Optimize shift for dense arrays (#3405)

* Optimize `Array.prototype.shift` for dense arrays

* Optimize shifts for dense arrays

boa_engine/src/builtins/array/mod.rs

@@ -1157,6 +1157,22 @@ impl Array {
             // b. Return undefined.
             return Ok(JsValue::undefined());
         }
+
+        // Small optimization for arrays using dense properties
+        // TODO: this optimization could be generalized to many other objects with
+        // slot-based dense property maps.
+        if o.is_array() {
+            let mut o_borrow = o.borrow_mut();
+            if let Some(dense) = o_borrow.properties_mut().dense_indexed_properties_mut() {
+                if len <= dense.len() as u64 {
+                    let v = dense.remove(0);
+                    drop(o_borrow);
+                    Self::set_length(&o, len - 1, context)?;
+                    return Ok(v);
+                }
+            }
+        }
+
         // 4. Let first be ? Get(O, "0").
         let first = o.get(0, context)?;
         // 5. Let k be 1.

boa_engine/src/object/property_map.rs

@@ -55,7 +55,7 @@ enum IndexedProperties {
     /// are not data descriptors with with a value field, writable field set to `true`, configurable field set to `true`, enumerable field set to `true`.
     ///
     /// This method uses more space, since we also have to store the property descriptors, not just the value.
-    /// It is also slower because we need to to a hash lookup.
+    /// It is also slower because we need to do a hash lookup.
     Sparse(Box<FxHashMap<u32, PropertyDescriptor>>),
 }
 
@@ -150,7 +150,7 @@ impl IndexedProperties {
         replaced
     }
 
-    /// Inserts a property descriptor with the specified key.
+    /// Removes a property descriptor with the specified key.
     fn remove(&mut self, key: u32) -> bool {
         let vec = match self {
             Self::Sparse(map) => {