Saturday, February 18, 2017 [Tweets] [Favorites]

Swift Ownership Manifesto

Apple (mailing list):

The widespread use of copy-on-write value types in Swift has generally been a success. It does, however, come with some drawbacks:

  • Reference counting and uniqueness testing do impose some overhead.

  • Reference counting provides deterministic performance in most cases, but that performance can still be complex to analyze and predict.

  • The ability to copy a value at any time and thus “escape” it forces the underlying buffers to generally be heap-allocated. Stack allocation is far more efficient but does require some ability to prevent, or at least recognize, attempts to escape the value.

Certain kinds of low-level programming require stricter performance guarantees. Often these guarantees are less about absolute performance than predictable performance. For example, keeping up with an audio stream is not a taxing job for a modern processor, even with significant per-sample overheads, but any sort of unexpected hiccup is immediately noticeable by users.

Another common programming task is to optimize existing code when something about it falls short of a performance target. Often this means finding ”hot spots” in execution time or memory use and trying to fix them in some way. When those hot spots are due to implicit copies, Swift’s current tools for fixing the problem are relatively poor; for example, a programmer can fall back on using unsafe pointers, but this loses a lot of the safety benefits and expressivity advantages of the library collection types.

We believe that these problems can be addressed with an opt-in set of features that we collectively call ownership.

Previously: Chris Lattner ATP Interview, Swift Plans.

2 Comments

Instead of adding yet another feature to an already bloated language, how about realizing that designing a single language for low level programming and complex UI applications isn't a good idea?

@Ilja I’m not sure the single language thing is going to work, but given the design goals this seems like an essential feature that won’t really get in the way if you aren’t using it.

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