Breaking the JavaScript Speed Limit with V8 - Google IO 2012

The Optimization Checklist

• Be prepared before you have a problem
• Identify and understand the crux of your problem
• Fix what matters

What does “Be Prepared” mean for V8?

• Understand how V8 optimizes JavaScript
• Write code mindfully
• Learn your tools and how they can help you

Hidden Classes Make JavaScript Faster

• V8 internally creates hidden classes for objects at runtime
• Objects with the same hidden class can use the same optimized generated code

Avoid the speed trap

• Initialize all object members in constructor functions
• Always initialize object members in the same order
• Prefer numeric values that can be represented as 31-bit signed integers
• Use contiguous keys starting at 0 for Arrays
• Don’t pre-allocate large Arrays (e.g. > 64K elements) to their maximum size, instead grow as you go
• Don’t delete elements in arrays, especially numeric arrays
• Don’t load uninitialized or deleted elements
• Initialize using array literals for small fixed-sized arrays
• Preallocate small arrays to correct size before using them
• Don’t store non-numeric values (objects) in numeric arrays
• Prefer monomorphic over polymorphic wherever possible
• Optimizing compiler “bails-out” on functions with try {} catch {} blocks
• Avoid hidden class changes in functions after they are optimized

Double (type) Array Unboxing

• Array’s hidden class tracks element types
• Arrays containing only doubles are unboxed
• Unboxing causes hidden class change
• Careless manipulation of Arrays can cause extra work due to boxing and unboxing, initialize a mixed type array other than set items one by one

Monomorphic Better Than Polymorphic

• Operations are monomorphic if the hidden class is always the same
• Otherwise they are polymorphic