[swift-evolution] SE-184 Improved Pointers

Andrew Trick atrick at apple.com
Tue Aug 8 20:38:24 CDT 2017

> On Aug 8, 2017, at 5:49 PM, Taylor Swift <kelvin13ma at gmail.com> wrote:
> On Tue, Aug 8, 2017 at 7:53 PM, Andrew Trick <atrick at apple.com <mailto:atrick at apple.com>> wrote:
>> On Aug 8, 2017, at 9:52 AM, Taylor Swift via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> Since Swift 5 just got opened up for proposals, SE-184 Improved Pointers is ready for community review, and I encourage everyone to look it over and provide feedback. Thank you!
>> <https://github.com/apple/swift-evolution/blob/master/proposals/0184-improved-pointers.md <https://github.com/apple/swift-evolution/blob/master/proposals/0184-improved-pointers.md>>
> Excellent. Thanks for patiently iterating on this. I know it's time consuming.
> > add a default value of 1 to all size parameters on
> > UnsafeMutablePointer and applicable size parameters on
> > UnsafeMutableRawPointer
> I'm generally ok with this if you have seen the benefit of it in real
> code. However, I do not think any `repeating:` methods should have a
> default count.
> Actually, i believe initialize(to:count:) is currently the one method that already has a default count. That’s probably because the standard library calls this method with a count argument of 1 more than any other memorystate method. I don’t know if this decision was only made for the sake of the stdlib or if it had an API justification.

Right you are. I had just noticed that none of the other `repeating` APIs had a default. But if this default argument simplifies real code patterns then I’m fine with it.

> > UnsafeMutableRawBufferPointer.allocate(bytes:alignedTo:)
> Well, I think it's somewhat ridiculous for users to write this every time they allocate a buffer:
> `UnsafeMutableRawBufferPointer.allocate(bytes: size, alignedTo: MemoryLayout<UInt>.alignment)`
> If anyone reading the code is unsure about the Swift API's alignment
> guarantee, it's trivial to check the API docs.
> You could introduce a clearly documented default `alignedTo`
> argument. The reason I didn't do that is that the runtime won't
> respect it anyway. But I think it would be fair to go ahead with the
> API and file a bug against the runtime.
> Default argument of MemoryLayout<Int>.alignment is the way to go but as you said i don’t know if that is actually allowed/works. An alternative is to have two allocate methods each, one that takes an alignment argument and one that doesn’t (and aligns to pointer alignment) but that feels inelegant. Default arguments would be better.

Default argument makes sense to me too. Then the raw buffer pointer and regular raw pointer APIs can be consistent with each other.

Runtime bug: https://bugs.swift.org/browse/SR-5664

> > and initializeMemory<Element>(as:at:repeating:count:),
> > initializeMemory<Element>(as:from:count:)
> > moveInitializeMemory<Element>(as:from:count:), and
> > bindMemory<Element>(to:count:) to UnsafeMutableRawBufferPointer
> I think you should move the raw pointer changes to a separate bullet point.
> Presumably the raw buffer capacity must match or exceed count * stride?
> -Andy
> The reason the raw buffer pointers don’t fill in their own size is the destination type might not line up with the raw buffer size, and then there’s questions of rounding and whatnot. bindMemory(to:count:) would also have to perform integer division or some other defined behavior. Though if people think computing the strided count inside the buffer pointer method is the right way to go, I’m open to that.

No, I think your API is fine. I just wanted to clarify that we will trap if the raw buffer is too small to fit the requested elements.


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