Comment by austinz | Hacker News Reader

Comment by austinz | Hacker News Reader

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> told to use Swift arrays as we would C arrays

Until you can declare a fixed-length Swift array, or a Swift array that isn't copy-on-write by default, or a fixed-length Swift array that doesn't live on the heap, this can't possibly be true, no matter what you think you've been told.

> it should be usable for low level, high performance programming but "safer than C".

With optimizations turned on, the 'Swift []' code comes within 3x of the performance of the ObjC code. Lots of room for improvement, but not abysmal either.

> Furthermore, using UnsafeMutablePointers in Swift makes the program a magnitude larger than the corresponding C program...It is a last resort, not something you should use all of the time.

I agree.

From what we're told, copy-on-write doesn't happen unless it has to (i.e. the compiler discovered that we use it elsewhere).

In any case, the issue here with debug builds does not seem to be due to memory access nor copy-on-write, but simply because of ARC sprinkling the accesses to the array with release/retain pairs.

The reason it works ok with optimizations turned on, is because Swift will not remove unnecessary release-retain in -Onone.

This was not a great problem in ObjC, since it only affected ObjC method dispatch - which you were unlikely to do in a tight loop. Plus, ObjC ARC actually emits retain/release pairs differently.

The issue with ARC emits was discovered with the very first beta, but as of yet there has been no fix.

The problem here is probably this:

1. In order to perform a local optimization on ARC retain/release, even at -Onone, you need to change the retain/release emits to the style used by ObjC.

2. But that retain/release often incurs an unnecessary autorelease, which would prevent even an optimized Swift build from eliminating certain retain/releases.

In other words, they either get significantly better -Onone performance, or slightly worse -O/-Ounchecked performance.

That's quite plausible.

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