On Mon, May 9, 2016 at 10:25 AM, Joe Groff via swift-users < swift-users@swift.org> wrote:
> > > On May 7, 2016, at 10:39 AM, Austin Zheng via swift-users < > swift-users@swift.org> wrote: > > > > Hello Swift users, > > > > I wanted to run something past you folks and get some opinions/feedback. > > > > About a month ago on Hacker News I saw someone commenting about how > Swift's string-handling code was unbearably slow (3 seconds to run a code > sample, vs. 0.8 in Java). I asked him to provide the code, and he obliged. > Unfortunately, I didn't have time to dig into it until this morning. The > code in its entirety can be found here: > https://gist.github.com/austinzheng/d6c674780a58cb63832c4df3f809e683 > > > > At line 26 we have the following code: > > > > result.append(begin == eos ? "" : String(cs[begin..<end.successor()])) > > > > 'cs' is a UTF16 view into an input string, while 'result' is a [String]. > When I profiled the code in Instruments, I noticed that it was spending > significant time within the reflection machinery. > > > > It turns out that the initializer to make a String out of a utf16 view > looks like this, and I believe this is the initializer the author intended > to call: > > > > init?(_: String.UTF16View) > > > > However, the actual initializer being called was this String initializer > in the Mirror code: > > > > public init<Subject>(_ instance: Subject) > > > > This seems like a tricky gotcha for developers who aren't extremely > familiar with both the String and reflection APIs. His code looked > reasonable at a first glance and I didn't suspect anything was wrong until > I profiled it. Even so, I only made the connection because I recognized the > name of the standard library function from poking around inside the source > files. > > > > What do other people think? Is this something worth worrying about, or > is it so rare that it shouldn't matter? Also, any suggestions as to how > that code sample might be improved would be appreciated - my naive first > attempt wasn't any better. > > This definitely strikes me as a problem. The String<T>(_:) constructor is > very easy to call by accident if you're trying to hit another unlabeled > initializer. It also strikes me as not particularly "value-preserving", > since stringifying many types loses information. Perhaps we should propose > giving it a label, String(printing:) maybe? > +1 > > -Joe > _______________________________________________ > swift-users mailing list > swift-users@swift.org > https://lists.swift.org/mailman/listinfo/swift-users >
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