[swift-evolution] SE-184 Improved Pointers

[Taylor Swift]

On Tue, Aug 8, 2017 at 11:24 PM, Andrew Trick <atrick at apple.com> wrote:

>
> On Aug 8, 2017, at 6:51 PM, Taylor Swift <kelvin13ma at gmail.com> wrote:
>
>
>
> On Tue, Aug 8, 2017 at 9:38 PM, Andrew Trick <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
>>
>>
> 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.
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