Shrink objects by using `ThinVec`s (#2752)

The fields like `[[PrivateElements]]` are hardly used but they occupy `24` bytes (on 64-bit arch.) the `ThinVec` type stores the `len` and `cap` right before the elements (like our `JsString` implementation) and only a pointer is kept (if not used it does not allocate!), was going to use this in #2723 , since it uses a `Vec` as a dense storage, but the PR is already too big.

It changes the following:

- Shrink object from `328` to `288` bytes (40 bytes reduction)
- Add the `thin_vec` lightweight crate (single file)

Cargo.lock

@@ -373,6 +373,7 @@ dependencies = [
  "sys-locale",
  "tap",
  "textwrap 0.16.0",
+ "thin-vec",
  "thiserror",
  "unicode-normalization",
  "writeable",
@@ -3984,6 +3985,12 @@ dependencies = [
  "unicode-width",
 ]
 
+[[package]]
+name = "thin-vec"
+version = "0.2.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "aac81b6fd6beb5884b0cf3321b8117e6e5d47ecb6fc89f414cfdcca8b2fe2dd8"
+
 [[package]]
 name = "thiserror"
 version = "1.0.40"

boa_engine/Cargo.toml

@@ -68,6 +68,7 @@ thiserror = "1.0.40"
 dashmap = "5.4.0"
 num_enum = "0.5.11"
 pollster = "0.3.0"
+thin-vec = "0.2.12"
 
 # intl deps
 boa_icu_provider = { workspace = true, optional = true }

boa_engine/src/builtins/function/mod.rs

@@ -36,6 +36,7 @@ use boa_gc::{self, custom_trace, Finalize, Gc, Trace};
 use boa_interner::Sym;
 use boa_parser::{Parser, Source};
 use boa_profiler::Profiler;
+use thin_vec::ThinVec;
 
 use std::fmt;
 
@@ -172,10 +173,10 @@ pub enum Function {
         home_object: Option<JsObject>,
 
         /// The `[[Fields]]` internal slot.
-        fields: Vec<ClassFieldDefinition>,
+        fields: ThinVec<ClassFieldDefinition>,
 
         /// The `[[PrivateMethods]]` internal slot.
-        private_methods: Vec<(PrivateName, PrivateElement)>,
+        private_methods: ThinVec<(PrivateName, PrivateElement)>,
 
         /// The class object that this function is associated with.
         class_object: Option<JsObject>,
@@ -238,7 +239,9 @@ unsafe impl Trace for Function {
                 mark(code);
                 mark(environments);
                 mark(home_object);
-                mark(fields);
+                for elem in fields {
+                    mark(elem);
+                }
                 for (_, elem) in private_methods {
                     mark(elem);
                 }

boa_engine/src/object/jsobject.rs

@@ -19,6 +19,7 @@ use std::{
     fmt::{self, Debug, Display},
     result::Result as StdResult,
 };
+use thin_vec::ThinVec;
 
 /// A wrapper type for an immutably borrowed type T.
 pub type Ref<'a, T> = boa_gc::GcRef<'a, T>;
@@ -74,7 +75,7 @@ impl JsObject {
                 prototype: prototype.into(),
                 extensible: true,
                 properties: PropertyMap::default(),
-                private_elements: Vec::new(),
+                private_elements: ThinVec::new(),
             })),
         }
     }

boa_engine/src/object/mod.rs

@@ -6,6 +6,7 @@ use boa_ast::function::PrivateName;
 pub use jsobject::{RecursionLimiter, Ref, RefMut};
 pub use operations::IntegrityLevel;
 pub use property_map::*;
+use thin_vec::ThinVec;
 
 use self::internal_methods::{
     arguments::ARGUMENTS_EXOTIC_INTERNAL_METHODS,
@@ -128,7 +129,7 @@ pub struct Object {
     /// Whether it can have new properties added to it.
     extensible: bool,
     /// The `[[PrivateElements]]` internal slot.
-    private_elements: Vec<(PrivateName, PrivateElement)>,
+    private_elements: ThinVec<(PrivateName, PrivateElement)>,
 }
 
 unsafe impl Trace for Object {
@@ -782,7 +783,7 @@ impl Default for Object {
             properties: PropertyMap::default(),
             prototype: None,
             extensible: true,
-            private_elements: Vec::default(),
+            private_elements: ThinVec::default(),
         }
     }
 }

boa_engine/src/vm/code_block.rs

@@ -29,6 +29,7 @@ use boa_gc::{Finalize, Gc, GcRefCell, Trace};
 use boa_interner::Sym;
 use boa_profiler::Profiler;
 use std::{collections::VecDeque, mem::size_of};
+use thin_vec::ThinVec;
 
 #[cfg(any(feature = "trace", feature = "flowgraph"))]
 use crate::vm::Opcode;
@@ -597,8 +598,8 @@ pub(crate) fn create_function_object(
             environments: context.realm.environments.clone(),
             constructor_kind: ConstructorKind::Base,
             home_object: None,
-            fields: Vec::new(),
+            fields: ThinVec::new(),
-            private_methods: Vec::new(),
+            private_methods: ThinVec::new(),
             class_object: None,
         }
     };