On Tue, Aug 8, 2017 at 11:24 PM, Andrew Trick <atr...@apple.com> wrote:
> > On Aug 8, 2017, at 6:51 PM, Taylor Swift <kelvin1...@gmail.com> wrote: > > > > On Tue, Aug 8, 2017 at 9:38 PM, Andrew Trick <atr...@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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