bool isPreIncDec() const {
+ return kind_ == Kind::PreIncrement || kind_ == Kind::PreDecrement;
+ }
+
+ [[nodiscard]] bool isInc() const {
+ return kind_ == Kind::PostIncrement || kind_ == Kind::PreIncrement;
+ }
+
+ [[nodiscard]] bool isSuper() const { return objKind_ == ObjKind::Super; }
+
+ public:
+ [[nodiscard]] bool prepareForObj();
+ [[nodiscard]] bool prepareForKey();
+
+ [[nodiscard]] bool emitGet();
+
+ [[nodiscard]] bool prepareForRhs();
+ [[nodiscard]] bool skipObjAndKeyAndRhs();
+
+ [[nodiscard]] bool emitDelete();
+
+ [[nodiscard]] bool emitAssignment();
+
+ [[nodiscard]] bool emitIncDec(ValueUsage valueUsage);
+};
+
+} /* namespace frontend */
+} // namespace js
+
+#endif /* frontend_ElemOpEmitter_h */