[swift-users] unsafe memory model questions

[Jordan Rose]

Hi, Ray. Most of the pointer model is covered in the original proposal for the UnsafeRawPointer type, SE-0107 <https://github.com/apple/swift-evolution/blob/master/proposals/0107-unsaferawpointer.md>. That's a good place to find answers to pointer- and memory-related questions in general.

The short forms are "yes, binding memory to types is mostly about communication with the optimizer, but it may also be checkable at run-time some day", and "it's okay to skip 'deinitialize' and use assignment instead of 'initialize' if and only if the element type is trivial <https://github.com/apple/swift-evolution/blob/master/proposals/0107-unsaferawpointer.md#trivial-types>".

Additionally, as far as I know, mutation through UnsafeMutableRawBufferPointer doesn't count as binding any type to the memory, but Andy will have to check me on that.

Best,
Jordan


> On Dec 11, 2016, at 14:51, Ray Fix via swift-users <swift-users at swift.org> wrote:
> 
> Hello,
> 
> So bindMemory is part of the memory model and memory can only bind to one type at a time to avoid aliasing. But what does binding actually do? Is it somehow communicating with the optimizer?
> 
> A tangentially related second question, in the following example:
> 
>   let count = 3
>   let mutablePointer = UnsafeMutablePointer<Int16>.allocate(capacity: count)
>   defer {
>     mutablePointer.deallocate(capacity: count)
>   }
> 
>   mutablePointer.initialize(to: 1234, count: count)
>   defer {
>     mutablePointer.deinitialize(count: count)  // must I do this?
>   }
> 
> Is it bad form if I don’t deinitialize and go straight to deallocate?  I can see where it is important for ref types (to update ref counts, etc).  Is it one of those things that the optimizer can remove for the case of value types?
> 
> Finally, if I initalize with some other means, such as with a raw buffer pointer subscript, is there any need to deinitialize?  Can such memory be considered initialized if I bind it with a type?
> 
>   // 1
>   let pointer = malloc(byteCount)
>   defer {
>     free(pointer)
>   }
> 
>   let mutableRawBufferPointer = UnsafeMutableRawBufferPointer(start: pointer, count: byteCount)
>   
>   for index in mutableRawBufferPointer.indices {
>     mutableRawBufferPointer[index] = 42 + UInt8(index)
>   }
> 
> Perhaps there is a document or proposal somewhere that talks about these things. Sorry if I missed it.
> 
> Thanks as always,
> Ray
> 
> 
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