LibJS: Add basic "if" statement support to the bytecode VM :^)

This also required making Bytecode::Op::Jump support lazy linking
to a target label.

I left a FIXME here about having the "if" statement return the result
value from the taken branch statement. That's what the AST interpreter
does but I'm not sure if it's actually required.

4 files changed, 30 insertions, 5 deletions

diff --git a/Userland/Libraries/LibJS/AST.h b/Userland/Libraries/LibJS/AST.h
index 4e156ef655..5dc8e73911 100644
--- a/Userland/Libraries/LibJS/AST.h
+++ b/Userland/Libraries/LibJS/AST.h
@@ -351,6 +351,7 @@ public:
 
     virtual Value execute(Interpreter&, GlobalObject&) const override;
     virtual void dump(int indent) const override;
+    virtual Optional<Bytecode::Register> generate_bytecode(Bytecode::Generator&) const override;
 
 private:
     NonnullRefPtr<Expression> m_predicate;
diff --git a/Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp b/Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp
index 18d4e24700..e53658f2ca 100644
--- a/Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp
+++ b/Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp
@@ -187,4 +187,26 @@ Optional<Bytecode::Register> ReturnStatement::generate_bytecode(Bytecode::Genera
     return argument_reg;
 }
 
+Optional<Bytecode::Register> IfStatement::generate_bytecode(Bytecode::Generator& generator) const
+{
+    auto predicate_reg = m_predicate->generate_bytecode(generator);
+    auto& if_jump = generator.emit<Bytecode::Op::JumpIfTrue>(*predicate_reg);
+    auto& else_jump = generator.emit<Bytecode::Op::JumpIfFalse>(*predicate_reg);
+
+    if_jump.set_target(generator.make_label());
+    (void)m_consequent->generate_bytecode(generator);
+    auto& end_jump = generator.emit<Bytecode::Op::Jump>();
+
+    else_jump.set_target(generator.make_label());
+    if (m_alternate) {
+        (void)m_alternate->generate_bytecode(generator);
+    }
+
+    end_jump.set_target(generator.make_label());
+
+    // FIXME: Do we need IfStatement to return the consequent/alternate result value?
+    //        (That's what the AST interpreter currently does)
+    return {};
+}
+
 }
diff --git a/Userland/Libraries/LibJS/Bytecode/Op.cpp b/Userland/Libraries/LibJS/Bytecode/Op.cpp
index 869dd169ce..0297533b11 100644
--- a/Userland/Libraries/LibJS/Bytecode/Op.cpp
+++ b/Userland/Libraries/LibJS/Bytecode/Op.cpp
@@ -72,7 +72,7 @@ void PutById::execute(Bytecode::Interpreter& interpreter) const
 
 void Jump::execute(Bytecode::Interpreter& interpreter) const
 {
-    interpreter.jump(m_target);
+    interpreter.jump(*m_target);
 }
 
 void JumpIfFalse::execute(Bytecode::Interpreter& interpreter) const
@@ -196,7 +196,7 @@ String GetById::to_string() const
 
 String Jump::to_string() const
 {
-    return String::formatted("Jump {}", m_target);
+    return String::formatted("Jump {}", *m_target);
 }
 
 String JumpIfFalse::to_string() const
diff --git a/Userland/Libraries/LibJS/Bytecode/Op.h b/Userland/Libraries/LibJS/Bytecode/Op.h
index 1d57fed10b..9769a6b5a6 100644
--- a/Userland/Libraries/LibJS/Bytecode/Op.h
+++ b/Userland/Libraries/LibJS/Bytecode/Op.h
@@ -214,17 +214,19 @@ private:
 
 class Jump final : public Instruction {
 public:
-    explicit Jump(Label target)
-        : m_target(target)
+    explicit Jump(Optional<Label> target = {})
+        : m_target(move(target))
     {
     }
 
+    void set_target(Optional<Label> target) { m_target = move(target); }
+
     virtual ~Jump() override { }
     virtual void execute(Bytecode::Interpreter&) const override;
     virtual String to_string() const override;
 
 private:
-    Label m_target;
+    Optional<Label> m_target;
 };
 
 class JumpIfFalse final : public Instruction {