From 4bdfe738955c8de70c6266ab58addec7ef41b31e Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Sun, 6 Jun 2021 13:33:02 +0200
Subject: [PATCH] LibJS: Add basic support for "continue" in the bytecode VM

Unlike the convoluted unwind-until-scope-type mechanism in the AST
interpreter, "continue" maps to a simple Bytecode::Op::Jump here. :^)

We know where to jump based on a stack of "continuable scopes" that
we now maintain on the Bytecode::Generator as we go.

Note that this only supports bare "continue", not continue-with-label.
---
 Userland/Libraries/LibJS/AST.h                   |  1 +
 Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp | 10 ++++++++++
 Userland/Libraries/LibJS/Bytecode/Generator.cpp  | 15 +++++++++++++++
 Userland/Libraries/LibJS/Bytecode/Generator.h    |  6 ++++++
 4 files changed, 32 insertions(+)

diff --git a/Userland/Libraries/LibJS/AST.h b/Userland/Libraries/LibJS/AST.h
index 5dc8e73911..b1f5332600 100644
--- a/Userland/Libraries/LibJS/AST.h
+++ b/Userland/Libraries/LibJS/AST.h
@@ -1300,6 +1300,7 @@ public:
     }
 
     virtual Value execute(Interpreter&, GlobalObject&) const override;
+    virtual Optional<Bytecode::Register> generate_bytecode(Bytecode::Generator&) const override;
 
     const FlyString& target_label() const { return m_target_label; }
 
diff --git a/Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp b/Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp
index e53658f2ca..6bb367186b 100644
--- a/Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp
+++ b/Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp
@@ -110,6 +110,7 @@ Optional<Bytecode::Register> AssignmentExpression::generate_bytecode(Bytecode::G
 
 Optional<Bytecode::Register> WhileStatement::generate_bytecode(Bytecode::Generator& generator) const
 {
+    generator.begin_continuable_scope();
     auto test_label = generator.make_label();
     auto test_result_reg = m_test->generate_bytecode(generator);
     VERIFY(test_result_reg.has_value());
@@ -117,13 +118,16 @@ Optional<Bytecode::Register> WhileStatement::generate_bytecode(Bytecode::Generat
     auto body_result_reg = m_body->generate_bytecode(generator);
     generator.emit<Bytecode::Op::Jump>(test_label);
     test_jump.set_target(generator.make_label());
+    generator.end_continuable_scope();
     return body_result_reg;
 }
 
 Optional<Bytecode::Register> DoWhileStatement::generate_bytecode(Bytecode::Generator& generator) const
 {
+    generator.begin_continuable_scope();
     auto head_label = generator.make_label();
     auto body_result_reg = m_body->generate_bytecode(generator);
+    generator.end_continuable_scope();
     auto test_result_reg = m_test->generate_bytecode(generator);
     VERIFY(test_result_reg.has_value());
     generator.emit<Bytecode::Op::JumpIfTrue>(*test_result_reg, head_label);
@@ -209,4 +213,10 @@ Optional<Bytecode::Register> IfStatement::generate_bytecode(Bytecode::Generator&
     return {};
 }
 
+Optional<Bytecode::Register> ContinueStatement::generate_bytecode(Bytecode::Generator& generator) const
+{
+    generator.emit<Bytecode::Op::Jump>(generator.nearest_continuable_scope());
+    return {};
+}
+
 }
diff --git a/Userland/Libraries/LibJS/Bytecode/Generator.cpp b/Userland/Libraries/LibJS/Bytecode/Generator.cpp
index 824f9de063..8a58af5b09 100644
--- a/Userland/Libraries/LibJS/Bytecode/Generator.cpp
+++ b/Userland/Libraries/LibJS/Bytecode/Generator.cpp
@@ -47,4 +47,19 @@ Label Generator::make_label() const
     return Label { m_block->instructions().size() };
 }
 
+Label Generator::nearest_continuable_scope() const
+{
+    return m_continuable_scopes.last();
+}
+
+void Generator::begin_continuable_scope()
+{
+    m_continuable_scopes.append(make_label());
+}
+
+void Generator::end_continuable_scope()
+{
+    m_continuable_scopes.take_last();
+}
+
 }
diff --git a/Userland/Libraries/LibJS/Bytecode/Generator.h b/Userland/Libraries/LibJS/Bytecode/Generator.h
index e0b4ab8509..3a19525e47 100644
--- a/Userland/Libraries/LibJS/Bytecode/Generator.h
+++ b/Userland/Libraries/LibJS/Bytecode/Generator.h
@@ -29,6 +29,11 @@ public:
 
     Label make_label() const;
 
+    void begin_continuable_scope();
+    void end_continuable_scope();
+
+    Label nearest_continuable_scope() const;
+
 private:
     Generator();
     ~Generator();
@@ -37,6 +42,7 @@ private:
 
     OwnPtr<Block> m_block;
     u32 m_next_register { 1 };
+    Vector<Label> m_continuable_scopes;
 };
 
 }