Remove `FormalParameterList` from CodeBlock (#3882)

It is not used in strict mode and in non-strict mode it's only used to
construct the binding indices for the `MappedArguments`.

core/engine/src/builtins/function/arguments.rs

@@ -13,6 +13,7 @@ use crate::{
 use boa_ast::{function::FormalParameterList, operations::bound_names};
 use boa_gc::{Finalize, Gc, Trace};
 use rustc_hash::FxHashMap;
+use thin_vec::{thin_vec, ThinVec};
 
 #[derive(Debug, Copy, Clone, Trace, Finalize, JsData)]
 #[boa_gc(empty_trace)]
@@ -137,30 +138,7 @@ impl MappedArguments {
 }
 
 impl MappedArguments {
-    /// Creates a new mapped Arguments exotic object.
+    pub(crate) fn binding_indices(formals: &FormalParameterList) -> ThinVec<Option<u32>> {
-    ///
-    /// <https://tc39.es/ecma262/#sec-createmappedargumentsobject>
-    #[allow(clippy::new_ret_no_self)]
-    pub(crate) fn new(
-        func: &JsObject,
-        formals: &FormalParameterList,
-        arguments_list: &[JsValue],
-        env: &Gc<DeclarativeEnvironment>,
-        context: &mut Context,
-    ) -> JsObject {
-        // 1. Assert: formals does not contain a rest parameter, any binding patterns, or any initializers.
-        // It may contain duplicate identifiers.
-        // 2. Let len be the number of elements in argumentsList.
-        let len = arguments_list.len();
-
-        // 3. Let obj be ! MakeBasicObject(« [[Prototype]], [[Extensible]], [[ParameterMap]] »).
-        // 4. Set obj.[[GetOwnProperty]] as specified in 10.4.4.1.
-        // 5. Set obj.[[DefineOwnProperty]] as specified in 10.4.4.2.
-        // 6. Set obj.[[Get]] as specified in 10.4.4.3.
-        // 7. Set obj.[[Set]] as specified in 10.4.4.4.
-        // 8. Set obj.[[Delete]] as specified in 10.4.4.5.
-        // 9. Set obj.[[Prototype]] to %Object.prototype%.
-
         // Section 17-19 are done first, for easier object creation in 11.
         //
         // The section 17-19 differs from the spec, due to the way the runtime environments work.
@@ -196,14 +174,9 @@ impl MappedArguments {
         // In the case of duplicate parameter names, the last one is bound as the environment binding.
         //
         // The following logic implements the steps 17-19 adjusted for our environment structure.
-
         let mut bindings = FxHashMap::default();
         let mut property_index = 0;
         for name in bound_names(formals) {
-            if property_index >= len {
-                break;
-            }
-
             // NOTE(HalidOdat): Offset by +1 to account for the first binding ("argument").
             let binding_index = bindings.len() as u32 + 1;
 
@@ -215,15 +188,44 @@ impl MappedArguments {
             property_index += 1;
         }
 
-        let mut map = MappedArguments {
+        let mut binding_indices = thin_vec![None; property_index];
-            binding_indices: vec![None; property_index],
-            environment: env.clone(),
-        };
-
         for (binding_index, property_index) in bindings.values() {
-            map.binding_indices[*property_index] = Some(*binding_index);
+            binding_indices[*property_index] = Some(*binding_index);
         }
 
+        binding_indices
+    }
+
+    /// Creates a new mapped Arguments exotic object.
+    ///
+    /// <https://tc39.es/ecma262/#sec-createmappedargumentsobject>
+    #[allow(clippy::new_ret_no_self)]
+    pub(crate) fn new(
+        func: &JsObject,
+        binding_indices: &[Option<u32>],
+        arguments_list: &[JsValue],
+        env: &Gc<DeclarativeEnvironment>,
+        context: &mut Context,
+    ) -> JsObject {
+        // 1. Assert: formals does not contain a rest parameter, any binding patterns, or any initializers.
+        // It may contain duplicate identifiers.
+        // 2. Let len be the number of elements in argumentsList.
+        let len = arguments_list.len();
+
+        // 3. Let obj be ! MakeBasicObject(« [[Prototype]], [[Extensible]], [[ParameterMap]] »).
+        // 4. Set obj.[[GetOwnProperty]] as specified in 10.4.4.1.
+        // 5. Set obj.[[DefineOwnProperty]] as specified in 10.4.4.2.
+        // 6. Set obj.[[Get]] as specified in 10.4.4.3.
+        // 7. Set obj.[[Set]] as specified in 10.4.4.4.
+        // 8. Set obj.[[Delete]] as specified in 10.4.4.5.
+        // 9. Set obj.[[Prototype]] to %Object.prototype%.
+
+        let range = binding_indices.len().min(len);
+        let map = MappedArguments {
+            binding_indices: binding_indices[..range].to_vec(),
+            environment: env.clone(),
+        };
+
         // %Array.prototype.values%
         let values_function = context.intrinsics().objects().array_prototype_values();
 

core/engine/src/bytecompiler/declarations.rs

@@ -1182,6 +1182,7 @@ impl ByteCompiler<'_> {
                 //          default value initializers, or any destructured parameters.
                 // ii. Let ao be CreateMappedArgumentsObject(func, formals, argumentsList, env).
                 self.emit_opcode(Opcode::CreateMappedArgumentsObject);
+                self.emitted_mapped_arguments_object_opcode = true;
             }
 
             // c. If strict is true, then

core/engine/src/bytecompiler/mod.rs

@@ -14,7 +14,7 @@ mod utils;
 use std::{cell::Cell, rc::Rc};
 
 use crate::{
-    builtins::function::ThisMode,
+    builtins::function::{arguments::MappedArguments, ThisMode},
     environments::{BindingLocator, BindingLocatorError, CompileTimeEnvironment},
     js_string,
     vm::{
@@ -307,6 +307,9 @@ pub struct ByteCompiler<'ctx> {
     /// Whether the function is in a `with` statement.
     pub(crate) in_with: bool,
 
+    /// Used to determine if a we emited a `CreateUnmappedArgumentsObject` opcode
+    pub(crate) emitted_mapped_arguments_object_opcode: bool,
+
     pub(crate) interner: &'ctx mut Interner,
 
     #[cfg(feature = "annex-b")]
@@ -361,6 +364,7 @@ impl<'ctx> ByteCompiler<'ctx> {
             #[cfg(feature = "annex-b")]
             annex_b_function_names: Vec::new(),
             in_with,
+            emitted_mapped_arguments_object_opcode: false,
         }
     }
 
@@ -1575,12 +1579,19 @@ impl<'ctx> ByteCompiler<'ctx> {
             handler.stack_count += self.register_count;
         }
 
+        let mapped_arguments_binding_indices = if self.emitted_mapped_arguments_object_opcode {
+            MappedArguments::binding_indices(&self.params)
+        } else {
+            ThinVec::new()
+        };
+
         CodeBlock {
             name: self.function_name,
             length: self.length,
             register_count: self.register_count,
             this_mode: self.this_mode,
-            params: self.params,
+            parameter_length: self.params.as_ref().len() as u32,
+            mapped_arguments_binding_indices,
             bytecode: self.bytecode.into_boxed_slice(),
             constants: self.constants,
             bindings: self.bindings.into_boxed_slice(),

core/engine/src/vm/code_block.rs

@@ -12,7 +12,6 @@ use crate::{
     Context, JsBigInt, JsString, JsValue,
 };
 use bitflags::bitflags;
-use boa_ast::function::FormalParameterList;
 use boa_gc::{empty_trace, Finalize, Gc, Trace};
 use boa_profiler::Profiler;
 use std::{cell::Cell, fmt::Display, mem::size_of, rc::Rc};
@@ -137,14 +136,16 @@ pub struct CodeBlock {
     /// The number of arguments expected.
     pub(crate) length: u32,
 
+    pub(crate) parameter_length: u32,
+
     pub(crate) register_count: u32,
 
     /// `[[ThisMode]]`
     pub(crate) this_mode: ThisMode,
 
-    /// Parameters passed to this function.
+    /// Used for constructing a `MappedArguments` object.
     #[unsafe_ignore_trace]
-    pub(crate) params: FormalParameterList,
+    pub(crate) mapped_arguments_binding_indices: ThinVec<Option<u32>>,
 
     /// Bytecode
     #[unsafe_ignore_trace]
@@ -180,7 +181,8 @@ impl CodeBlock {
             length,
             register_count: 0,
             this_mode: ThisMode::Global,
-            params: FormalParameterList::default(),
+            mapped_arguments_binding_indices: ThinVec::new(),
+            parameter_length: 0,
             handlers: ThinVec::default(),
             ic: Box::default(),
         }

core/engine/src/vm/opcode/arguments.rs

@@ -31,7 +31,7 @@ impl Operation for CreateMappedArgumentsObject {
         let env = context.vm.environments.current();
         let arguments = MappedArguments::new(
             &function_object,
-            &code.params,
+            &code.mapped_arguments_binding_indices,
             &args,
             env.declarative_expect(),
             context,

core/engine/src/vm/opcode/rest_parameter/mod.rs

@@ -17,12 +17,12 @@ impl Operation for RestParameterInit {
     const COST: u8 = 6;
 
     fn execute(context: &mut Context) -> JsResult<CompletionType> {
-        let argument_count = context.vm.frame().argument_count as usize;
+        let argument_count = context.vm.frame().argument_count;
-        let param_count = context.vm.frame().code_block().params.as_ref().len();
+        let param_count = context.vm.frame().code_block().parameter_length;
 
         let array = if argument_count >= param_count {
             let rest_count = argument_count - param_count + 1;
-            let args = context.vm.pop_n_values(rest_count);
+            let args = context.vm.pop_n_values(rest_count as usize);
             Array::create_array_from_list(args, context)
         } else {
             Array::array_create(0, None, context).expect("could not create an empty array")