Skip environment creation when all bindings in the scope are local

core/ast/src/scope.rs

@@ -50,8 +50,9 @@ impl<'a> arbitrary::Arbitrary<'a> for Scope {
 
 #[derive(Debug, PartialEq)]
 pub(crate) struct Inner {
+    unique_id: u32,
     outer: Option<Scope>,
-    index: u32,
+    index: RefCell<u32>,
     bindings: RefCell<FxHashMap<JsString, Binding>>,
     function: bool,
 }
@@ -62,8 +63,9 @@ impl Scope {
     pub fn new_global() -> Self {
         Self {
             inner: Rc::new(Inner {
+                unique_id: 0,
                 outer: None,
-                index: 0,
+                index: RefCell::default(),
                 bindings: RefCell::default(),
                 function: true,
             }),
@@ -73,17 +75,29 @@ impl Scope {
     /// Creates a new scope.
     #[must_use]
     pub fn new(parent: Self, function: bool) -> Self {
-        let index = parent.inner.index + 1;
+        let index = *parent.inner.index.borrow() + 1;
         Self {
             inner: Rc::new(Inner {
+                unique_id: index,
                 outer: Some(parent),
-                index,
+                index: RefCell::new(index),
                 bindings: RefCell::default(),
                 function,
             }),
         }
     }
 
+    /// Checks if the scope has only local bindings.
+    #[must_use]
+    pub fn all_bindings_local(&self) -> bool {
+        // if self.inner.function && self.inn
+        self.inner
+            .bindings
+            .borrow()
+            .values()
+            .all(|binding| !binding.escapes)
+    }
+
     /// Marks all bindings in this scope as escaping.
     pub fn escape_all_bindings(&self) {
         for binding in self.inner.bindings.borrow_mut().values_mut() {
@@ -113,7 +127,12 @@ impl Scope {
     pub fn get_identifier_reference(&self, name: JsString) -> IdentifierReference {
         if let Some(binding) = self.inner.bindings.borrow().get(&name) {
             IdentifierReference::new(
-                BindingLocator::declarative(name, self.inner.index, binding.index),
+                BindingLocator::declarative(
+                    name,
+                    *self.inner.index.borrow(),
+                    binding.index,
+                    self.inner.unique_id,
+                ),
                 binding.lex,
                 binding.escapes,
             )
@@ -134,7 +153,12 @@ impl Scope {
     /// Returns the index of this scope.
     #[must_use]
     pub fn scope_index(&self) -> u32 {
-        self.inner.index
+        *self.inner.index.borrow()
+    }
+
+    /// Set the index of this scope.
+    pub(crate) fn set_index(&self, index: u32) {
+        *self.inner.index.borrow_mut() = index;
     }
 
     /// Check if the scope is a function scope.
@@ -153,7 +177,12 @@ impl Scope {
     #[must_use]
     pub fn get_binding(&self, name: &JsString) -> Option<BindingLocator> {
         self.inner.bindings.borrow().get(name).map(|binding| {
-            BindingLocator::declarative(name.clone(), self.inner.index, binding.index)
+            BindingLocator::declarative(
+                name.clone(),
+                *self.inner.index.borrow(),
+                binding.index,
+                self.inner.unique_id,
+            )
         })
     }
 
@@ -162,7 +191,12 @@ impl Scope {
     pub fn get_binding_reference(&self, name: &JsString) -> Option<IdentifierReference> {
         self.inner.bindings.borrow().get(name).map(|binding| {
             IdentifierReference::new(
-                BindingLocator::declarative(name.clone(), self.inner.index, binding.index),
+                BindingLocator::declarative(
+                    name.clone(),
+                    *self.inner.index.borrow(),
+                    binding.index,
+                    self.inner.unique_id,
+                ),
                 binding.lex,
                 binding.escapes,
             )
@@ -209,7 +243,12 @@ impl Scope {
                 escapes: self.is_global(),
             },
         );
-        BindingLocator::declarative(name, self.inner.index, binding_index)
+        BindingLocator::declarative(
+            name,
+            *self.inner.index.borrow(),
+            binding_index,
+            self.inner.unique_id,
+        )
     }
 
     /// Crate an immutable binding.
@@ -238,7 +277,12 @@ impl Scope {
     ) -> Result<IdentifierReference, BindingLocatorError> {
         Ok(match self.inner.bindings.borrow().get(&name) {
             Some(binding) if binding.mutable => IdentifierReference::new(
-                BindingLocator::declarative(name, self.inner.index, binding.index),
+                BindingLocator::declarative(
+                    name,
+                    *self.inner.index.borrow(),
+                    binding.index,
+                    self.inner.unique_id,
+                ),
                 binding.lex,
                 binding.escapes,
             ),
@@ -281,7 +325,12 @@ impl Scope {
 
         Ok(match self.inner.bindings.borrow().get(&name) {
             Some(binding) if binding.mutable => IdentifierReference::new(
-                BindingLocator::declarative(name, self.inner.index, binding.index),
+                BindingLocator::declarative(
+                    name,
+                    *self.inner.index.borrow(),
+                    binding.index,
+                    self.inner.unique_id,
+                ),
                 binding.lex,
                 binding.escapes,
             ),
@@ -358,15 +407,23 @@ pub struct BindingLocator {
 
     /// Index of the binding in the scope.
     binding_index: u32,
+
+    unique_scope_id: u32,
 }
 
 impl BindingLocator {
     /// Creates a new declarative binding locator that has knows indices.
-    pub(crate) const fn declarative(name: JsString, scope_index: u32, binding_index: u32) -> Self {
+    pub(crate) const fn declarative(
+        name: JsString,
+        scope_index: u32,
+        binding_index: u32,
+        unique_scope_id: u32,
+    ) -> Self {
         Self {
             name,
             scope: scope_index + 1,
             binding_index,
+            unique_scope_id,
         }
     }
 
@@ -376,6 +433,7 @@ impl BindingLocator {
             name,
             scope: 0,
             binding_index: 0,
+            unique_scope_id: 0,
         }
     }
 

core/ast/src/scope_analyzer.rs

@@ -1149,6 +1149,423 @@ impl BindingCollectorVisitor<'_> {
     }
 }
 
+/// Optimize scope indicies when scopes only contain local bindings.
+pub(crate) fn optimize_scope_indicies<'a, N>(node: &'a mut N, scope: &Scope)
+where
+    &'a mut N: Into<NodeRefMut<'a>>,
+{
+    let mut visitor = ScopeIndexVisitor {
+        index: scope.scope_index(),
+    };
+    visitor.visit(node.into());
+}
+
+struct ScopeIndexVisitor {
+    index: u32,
+}
+
+impl<'ast> VisitorMut<'ast> for ScopeIndexVisitor {
+    type BreakTy = ();
+
+    fn visit_function_declaration_mut(
+        &mut self,
+        node: &'ast mut FunctionDeclaration,
+    ) -> ControlFlow<Self::BreakTy> {
+        self.visit_function_like(
+            &mut node.body,
+            &mut node.parameters,
+            &mut node.scopes,
+            &mut None,
+        )
+    }
+
+    fn visit_generator_declaration_mut(
+        &mut self,
+        node: &'ast mut GeneratorDeclaration,
+    ) -> ControlFlow<Self::BreakTy> {
+        self.visit_function_like(
+            &mut node.body,
+            &mut node.parameters,
+            &mut node.scopes,
+            &mut None,
+        )
+    }
+
+    fn visit_async_function_declaration_mut(
+        &mut self,
+        node: &'ast mut AsyncFunctionDeclaration,
+    ) -> ControlFlow<Self::BreakTy> {
+        self.visit_function_like(
+            &mut node.body,
+            &mut node.parameters,
+            &mut node.scopes,
+            &mut None,
+        )
+    }
+
+    fn visit_async_generator_declaration_mut(
+        &mut self,
+        node: &'ast mut AsyncGeneratorDeclaration,
+    ) -> ControlFlow<Self::BreakTy> {
+        self.visit_function_like(
+            &mut node.body,
+            &mut node.parameters,
+            &mut node.scopes,
+            &mut None,
+        )
+    }
+
+    fn visit_function_expression_mut(
+        &mut self,
+        node: &'ast mut FunctionExpression,
+    ) -> ControlFlow<Self::BreakTy> {
+        self.visit_function_like(
+            &mut node.body,
+            &mut node.parameters,
+            &mut node.scopes,
+            &mut node.name_scope,
+        )
+    }
+
+    fn visit_generator_expression_mut(
+        &mut self,
+        node: &'ast mut GeneratorExpression,
+    ) -> ControlFlow<Self::BreakTy> {
+        self.visit_function_like(
+            &mut node.body,
+            &mut node.parameters,
+            &mut node.scopes,
+            &mut node.name_scope,
+        )
+    }
+
+    fn visit_async_function_expression_mut(
+        &mut self,
+        node: &'ast mut AsyncFunctionExpression,
+    ) -> ControlFlow<Self::BreakTy> {
+        self.visit_function_like(
+            &mut node.body,
+            &mut node.parameters,
+            &mut node.scopes,
+            &mut node.name_scope,
+        )
+    }
+
+    fn visit_async_generator_expression_mut(
+        &mut self,
+        node: &'ast mut AsyncGeneratorExpression,
+    ) -> ControlFlow<Self::BreakTy> {
+        self.visit_function_like(
+            &mut node.body,
+            &mut node.parameters,
+            &mut node.scopes,
+            &mut node.name_scope,
+        )
+    }
+
+    fn visit_arrow_function_mut(
+        &mut self,
+        node: &'ast mut ArrowFunction,
+    ) -> ControlFlow<Self::BreakTy> {
+        self.visit_function_like(
+            &mut node.body,
+            &mut node.parameters,
+            &mut node.scopes,
+            &mut None,
+        )
+    }
+
+    fn visit_async_arrow_function_mut(
+        &mut self,
+        node: &'ast mut AsyncArrowFunction,
+    ) -> ControlFlow<Self::BreakTy> {
+        self.visit_function_like(
+            &mut node.body,
+            &mut node.parameters,
+            &mut node.scopes,
+            &mut None,
+        )
+    }
+
+    fn visit_class_declaration_mut(
+        &mut self,
+        node: &'ast mut ClassDeclaration,
+    ) -> ControlFlow<Self::BreakTy> {
+        let index = self.index;
+        if !node.name_scope.all_bindings_local() {
+            self.index += 1;
+        }
+        node.name_scope.set_index(self.index);
+        if let Some(super_ref) = &mut node.super_ref {
+            try_break!(self.visit_expression_mut(super_ref));
+        }
+        if let Some(constructor) = &mut node.constructor {
+            try_break!(self.visit_function_expression_mut(constructor));
+        }
+        for element in &mut *node.elements {
+            try_break!(self.visit_class_element_mut(element));
+        }
+        self.index = index;
+        ControlFlow::Continue(())
+    }
+
+    fn visit_class_expression_mut(
+        &mut self,
+        node: &'ast mut ClassExpression,
+    ) -> ControlFlow<Self::BreakTy> {
+        let index = self.index;
+        if let Some(scope) = &node.name_scope {
+            if !scope.all_bindings_local() {
+                self.index += 1;
+            }
+            scope.set_index(self.index);
+        }
+        if let Some(super_ref) = &mut node.super_ref {
+            try_break!(self.visit_expression_mut(super_ref));
+        }
+        if let Some(constructor) = &mut node.constructor {
+            try_break!(self.visit_function_expression_mut(constructor));
+        }
+        for element in &mut *node.elements {
+            try_break!(self.visit_class_element_mut(element));
+        }
+        self.index = index;
+        ControlFlow::Continue(())
+    }
+
+    fn visit_class_element_mut(
+        &mut self,
+        node: &'ast mut ClassElement,
+    ) -> ControlFlow<Self::BreakTy> {
+        match node {
+            ClassElement::MethodDefinition(node) => self.visit_function_like(
+                &mut node.body,
+                &mut node.parameters,
+                &mut node.scopes,
+                &mut None,
+            ),
+            ClassElement::FieldDefinition(field) | ClassElement::StaticFieldDefinition(field) => {
+                try_break!(self.visit_property_name_mut(&mut field.name));
+                let index = self.index;
+                self.index += 1;
+                field.scope.set_index(self.index);
+                if let Some(e) = &mut field.field {
+                    try_break!(self.visit_expression_mut(e));
+                }
+                self.index = index;
+                ControlFlow::Continue(())
+            }
+            ClassElement::PrivateFieldDefinition(field) => {
+                let index = self.index;
+                self.index += 1;
+                field.scope.set_index(self.index);
+                if let Some(e) = &mut field.field {
+                    try_break!(self.visit_expression_mut(e));
+                }
+                self.index = index;
+                ControlFlow::Continue(())
+            }
+            ClassElement::PrivateStaticFieldDefinition(_, e) => {
+                if let Some(e) = e {
+                    try_break!(self.visit_expression_mut(e));
+                }
+                ControlFlow::Continue(())
+            }
+            ClassElement::StaticBlock(node) => self.visit_function_like(
+                &mut node.body,
+                &mut FormalParameterList::default(),
+                &mut node.scopes,
+                &mut None,
+            ),
+        }
+    }
+
+    fn visit_object_method_definition_mut(
+        &mut self,
+        node: &'ast mut ObjectMethodDefinition,
+    ) -> ControlFlow<Self::BreakTy> {
+        match &mut node.name {
+            PropertyName::Literal(_) => {}
+            PropertyName::Computed(name) => {
+                try_break!(self.visit_expression_mut(name));
+            }
+        }
+        self.visit_function_like(
+            &mut node.body,
+            &mut node.parameters,
+            &mut node.scopes,
+            &mut None,
+        )
+    }
+
+    fn visit_block_mut(&mut self, node: &'ast mut Block) -> ControlFlow<Self::BreakTy> {
+        let index = self.index;
+        if let Some(scope) = &node.scope {
+            if !scope.all_bindings_local() {
+                self.index += 1;
+            }
+            scope.set_index(self.index);
+        }
+        try_break!(self.visit_statement_list_mut(&mut node.statements));
+        self.index = index;
+        ControlFlow::Continue(())
+    }
+
+    fn visit_switch_mut(&mut self, node: &'ast mut Switch) -> ControlFlow<Self::BreakTy> {
+        let index = self.index;
+        try_break!(self.visit_expression_mut(&mut node.val));
+        if let Some(scope) = &node.scope {
+            if !scope.all_bindings_local() {
+                self.index += 1;
+            }
+            scope.set_index(self.index);
+        }
+        for case in &mut *node.cases {
+            try_break!(self.visit_case_mut(case));
+        }
+        self.index = index;
+        ControlFlow::Continue(())
+    }
+
+    fn visit_with_mut(&mut self, node: &'ast mut With) -> ControlFlow<Self::BreakTy> {
+        let index = self.index;
+        try_break!(self.visit_expression_mut(&mut node.expression));
+        self.index += 1;
+        node.scope.set_index(self.index);
+        try_break!(self.visit_statement_mut(&mut node.statement));
+        self.index = index;
+        ControlFlow::Continue(())
+    }
+
+    fn visit_catch_mut(&mut self, node: &'ast mut Catch) -> ControlFlow<Self::BreakTy> {
+        let index = self.index;
+        if !node.scope.all_bindings_local() {
+            self.index += 1;
+        }
+        node.scope.set_index(self.index);
+        if let Some(binding) = &mut node.parameter {
+            try_break!(self.visit_binding_mut(binding));
+        }
+        try_break!(self.visit_block_mut(&mut node.block));
+        self.index = index;
+        ControlFlow::Continue(())
+    }
+
+    fn visit_for_loop_mut(&mut self, node: &'ast mut ForLoop) -> ControlFlow<Self::BreakTy> {
+        let index = self.index;
+        if let Some(ForLoopInitializer::Lexical(decl)) = &mut node.inner.init {
+            if !decl.scope.all_bindings_local() {
+                self.index += 1;
+            }
+            decl.scope.set_index(self.index);
+        }
+        if let Some(fli) = &mut node.inner.init {
+            try_break!(self.visit_for_loop_initializer_mut(fli));
+        }
+        if let Some(expr) = &mut node.inner.condition {
+            try_break!(self.visit_expression_mut(expr));
+        }
+        if let Some(expr) = &mut node.inner.final_expr {
+            try_break!(self.visit_expression_mut(expr));
+        }
+        self.visit_statement_mut(&mut node.inner.body);
+        self.index = index;
+        ControlFlow::Continue(())
+    }
+
+    fn visit_for_in_loop_mut(&mut self, node: &'ast mut ForInLoop) -> ControlFlow<Self::BreakTy> {
+        {
+            let index = self.index;
+            if let Some(scope) = &node.target_scope {
+                if !scope.all_bindings_local() {
+                    self.index += 1;
+                }
+                scope.set_index(self.index);
+            }
+            try_break!(self.visit_expression_mut(&mut node.target));
+            self.index = index;
+        }
+        let index = self.index;
+        if let Some(scope) = &node.scope {
+            if !scope.all_bindings_local() {
+                self.index += 1;
+            }
+            scope.set_index(self.index);
+        }
+        try_break!(self.visit_iterable_loop_initializer_mut(&mut node.initializer));
+        try_break!(self.visit_statement_mut(&mut node.body));
+        self.index = index;
+        ControlFlow::Continue(())
+    }
+
+    fn visit_for_of_loop_mut(&mut self, node: &'ast mut ForOfLoop) -> ControlFlow<Self::BreakTy> {
+        {
+            let index = self.index;
+            if let Some(scope) = &node.iterable_scope {
+                if !scope.all_bindings_local() {
+                    self.index += 1;
+                }
+                scope.set_index(self.index);
+            }
+            try_break!(self.visit_expression_mut(&mut node.iterable));
+            self.index = index;
+        }
+        let index = self.index;
+        if let Some(scope) = &node.scope {
+            if !scope.all_bindings_local() {
+                self.index += 1;
+            }
+            scope.set_index(self.index);
+        }
+        try_break!(self.visit_iterable_loop_initializer_mut(&mut node.init));
+        try_break!(self.visit_statement_mut(&mut node.body));
+        self.index = index;
+        ControlFlow::Continue(())
+    }
+}
+
+impl ScopeIndexVisitor {
+    fn visit_function_like(
+        &mut self,
+        body: &mut FunctionBody,
+        parameters: &mut FormalParameterList,
+        scopes: &mut FunctionScopes,
+        name_scope: &mut Option<Scope>,
+    ) -> ControlFlow<()> {
+        let index = self.index;
+        if let Some(scope) = name_scope {
+            if !scope.all_bindings_local() {
+                self.index += 1;
+            }
+            scope.set_index(self.index);
+        }
+        self.index += 1;
+        scopes.function_scope.set_index(self.index);
+        if let Some(scope) = &scopes.parameters_eval_scope {
+            if !scope.all_bindings_local() {
+                self.index += 1;
+            }
+            scope.set_index(self.index);
+        }
+        try_break!(self.visit_formal_parameter_list_mut(parameters));
+        if let Some(scope) = &scopes.parameters_scope {
+            if !scope.all_bindings_local() {
+                self.index += 1;
+            }
+            scope.set_index(self.index);
+        }
+        if let Some(scope) = &scopes.lexical_scope {
+            if !scope.all_bindings_local() {
+                self.index += 1;
+            }
+            scope.set_index(self.index);
+        }
+        try_break!(self.visit_function_body_mut(body));
+        self.index = index;
+        ControlFlow::Continue(())
+    }
+}
+
 /// `GlobalDeclarationInstantiation ( script, env )`
 ///
 /// More information:

core/ast/src/source.rs

@@ -7,7 +7,7 @@ use crate::{
     scope::Scope,
     scope_analyzer::{
         analyze_binding_escapes, collect_bindings, eval_declaration_instantiation_scope,
-        EvalDeclarationBindings,
+        optimize_scope_indicies, EvalDeclarationBindings,
     },
     visitor::{VisitWith, Visitor, VisitorMut},
     ModuleItemList, StatementList,
@@ -56,7 +56,11 @@ impl Script {
         if !collect_bindings(self, self.strict(), false, scope, interner) {
             return false;
         }
-        analyze_binding_escapes(self, false, scope.clone(), interner)
+        if !analyze_binding_escapes(self, false, scope.clone(), interner) {
+            return false;
+        }
+        optimize_scope_indicies(self, scope);
+        true
     }
 
     /// Analyze the scope of the script in eval mode.
@@ -84,10 +88,10 @@ impl Script {
         if !collect_bindings(self, strict, true, lexical_scope, interner) {
             return Err(String::from("Failed to analyze scope"));
         }
-
         if !analyze_binding_escapes(self, true, lexical_scope.clone(), interner) {
             return Err(String::from("Failed to analyze scope"));
         }
+        optimize_scope_indicies(self, lexical_scope);
 
         Ok(bindings)
     }
@@ -158,7 +162,11 @@ impl Module {
         if !collect_bindings(self, true, false, scope, interner) {
             return false;
         }
-        analyze_binding_escapes(self, false, scope.clone(), interner)
+        if !analyze_binding_escapes(self, false, scope.clone(), interner) {
+            return false;
+        }
+        optimize_scope_indicies(self, &self.scope.clone());
+        true
     }
 }
 

core/engine/src/bytecompiler/class.rs

@@ -79,14 +79,7 @@ impl ByteCompiler<'_> {
             .map_or(Sym::EMPTY_STRING, Identifier::sym)
             .to_js_string(self.interner());
 
-        let outer_scope = if let Some(name_scope) = class.name_scope {
+        let outer_scope = self.push_declarative_scope(class.name_scope);
-            let outer_scope = self.lexical_scope.clone();
-            let scope_index = self.push_scope(name_scope);
-            self.emit_with_varying_operand(Opcode::PushScope, scope_index);
-            Some(outer_scope)
-        } else {
-            None
-        };
 
         let mut compiler = ByteCompiler::new(
             class_name.clone(),
@@ -181,9 +174,11 @@ impl ByteCompiler<'_> {
         let mut static_elements = Vec::new();
         let mut static_field_name_count = 0;
 
-        if outer_scope.is_some() {
+        if let Some(scope) = class.name_scope {
+            let binding = scope.get_identifier_reference(class_name.clone());
+            let index = self.get_or_insert_binding(binding);
             self.emit_opcode(Opcode::Dup);
-            self.emit_binding(BindingOpcode::InitLexical, class_name.clone());
+            self.emit_binding_access(Opcode::PutLexicalValue, &index);
         }
 
         for element in class.elements {
@@ -474,12 +469,7 @@ impl ByteCompiler<'_> {
         self.emit_opcode(Opcode::Swap);
         self.emit_opcode(Opcode::Pop);
 
-        if let Some(outer_scope) = outer_scope {
+        self.pop_declarative_scope(outer_scope);
-            self.pop_scope();
-            self.lexical_scope = outer_scope;
-            self.emit_opcode(Opcode::PopEnvironment);
-        }
-
         self.emit_opcode(Opcode::PopPrivateEnvironment);
 
         if !expression {

core/engine/src/bytecompiler/declarations.rs

@@ -963,10 +963,7 @@ impl ByteCompiler<'_> {
         }
 
         // 19-20
-        if let Some(scope) = scopes.parameters_eval_scope() {
+        drop(self.push_declarative_scope(scopes.parameters_eval_scope()));
-            let scope_index = self.push_scope(scope);
-            self.emit_with_varying_operand(Opcode::PushScope, scope_index);
-        }
 
         let scope = self.lexical_scope.clone();
 
@@ -1057,8 +1054,7 @@ impl ByteCompiler<'_> {
                 //          visibility of declarations in the function body.
                 // b. Let varEnv be NewDeclarativeEnvironment(env).
                 // c. Set the VariableEnvironment of calleeContext to varEnv.
-                let scope_index = self.push_scope(scope);
+                drop(self.push_declarative_scope(Some(scope)));
-                self.emit_with_varying_operand(Opcode::PushScope, scope_index);
 
                 let mut variable_scope = self.lexical_scope.clone();
 
@@ -1177,10 +1173,7 @@ impl ByteCompiler<'_> {
         }
 
         // 30-31
-        if let Some(scope) = scopes.lexical_scope() {
+        drop(self.push_declarative_scope(scopes.lexical_scope()));
-            let scope_index = self.push_scope(scope);
-            self.emit_with_varying_operand(Opcode::PushScope, scope_index);
-        }
 
         // 35. Let privateEnv be the PrivateEnvironment of calleeContext.
         // 36. For each Parse Node f of functionsToInitialize, do

core/engine/src/bytecompiler/env.rs

@@ -1,5 +1,7 @@
 use boa_ast::scope::Scope;
 
+use crate::vm::{Constant, Opcode};
+
 use super::ByteCompiler;
 
 impl ByteCompiler<'_> {
@@ -9,8 +11,7 @@ impl ByteCompiler<'_> {
         self.current_open_environments_count += 1;
 
         let index = self.constants.len() as u32;
-        self.constants
+        self.constants.push(Constant::Scope(scope.clone()));
-            .push(crate::vm::Constant::Scope(scope.clone()));
 
         if scope.is_function() {
             self.variable_scope = scope.clone();
@@ -21,6 +22,33 @@ impl ByteCompiler<'_> {
         index
     }
 
+    /// Push a declarative scope.
+    ///
+    /// Returns the outer scope.
+    #[must_use]
+    pub(crate) fn push_declarative_scope(&mut self, scope: Option<&Scope>) -> Option<Scope> {
+        let mut scope = scope?.clone();
+        if !scope.all_bindings_local() {
+            self.current_open_environments_count += 1;
+            let index = self.constants.len() as u32;
+            self.constants.push(Constant::Scope(scope.clone()));
+            self.emit_with_varying_operand(Opcode::PushScope, index);
+        }
+        std::mem::swap(&mut self.lexical_scope, &mut scope);
+        Some(scope)
+    }
+
+    /// Pop a declarative scope.
+    pub(crate) fn pop_declarative_scope(&mut self, scope: Option<Scope>) {
+        if let Some(mut scope) = scope {
+            std::mem::swap(&mut self.lexical_scope, &mut scope);
+            if !scope.all_bindings_local() {
+                self.current_open_environments_count -= 1;
+                self.emit_opcode(Opcode::PopEnvironment);
+            }
+        }
+    }
+
     /// Pops the top scope.
     pub(crate) fn pop_scope(&mut self) {
         self.current_open_environments_count -= 1;

core/engine/src/bytecompiler/function.rs

@@ -129,8 +129,10 @@ impl FunctionCompiler {
         }
 
         if let Some(scope) = self.name_scope {
-            compiler.code_block_flags |= CodeBlockFlags::HAS_BINDING_IDENTIFIER;
+            if !scope.all_bindings_local() {
-            let _ = compiler.push_scope(&scope);
+                compiler.code_block_flags |= CodeBlockFlags::HAS_BINDING_IDENTIFIER;
+                let _ = compiler.push_scope(&scope);
+            }
         }
         // Function environment
         let _ = compiler.push_scope(scopes.function_scope());

core/engine/src/bytecompiler/statement/block.rs

@@ -1,25 +1,12 @@
-use crate::{bytecompiler::ByteCompiler, vm::Opcode};
+use crate::bytecompiler::ByteCompiler;
 use boa_ast::statement::Block;
 
 impl ByteCompiler<'_> {
     /// Compile a [`Block`] `boa_ast` node
     pub(crate) fn compile_block(&mut self, block: &Block, use_expr: bool) {
-        let outer_scope = if let Some(scope) = block.scope() {
+        let scope = self.push_declarative_scope(block.scope());
-            let outer_scope = self.lexical_scope.clone();
-            let scope_index = self.push_scope(scope);
-            self.emit_with_varying_operand(Opcode::PushScope, scope_index);
-            Some(outer_scope)
-        } else {
-            None
-        };
-
         self.block_declaration_instantiation(block);
         self.compile_statement_list(block.statement_list(), use_expr, true);
-
+        self.pop_declarative_scope(scope);
-        if let Some(outer_scope) = outer_scope {
-            self.pop_scope();
-            self.lexical_scope = outer_scope;
-            self.emit_opcode(Opcode::PopEnvironment);
-        }
     }
 }

core/engine/src/bytecompiler/statement/loop.rs

@@ -22,6 +22,7 @@ impl ByteCompiler<'_> {
         use_expr: bool,
     ) {
         let mut let_binding_indices = None;
+        let mut outer_scope_local = None;
         let mut outer_scope = None;
 
         if let Some(init) = for_loop.init() {
@@ -31,9 +32,16 @@ impl ByteCompiler<'_> {
                     self.compile_var_decl(decl);
                 }
                 ForLoopInitializer::Lexical(decl) => {
-                    outer_scope = Some(self.lexical_scope.clone());
+                    let scope_index = if decl.scope().all_bindings_local() {
-                    let scope_index = self.push_scope(decl.scope());
+                        outer_scope_local = Some(self.lexical_scope.clone());
-                    self.emit_with_varying_operand(Opcode::PushScope, scope_index);
+                        self.lexical_scope = decl.scope().clone();
+                        None
+                    } else {
+                        outer_scope = Some(self.lexical_scope.clone());
+                        let scope_index = self.push_scope(decl.scope());
+                        self.emit_with_varying_operand(Opcode::PushScope, scope_index);
+                        Some(scope_index)
+                    };
 
                     let names = bound_names(decl.declaration());
                     if decl.declaration().is_const() {
@@ -41,8 +49,8 @@ impl ByteCompiler<'_> {
                         let mut indices = Vec::new();
                         for name in &names {
                             let name = name.to_js_string(self.interner());
-                            let binding = self
+                            let binding = decl
-                                .lexical_scope
+                                .scope()
                                 .get_binding_reference(&name)
                                 .expect("binding must exist");
                             let index = self.get_or_insert_binding(binding);
@@ -62,8 +70,10 @@ impl ByteCompiler<'_> {
                 self.emit_binding_access(Opcode::GetName, index);
             }
 
-            self.emit_opcode(Opcode::PopEnvironment);
+            if let Some(index) = scope_index {
-            self.emit_with_varying_operand(Opcode::PushScope, *scope_index);
+                self.emit_opcode(Opcode::PopEnvironment);
+                self.emit_with_varying_operand(Opcode::PushScope, *index);
+            }
 
             for index in let_binding_indices.iter().rev() {
                 self.emit_binding_access(Opcode::PutLexicalValue, index);
@@ -85,8 +95,10 @@ impl ByteCompiler<'_> {
                 self.emit_binding_access(Opcode::GetName, index);
             }
 
-            self.emit_opcode(Opcode::PopEnvironment);
+            if let Some(index) = scope_index {
-            self.emit_with_varying_operand(Opcode::PushScope, *scope_index);
+                self.emit_opcode(Opcode::PopEnvironment);
+                self.emit_with_varying_operand(Opcode::PushScope, *index);
+            }
 
             for index in let_binding_indices.iter().rev() {
                 self.emit_binding_access(Opcode::PutLexicalValue, index);
@@ -115,11 +127,10 @@ impl ByteCompiler<'_> {
         self.patch_jump(exit);
         self.pop_loop_control_info();
 
-        if let Some(outer_scope) = outer_scope {
+        if let Some(outer_scope_local) = outer_scope_local {
-            self.pop_scope();
+            self.lexical_scope = outer_scope_local;
-            self.lexical_scope = outer_scope;
-            self.emit_opcode(Opcode::PopEnvironment);
         }
+        self.pop_declarative_scope(outer_scope);
     }
 
     pub(crate) fn compile_for_in_loop(
@@ -138,17 +149,9 @@ impl ByteCompiler<'_> {
                 }
             }
         }
-        if let Some(scope) = for_in_loop.target_scope() {
+        let outer_scope = self.push_declarative_scope(for_in_loop.target_scope());
-            let outer_scope = self.lexical_scope.clone();
+        self.compile_expr(for_in_loop.target(), true);
-            let scope_index = self.push_scope(scope);
+        self.pop_declarative_scope(outer_scope);
-            self.emit_with_varying_operand(Opcode::PushScope, scope_index);
-            self.compile_expr(for_in_loop.target(), true);
-            self.pop_scope();
-            self.lexical_scope = outer_scope;
-            self.emit_opcode(Opcode::PopEnvironment);
-        } else {
-            self.compile_expr(for_in_loop.target(), true);
-        }
 
         let early_exit = self.jump_if_null_or_undefined();
         self.emit_opcode(Opcode::CreateForInIterator);
@@ -163,13 +166,7 @@ impl ByteCompiler<'_> {
 
         self.emit_opcode(Opcode::IteratorValue);
 
-        let mut outer_scope = None;
+        let outer_scope = self.push_declarative_scope(for_in_loop.scope());
-
-        if let Some(scope) = for_in_loop.scope() {
-            outer_scope = Some(self.lexical_scope.clone());
-            let scope_index = self.push_scope(scope);
-            self.emit_with_varying_operand(Opcode::PushScope, scope_index);
-        }
 
         match for_in_loop.initializer() {
             IterableLoopInitializer::Identifier(ident) => {
@@ -208,12 +205,7 @@ impl ByteCompiler<'_> {
         }
 
         self.compile_stmt(for_in_loop.body(), use_expr, true);
-
+        self.pop_declarative_scope(outer_scope);
-        if let Some(outer_scope) = outer_scope {
-            self.pop_scope();
-            self.lexical_scope = outer_scope;
-            self.emit_opcode(Opcode::PopEnvironment);
-        }
 
         self.emit(Opcode::Jump, &[Operand::U32(start_address)]);
 
@@ -234,17 +226,9 @@ impl ByteCompiler<'_> {
         label: Option<Sym>,
         use_expr: bool,
     ) {
-        if let Some(scope) = for_of_loop.iterable_scope() {
+        let outer_scope = self.push_declarative_scope(for_of_loop.iterable_scope());
-            let outer_scope = self.lexical_scope.clone();
+        self.compile_expr(for_of_loop.iterable(), true);
-            let scope_index = self.push_scope(scope);
+        self.pop_declarative_scope(outer_scope);
-            self.emit_with_varying_operand(Opcode::PushScope, scope_index);
-            self.compile_expr(for_of_loop.iterable(), true);
-            self.pop_scope();
-            self.lexical_scope = outer_scope;
-            self.emit_opcode(Opcode::PopEnvironment);
-        } else {
-            self.compile_expr(for_of_loop.iterable(), true);
-        }
 
         if for_of_loop.r#await() {
             self.emit_opcode(Opcode::GetAsyncIterator);
@@ -271,14 +255,7 @@ impl ByteCompiler<'_> {
         let exit = self.jump_if_true();
         self.emit_opcode(Opcode::IteratorValue);
 
-        let mut outer_scope = None;
+        let outer_scope = self.push_declarative_scope(for_of_loop.scope());
-
-        if let Some(scope) = for_of_loop.scope() {
-            outer_scope = Some(self.lexical_scope.clone());
-            let scope_index = self.push_scope(scope);
-            self.emit_with_varying_operand(Opcode::PushScope, scope_index);
-        }
-
         let handler_index = self.push_handler();
 
         match for_of_loop.initializer() {
@@ -354,12 +331,7 @@ impl ByteCompiler<'_> {
             self.patch_jump(exit);
         }
 
-        if let Some(outer_scope) = outer_scope {
+        self.pop_declarative_scope(outer_scope);
-            self.pop_scope();
-            self.lexical_scope = outer_scope;
-            self.emit_opcode(Opcode::PopEnvironment);
-        }
-
         self.emit(Opcode::Jump, &[Operand::U32(start_address)]);
 
         self.patch_jump(exit);

core/engine/src/bytecompiler/statement/switch.rs

@@ -5,15 +5,7 @@ impl ByteCompiler<'_> {
     /// Compile a [`Switch`] `boa_ast` node
     pub(crate) fn compile_switch(&mut self, switch: &Switch, use_expr: bool) {
         self.compile_expr(switch.val(), true);
-
+        let outer_scope = self.push_declarative_scope(switch.scope());
-        let outer_scope = if let Some(scope) = switch.scope() {
-            let outer_scope = self.lexical_scope.clone();
-            let scope_index = self.push_scope(scope);
-            self.emit_with_varying_operand(Opcode::PushScope, scope_index);
-            Some(outer_scope)
-        } else {
-            None
-        };
 
         self.block_declaration_instantiation(switch);
 
@@ -55,11 +47,6 @@ impl ByteCompiler<'_> {
         }
 
         self.pop_switch_control_info();
-
+        self.pop_declarative_scope(outer_scope);
-        if let Some(outer_scope) = outer_scope {
-            self.pop_scope();
-            self.lexical_scope = outer_scope;
-            self.emit_opcode(Opcode::PopEnvironment);
-        }
     }
 }

core/engine/src/bytecompiler/statement/try.rs

@@ -108,11 +108,7 @@ impl ByteCompiler<'_> {
     }
 
     pub(crate) fn compile_catch_stmt(&mut self, catch: &Catch, _has_finally: bool, use_expr: bool) {
-        // stack: exception
+        let outer_scope = self.push_declarative_scope(Some(catch.scope()));
-
-        let outer_scope = self.lexical_scope.clone();
-        let scope_index = self.push_scope(catch.scope());
-        self.emit_with_varying_operand(Opcode::PushScope, scope_index);
 
         if let Some(binding) = catch.parameter() {
             match binding {
@@ -130,9 +126,7 @@ impl ByteCompiler<'_> {
 
         self.compile_catch_finally_block(catch.block(), use_expr);
 
-        self.pop_scope();
+        self.pop_declarative_scope(outer_scope);
-        self.lexical_scope = outer_scope;
-        self.emit_opcode(Opcode::PopEnvironment);
     }
 
     pub(crate) fn compile_finally_stmt(&mut self, finally: &Finally, has_catch: bool) {