What is the performance difference between blocks and callbacks?

Question

One of the things that block objects, introduced in Snow Leopard, are good for is situations that would previously have been handled with callbacks. The syntax is much cleaner for passing context around. However, I haven't seen any information on the performance implications of using blocks in this manner. What, if any, performance pitfalls should I look out for when using blocks, particularly as a replacement for a C-style callback?

Answer 1

You might want to check out this blog post and this one. Blocks are implemented as Objective-C objects, except they can be put on the stack, so they don't necessarily have to be malloc'd (if you retain a reference to a block, it will be copied onto the heap, though). They will thus probably perform better than most Objective-C objects, but will have a slight performance hit compared to a simple callback--I'd guess it shouldn't be a problem 95% of the time.

Answer 2

The blocks runtime looks pretty tight. Block descriptors and functions are statically allocated, so they could enlarge the working set of your program, but you only "pay" in storage for the variables you reference from the enclosing scope. Non-global block literals and __block variables are constructed on the stack without any branching, so you're unlikely to run into much of a slowdown from that. Calling a block is just result = (*b->__FuncPtr)(b, arg1, arg2); this is comparable to result = (*callback_func_ptr)(callback_ctx, arg1, arg2).

If you think of blocks as "callbacks that write their own context structure and handle the ugly packing, memory management, casting, and dereferencing for you," I think you'll realize that blocks are a small cost at runtime and a huge savings in programming time.