BlitzMax optimization - Object casting

With the exception of numeric types (Byte, Short, Int, Float, Double and Long), all objects in BlitzMax extend the from base Object type. This comes in handy for passing around variables that can be different types, but there’s some overhead when converting (i.e. casting) an Object to a different type.

For example:

Type MyType
    Field something:String
     
    Method New()
         Self.something = "Hello, World!"
    End Method
End Type

' Create a new MyType but assign it to an Object variable.
Local o:Object = new MyType

' The following code is invalid:
' Print o.something

' The variable needs to be cast first:
Print MyType(o).something

There’s normally no need to use plain Object types, but collections like TMap and TList use them for storing generic objects. This makes it easier to create lists on the fly, but comes with some overhead.

Summary

1. Casting an object is approximately 8x slower than not casting.
2. The size of the object (i.e. the number of fields and methods) does not affect performance.
3. Casting happens automatically when iterating over a collection (such as a TList). If possible, use arrays or a strongly-typed list if frequently iterating over the same collection.

Casting benchmarks

All benchmarks are in release mode with threading enabled. Each test was executed 1,000,000,000 times.

The size of the object makes no difference. The same as the previous test, but with a type containing 2,000 fields:

Reducing casting

For collections that are large, or iterated over frequently, a strongly-typed collection will help with performance. If the size of the collection remains the same, an array will be fastest. A strongly-typed ObjectBag is almost as fast but can also be resized on the fly.

Test code

SuperStrict
Framework brl.basic

Const ITERATIONS:Int = 1000000000
Local startTime:Int
Local counter:int

' -- No Casting
Local result1:Object
Local o:Object = new MyType
startTime = millisecs()
For local i:int = 1 to ITERATIONS
	result1 = o
Next
Print "No Cast: " + (millisecs() - startTime) 

' -- Casting
Local result2:MyType
startTime = millisecs()
For local i:int = 1 to ITERATIONS
	result2 = MyType(o)
Next
Print "Cast: " + (millisecs() - startTime) 

Type MyType
     Field something:String
     Field something_else:Int
End Type