[swift-evolution] Pitch: Improved Swift pointers

[Taylor Swift]

For the sake of argument, I’m gonna claim that instead,
UnsafeMutableBufferPointer is the low level necessity, and
UnsafeMutablePointer is the low level convenience structure.

Suppose that every block of memory has a starting point, and a length. An
UnsafeBufferPointer represents that, since all an UnsafeBufferPointer is,
is a start address, and a length value, bundled together into one variable
instead of two. Then we can say that UnsafePointer is a *special case* of
UnsafeBufferPointer, where the length value is *unknown*.

Mind you I didn’t say that the length value *equals* *1*. But it *might as
well* be. Because when the length is unknown,* the only index where you can
reasonably expect there to be a valid value is index zero*. You wouldn’t
feel confident trying to do something to the memory address 3 strides down
from the pointer. If you did feel confident doing that, well now you have a
length value. The length value is 4. Your regular Pointer is now a
BufferPointer.

The only reason any of this matters, is because all of our memory and
memorystate -related functions take size arguments. You have to fill in the
size argument with *something*. That *something* is the buffer `count`.
Even if you don’t actually know the number for the `count`, you still have
to supply some value, in which case the “only” sensible choice is “1”.
That’s why you could argue that UnsafePointer is just a special case of a
buffer pointer where `count` is 1. The UnsafePointer API demands
information that only BufferPointers know. You could design an API where
buffer pointers are the only pointers that exist, and everything would
still work fine. Just because they have high-level *capabilities* doesn’t
mean they can’t do everything plain pointers can do just as efficiently. A
plain pointer takes up one word of storage, and you use another word of
storage yourself to track the size. A buffer pointer stores the two words
next to each other. You could ignore the size word and track the size
yourself, taking up three words of storage, but that would just be silly.

*But wait! I don’t need to track the size*, you say! *The pointer just
points to one item! I just need the address word, not the size word!* Well,
*yeah*. That’s why I proposed a sizeless singular pointer API, so you don’t
have to go around plugging “1”s everywhere and save yourself a word of
storage. You can argue that’s the *actual* high-level API, since it
abstracts away the length stuff. But if “1” isn’t your answer to the
question “on how many instances should this method operate on”, go back to
paragraph 3 and convince yourself that what you really have is an buffer
pointer, not a plain pointer.

Now I thought of an exception to this, like when you’re assigning 8-bit
RGBA values to an image buffer and it might make sense to write something
like this in the current API:

let size:Int = height * width << 2
let base = UnsafeMutablePointer<UInt8>.allocate(capacity: size)
defer
{
    base.deallocate(capacity: -314159) // anything goes
}
var pixel:UnsafeMutablePointer<UInt8> = base
while pixel < base + size
{
    pixel.initialize(from: bg_color_rgba, count: 4)
    pixel += 4
}

And it’s convenient to be able to initialize 4 instances at a time without
creating a 4-count buffer pointer. But this doesn’t really contradict my
point if you think carefully. The groups of 4 RGBA values are your atom
here, you’re not really working on 4 instances at a time with a stride 1
UInt8 big, you’re working on 1 single instance at a time with a stride 4
UInt8s big. Writing this is currently painful without the sized
UnsafeMutablePointer API, but I think this is a deficiency of the
UnsafeMutableBufferPointer API and that of its slice type, not a reason for
keeping the sized UnsafeMutablePointer API, which if you ask me is a hack
to get around the fact that we can’t use buffer pointer slices easily.

Anyway, I mention all of this only because everyone seems convinced that
UnsafePointer is supposed to be “edgier” than UnsafeBufferPointer. If you
have ideas to make this better, by all means share them


On Fri, Jul 14, 2017 at 2:27 PM, Michael Ilseman <milseman at apple.com> wrote:

>
> On Jul 13, 2017, at 6:55 PM, Taylor Swift via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>
>
> On Thu, Jul 13, 2017 at 6:56 PM, Andrew Trick <atrick at apple.com> wrote:
>
>>
>> On Jul 12, 2017, at 12:16 PM, Taylor Swift via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>> Hi all, I’ve written up a proposal to modify the unsafe pointer API for
>> greater consistency, safety, and ease of use.
>>
>> ~~~
>>
>> Swift currently offers two sets of pointer types — singular pointers such
>> as UnsafeMutablePointer, and vector (buffer) pointers such as
>> UnsafeMutable*Buffer*Pointer. This implies a natural separation of tasks
>> the two kinds of pointers are meant to do. For example, buffer pointers
>> implement Collection conformance, while singular pointers do not.
>>
>> However, some aspects of the pointer design contradict these implied
>> roles. It is possible to allocate an arbitrary number of instances from a
>> type method on a singular pointer, but not from a buffer pointer. The
>> result of such an operation returns a singular pointer, even though a
>> buffer pointer would be more appropriate to capture the information about
>> the *number* of instances allocated. It’s possible to subscript into a
>> singular pointer, even though they are not real Collections. Some parts
>> of the current design turn UnsafePointers into downright *Dangerous*Pointers,
>> leading users to believe that they have allocated or freed memory when in
>> fact, they have not.
>>
>> This proposal seeks to iron out these inconsistencies, and offer a more
>> convenient, more sensible, and less bug-prone API for Swift pointers.
>>
>> <https://gist.github.com/kelvin13/a9c033193a28b1d4960a89b25fbffb06>
>>
>> ~~~
>>
>>
>> Thanks for taking time to write this up.
>>
>> General comments:
>>
>> UnsafeBufferPointer is an API layer on top of UnsafePointer. The role
>> of UnsafeBufferPointer is direct memory access sans lifetime
>> management with Collection semantics. The role of UnsafePointer is
>> primarily C interop. Those C APIs should be wrapped in Swift APIs that
>> take UnsafeBufferPointer whenever the pointer represents a C array. I
>> suppose making UnsafePointer less convenient would push developers
>> toward UnsafeBufferPointer. I don't think that's worth outright
>> breaking source, but gradual deprecation of convenience methods, like
>> `susbscript` might be acceptable.
>>
>
> Gradual deprecation is exactly what I am proposing. As the document states
> <https://gist.github.com/kelvin13/a9c033193a28b1d4960a89b25fbffb06#proposed-solution>,
> the only methods which should be marked immediately as unavailable are the `
> deallocate(capacity:)` methods, for safety and source compatibility
> reasons. Removing `deallocate(capacity:)` now and forcing a loud compiler
> error prevents catastrophic *silent* source breakage in the future, or
> worse, from having to *support our own bug*.
>
>>
>>
>> I have mixed feelings about stripping UnsafePointer of basic
>> functionality. Besides breaking source, doing that would be
>> inconsistent with its role as a lower API layer. The advantage would
>> just be descreasing API surface area and forcing developers to use a
>> higher-level API.
>>
>
> UnsafePointer is as much a high level API as UnsafeBufferPointer is. You
> wouldn’t create a buffer pointer of length 1 just so you can “stick with
> the high level API”. UnsafePointer and UnsafeBufferPointer are two tools
> that do related but different things and they can exist at whatever
> abstract level you need them at. After all, UnsafeBufferPointer is nothing
> but an UnsafePointer? with a length value attached to it. If you’re
> allocating more than one instance of memory, you almost certainly need to
> track the length of the buffer anyway.
>
>
>
> I disagree. UnsafePointer can be viewed as the low level representation of
> Swift’s memory model. UnsafeBufferPointer is a convenient facility for
> programming at this level, as illustrated by e.g. RandomAccessCollection
> conformance. One could drop UnsafeBufferPointer from the standard library
> without compromising Swift’s core capabilities, it would just be a large
> convenience regression.
>
> UnsafePointer is a low level necessity; UnsafeBufferPointer is a low level
> convenience.
>
> The additive changes you propose are fairly obvious. See [SR-3088]
>> UnsafeMutableBufferPointer doesn't have an allocating init.
>>
>> I haven't wanted to waste review cycles on small additive
>> changes. It may make sense to batch them up into one coherent
>> proposal. Here are a few more to consider.
>>
>> - [SR-3929] UnsafeBufferPointer should have init from mutable
>> - [SR-4340] UnsafeBufferPointer needs a withMemoryRebound method
>> - [SR-3087] No way to arbitrarily initialise an Array's storage
>>
>
> The feature requests you mention are all very valuable, however with
> Michael’s point about fixing the memorystate API’s, the size of this
> proposal has already grown to encompass dozens of methods in five types. I
> think this says a lot about just how broken the current system is
>
>
> I fail to see how the current system is broken. It accurately reflects
> Swift’s memory model while providing the facilities to stay in the bounds
> of defined behavior. I do agree that it would be far more convenient for
> UnsafeBufferPointer to provide allocation/deallocation and initialization/deinitialization
> convenience functionality as well, as I’ve often wanted it myself.
>
> , but I think it’s better to try to fix one class of problems at a time,
> and save the less closely-related issues for separate proposals.
>
>
> These seem very closely related. Isn’t the overall goal of this proposal
> to have API parity for UnsafeBufferPointer?
>
>
>
>>
>> Point by point:
>>
>> > drop the capacity parameter from UnsafeMutablePointer.allocate() and
>> deallocate().
>>
>> I do not agree with removing the capacity parameter and adding a
>> single-instance allocation API. UnsafePointer was not designed for
>> single instances, it was primarily designed for C-style arrays. I
>> don't see the value in providing a different unsafe API for single
>> vs. multiple values.
>>
>
> Although it’s common to *receive* Unsafe__Pointers from C API’s, it’s rare
> to *create* them from the Swift side. 95% of the time your Swift data lives
> in a Swift Array, and you use withUnsafePointer(_:) to send them to the C
> API, or just pass them directly with Array bridging.
>
> The only example I can think of where I had to allocate memory from the
> Swift side to pass to a C API is when I was using the Cairo C library and I
> wanted the Swift code to own the image buffer backing the Cairo C structs
> and I wanted to manage the memory manually to prevent the buffer backing
> from getting deallocated prematurely. I think I ended up using
> UnsafeMutableBufferPointer and extracting baseAddresses to manage the
> memory. This proposal tries to mitigate that pain of extracting
> baseAddresses by giving buffer pointers their own memory management methods.
>
> As for the UnsafePointers you get from C APIs, they almost always come
> with a size (or you specify it beforehand with a parameter) so you’re
> probably going to be turning them into UnsafeBufferPointers anyway.
>
> I also have to say it’s not common to deallocate something in Swift that
> you didn’t previously allocate in Swift.
>
>
> I’m not sure it is even defined to use `deallocate` on memory that wasn’t
> `allocated` in Swift. Andy, thoughts here? Perhaps more clarity in
> deallocate’s documentation is needed?
>
>
>> I agree the primary allocation API should be
>> UnsafeMutableBufferPointer.allocate(capacity:). There is an argument
>> to be made for removing UnsafeMutablePointer.allocate(capacity:)
>> entirely. But, as Michael Ilseman pointed out, that would involve
>> reevaluating several other members of the UnsafePointer API. I think
>> it's reasonable for UnsafePointer to retain all its functionality as a
>> lower level API.
>>
>>
> I think duplication of functionality is something to be avoided if
> possible.
>
>
>
> You have to weigh duplication against convenience. If avoiding duplication
> is more important, then we should keep the APIs only on UnsafePointer. But,
> I would prefer convenience in this scenario.
>
> I don't understand what is misleading about
>> UnsafePointer.deallocate(capacity:). It *is* inconvenienent for the
>> user to keep track of memory capacity. Presumably that was done so
>> either the implementation can move away from malloc/free or some sort
>> of memory tracking can be implemented on the standard library
>> side. Obviously, UnsafeBufferPointer.deallocate() would be cleaner in
>> most cases.
>>
>
> It’s misleading because it plain doesn’t deallocate `capacity` instances.
> It deletes the whole memory block regardless of what you pass in the
> capacity argument. If the implementation is ever “fixed” so that it
> actually deallocates `capacity` instances, suddenly every source that uses
> `deallocate(capacity:)` will break, and *no one will know* until their app
> starts mysteriously crashing. If the method is not removed, we will have to
> support this behavior to avoid breaking sources, and basically say “yes the
> argument label says it deallocates a capacity, but what it *really* does is
> free the whole block and we can’t fix it because existing code assumes this
> behavior”.
>
>
>
> Note that many of these things are basically programming at the memory
> semantics level, not necessarily at a execution level.
>
>
>> > add an allocate(count:) type method to UnsafeMutableBufferPointer
>>
>> `capacity` should be used for allocating uninitialized memory not
>> `count`. `count` should only refer to a number of initialized objects!
>>
>
> We can decide on what the correct term should be, but the current state of
> Swift pointers is that *neither* convention is being followed. Just look at
> the API for UnsafeMutableRawPointer. It’s a mess. This proposal at the
> minimum establishes a consistent convention. It can be revised if you feel
> `capacity` is more appropriate than `count`. If what you mean is that it’s
> important to maintain the distinction between “initialized counts” and
> “uninitialized counts”, well that can be revised in too.
>
>
>
> So I just looked over all of the APIs, and they are very consistent in
> their use of `count` and `capacity`. Where are you seeing a violation?
>
> > add a deallocate() instance method to UnsafeMutableBufferPointer
>>
>> Yes, of course! I added a mention of that in SR-3088.
>>
>> > remove subscripts from UnsafePointer and UnsafeMutablePointer
>>
>> It's often more clear to perform arithmetic on C array indices rather
>> than pointers. That said, I'm happy to push developers to use
>> UnsafeBufferPointer whenever that have a known capacity. To me, this
>> is a question of whether the benefit of making a dangerous thing less
>> convenient is worth breaking source compatibility.
>>
>
> Again, I think this is more about what the real use patterns are. If you
> are subscripting into a C array with integers, then UnsafeBufferPointer is
> the tool for the job, since it give you Collection conformance. If you
> can’t make an UnsafeBufferPointer, it’s probably because you don’t know the
> length of the array, and so you’re probably iterating through it one
> element at a time. UnsafeMutablePointer.successor() is perfect for this
> job. If you want to extract or set fields at fixed but irregular offsets,
> UnsafeRawPointer is the tool for the job. But I’m hard-pressed to think of
> a use case for random access into a singular typed pointer.
>
>
> I’m also very interested in more convenience facilities for low level
> programming and I view UnsafeBufferPointer as the first low level
> convenience construct. The point of the very careful dance that the
> Unsafe*Pointers go through is to help the programmer avoid unwittingly
> falling into undefined behavior.
>
>
>
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>
>
>
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