> On Dec 11, 2016, at 2:51 PM, Ray Fix via swift-users <swift-users@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?
Binding memory to a type tells the compiler what type the memory can hold.
Normally that happens implicitly when you allocate memory for a particular
type, so you don’t need to think about it (your memory below is implicitly
bound to Int16). But if you’re playing around with raw memory/type punning and
get the bound type wrong, future versions of the optimizer could “break” your
code in very confusing ways.
> 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?
You don’t need to deinitialize trivial types—it’s semantically legal to omit
the deinitialize. But it doesn’t hurt—the optimizer will remove the call. I
think think it’s good form to deinitialize in examples where others are using
your code as reference for best practice, or if the type isn’t obviously
trivial; e.g. someone may add a reference to your previously trivial struct.
> 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)
The above is essentially equivalent to:
let mutableRawBufferPointer = UnsafeMutableRawBufferPointer.allocate(bytes:
byteCount, alignedTo: 16)
// alignment depends on the platform
defer {
mutableRawBufferPointer.deallocate(bytes: byteCount, alignedTo: 16)
}
> for index in mutableRawBufferPointer.indices {
> mutableRawBufferPointer[index] = 42 + UInt8(index)
> }
This is an interesting case because you’ve initialized memory to “raw bytes”.
The memory isn’t bound to any type. The documentation talks about initialized
memory always being bound to a type, but the underlying assumption is that
you’ve used a typed operation to initialize the memory. Assigning bytes via a
raw pointer subscript or storeBytes isn’t a typed operation.
It wouldn’t even make sense to deinitialize this memory because it doesn’t hold
any typed values!
You could bulk-reinterpret that memory as some type (without loading the
values), simply by binding the memory. It is only valid to reinterpret those
raw bytes as a trivial type though, so you still don’t need to deinitialize.
The UnsafeRawPointer API specifies that copying raw bytes to/from nontrivially
typed memory or is illegal. The only way to get around that would be calling
down to memcpy/memmove. Similarly, reinterpreting (rebinding) trivial to
nontrivial types is illegal.
> Perhaps there is a document or proposal somewhere that talks about these
> things. Sorry if I missed it.
https://github.com/apple/swift-evolution/blob/master/proposals/0107-unsaferawpointer.md
<https://github.com/apple/swift-evolution/blob/master/proposals/0107-unsaferawpointer.md>
https://swift.org/migration-guide/se-0107-migrate.html
<https://swift.org/migration-guide/se-0107-migrate.html>
The rules are codified in API doc comments, but the language is not very
user-friendly. Those API docs are being worked on, so in the future you
typically won’t need to refer back to the evolution proposal just to be able to
understand the comments.
-Andy
>
> Thanks as always,
> Ray
>
>
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