[swift-evolution] SE-184 Improved Pointers

[Andrew Trick]

> On Aug 8, 2017, at 8:29 PM, Taylor Swift <kelvin13ma at gmail.com> wrote:
> 
> 
> 
> On Tue, Aug 8, 2017 at 11:24 PM, Andrew Trick <atrick at apple.com <mailto:atrick at apple.com>> wrote:
> 
>> On Aug 8, 2017, at 6:51 PM, Taylor Swift <kelvin13ma at gmail.com <mailto:kelvin13ma at gmail.com>> wrote:
>> 
>> 
>> 
>> On Tue, Aug 8, 2017 at 9:38 PM, Andrew Trick <atrick at apple.com <mailto:atrick at apple.com>> wrote:
>> 
>>> > 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 <https://bugs.swift.org/browse/SR-5664>
>> 
>> yikes i was not aware of this. I don’t think it’s bad enough to warrant dropping the argument like with deallocate(capacity:) but I can imagine bad things happening to code that crams extra inhabitants into pointers.
> 
> If we ever need to do pointer adjustment during deallocation to accommodate alignment, then I think the Swift runtime can track that. I see no reason to muddy the UnsafeRawPointer API with it. So, I agree with your proposed change to drop `alignedTo` there.
> 
> -Andy
> 
> oh lol I was talking about assuming the pointer returned by allocate(bytes:alignedTo:) is a multiple of alignedTo. Some code might be relying on the last few bits of the pointer being zero; i.e. sticking bit flags there like how some implementations store the red/black color information in a red-black tree node.

Oh, sure. But I think it will be easy to fix the runtime. We could probably do it before the proposal is accepted if necessary.
-Andy

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