[swift-evolution] [Pitch] Retiring `where` from for-in loops

[Charlie Monroe]

> On Jun 9, 2016, at 10:29 AM, Brent Royal-Gordon <brent at architechies.com> wrote:
> 
>> I've taken the time to run a test, going through milion numbers (several times) using:
>> 
>> for i in arr { if i % 2 == 0 { continue } }
>> for i in arr where i % 2 == 0 { }
>> for i in arr.filter({ $0 % 2 == 0 }) { }
>> for i in arr.lazy.filter({ $0 % 2 == 0 }) { }
>> 
>> Results:
>> 
>> - plain for loop with if-continue: 27.19 seconds (+1.76%)
>> - with where: 26.72 seconds (+0.00%)
>> - .filter: 44.73 seconds (+67.40%)
>> - .lazy.filter: 31.66 seconds (+18.48%)
> 
> This is great data. I have a hard time imagining a little compiler work couldn't make if-continue as fast as for-where, but lazy.filter might be a taller order for it, and optimizing plain filter could actually change behavior.
> 
> A month or two ago, I actually fell into the "just use the higher-order functions" camp on this question, but I've been rethinking that more and more lately. Between the trailing closure incompatibility, the need to remember to use `lazy` to get decent performance, and now the noticeable speed difference even *with* lazy, I'm no longer convinced that answer is good enough.

There will IMHO always be noticeable overhead since you're calling a function which is then invoking a closure. When you look at what that means:

- thunks generated around the invocation, which are a few instructions
- new stack frame for each call (correct me if I'm wrong). 

So instead of a single `i % 2 == 0` (which is just 2-3 instructions, depending on the architecture and optimization settings), it will invoke the closure milion times, if the array contains a milion members.

Maybe I'm over-optimizing, but 18% seemed like a lot to me.


> 
> (Though I do think `while` is probably too niche to bother with as a first-class feature, and I am open to if-continue on the `where` clause.)
> 
> -- 
> Brent Royal-Gordon
> Architechies
>