Hello Brent,
Thanks for pointing it out - obvious now!
Regards,
Will Stanton
> On Mar 19, 2016, at 6:43 PM, Brent Royal-Gordon via swift-dev
> <swift-dev@swift.org> wrote:
>
> Actually, the code samples I gave you *do* subtly depend on
> underestimatedCount being an underestimate, because the initial fast loop
> force-unwraps the element returned by `next`. If the count is an
> overestimate, that force unwrap will fail. You can see it in my example:
>
> // Load all the fast elements
> for _ in 0..<fastCount {
> let elem = generator.next()!
>
> _appendElementQuicklyBecauseWeAlreadyKnowWeHaveTheCapacity(elem)
> }
>
> And in the actual standard library code:
>
> // Add elements up to the initial capacity without checking for
> regrowth.
> for _ in 0..<initialCapacity {
> builder.addWithExistingCapacity(generator.next()!)
> }
>
> These and similar constructs could be turned into `if let`s, but that would
> add a conditional branch which would make the loop slower, particularly in
> -Ounchecked mode where safety checks are disabled. In that mode, the force
> unwrap operator simply *assumes* the value is not `nil`, which makes this
> code run as fast as possible—it pretty much can just copy or retain the
> element, assign it into the next slot in the array, and increment the array's
> count.
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