[swift-evolution] [swift-evolution-announce] [Review] SE-0184: Unsafe[Mutable][Raw][Buffer]Pointer: add missing methods, adjust existing labels for clarity, and remove deallocation size

[Joe Groff]

> On Sep 6, 2017, at 1:06 PM, Taylor Swift <kelvin13ma at gmail.com> wrote:
> 
> 
> 
> On Wed, Sep 6, 2017 at 12:41 PM, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
> > Currently, memory is deallocated by an instance method on UnsafeMutablePointer, deallocate(count:). Like much of the Swift pointer API, performing this operation on a buffer pointer requires extracting baseAddress! and count. It is very common for the allocation code above to be immediately followed by:
> >
> > defer
> >
> > {
> >     buffer.
> > baseAddress?.deallocate(capacity: buffer.count
> > )
> > }
> >
> > This method is extremely problematic because nearly all users, on first seeing the signature of deallocate(capacity:), will naturally conclude from the capacity label that deallocate(capacity:) is equivalent to some kind of realloc()that can only shrink the buffer. However this is not the actual behavior — deallocate(capacity:) actually ignores the capacity argument and just calls free() on self. The current API is not only awkward and suboptimal, it is misleading. You can write perfectly legal Swift code that shouldn’t segfault, but still can, for example
> >
> > var ptr = UnsafeMutablePointer<UInt8>.allocate(capacity: 1000000
> > )
> > ptr.
> > initialize(to: 13, count: 1000000
> > )
> > ptr.
> > deallocate(capacity: 500000) // deallocate the second half of the memory block
> > ptr[0] // segmentation fault
> > where the first 500000 addresses should still be valid if the documentation is to be read literally.
> 
> The fact that the Swift runtime currently uses malloc/free is an implementation detail. Tracking deallocation size is a very useful optimization for better allocator backends, and C++ underwent an ABI break to make it so that sized delete can be supported. Maybe we can change the name to `deallocate(allocatedCapacity:)` to make it clear that it isn't resizing the memory, and/or make the capacity argument optional so that you can pay for the overhead of the allocator deriving the size if it's inconvenient for the calling code to carry the size around, but we shouldn't remove the functionality altogether.
> 
> -Joe
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> The idea is to get the house in order by removing all parameters from deallocate(), since that’s what it really does right now. Then, in the future, if Swift gets a more sophisticated allocator backend, a new method like deallocate(capacity:) or reallocate(toCapacity:) could be added without conflicting with the currently named deallocate(capacity:). However, using the function signature to pretend that it does something it can’t actually do right now is extremely dangerous.

I don't think that's a good idea in this case, because it's not unlikely we would explore an optimized allocator soon after ABI stability, and retrofitting these interfaces in a future version of Swift would put a deployment target limit on when they can be used, and mean that a lot of user code would need to be retrofitted to carry allocated capacities where needed to see any benefit.

-Joe