[swift-evolution] [Pitch] Normalize Slice Types for Unsafe Buffers

[Xiaodi Wu]

Can you elaborate on this? Why aren't you sure this is a wise idea?

On Thu, Dec 8, 2016 at 16:19 Dave Abrahams via swift-evolution <
swift-evolution at swift.org> wrote:

>
> on Thu Dec 08 2016, Alexis Beingessner <swift-evolution at swift.org> wrote:
>
> >> On Dec 8, 2016, at 3:50 PM, Dave Abrahams via swift-evolution <
> swift-evolution at swift.org> wrote:
> >>
> >>
> >> on Thu Dec 08 2016, Ben Cohen <ben_cohen-AT-apple.com> wrote:
> >>
> >
> >>>> On Dec 2, 2016, at 8:27 PM, Nate Cook <natecook at gmail.com> wrote:
> >>>>
> >>>>> On Dec 2, 2016, at 2:12 PM, Ben Cohen via swift-evolution
> >>>>> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>
> >>>>> wrote:
> >>>
> >>>>>
> >>>>>> On Dec 1, 2016, at 11:33 PM, Nate Cook via swift-evolution
> >>>>>> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>
> >>>>>> wrote:
> >>>>>>
> >>>>>> 3) Make all buffer pointers their own slices but use a different
> >>>>>> index type. If the indices were just wrapped pointers, that would
> >>>>>> handle the index sharing without needing an additional property on
> >>>>>> the buffer. We could also maintain integer-based stridable
> >>>>>> conformance (which greatly simplifies index arithmetic), since the
> >>>>>> indices would just offset by a byte for raw buffers or a stride
> >>>>>> for typed buffers.
> >>>>>>
> >>>>>
> >>>>> Unfortunately, switching to non-integer indices would change this
> >>>>> from being mildly source-breaking to being extremely
> >>>>> source-breaking, as there’s lots of code out there using buffers
> >>>>> today indexing them with integers (including integer literals).
> >>>>>
> >>>>> The big win with UnsafeBufferPointer having an integer index is
> >>>>> it’s a drop-in replacement for arrays, so when you hit a
> >>>>> performance problem using an array you can quickly switch to using
> >>>>> a buffer under most circumstances instead without having to change
> >>>>> much of your code – including code that uses for i in
> >>>>> 0..<myArray.count, of which there is a lot out there in the
> >>>>> wild. Switching to an opaque index would break anyone doing that.
> >>>>
> >>>> It is definitely very source-breaking, though with relatively simple
> fixits:
> >>>>
> >>>>    buf[0] ---> buf[buf.startIndex]
> >>>>    buf[3] ---> buf[buf.startIndex + 3]
> >>>>    buf[i] ---> buf[buf.startIndex + i]
> >>>>
> >>>> Any integer arithmetic happening outside the subscript could be left
> >>>> unchanged. If that cost isn't worth the benefit, then making
> >>>> UnsafeRawBufferPointer use Slice as its slice type is probably the
> >>>> best way to resolve that issue.
> >>>>
> >>>> Nate
> >>>
> >>> The fixits aren’t quite that simple for slices, though:
> >>>
> >>>    let slice = buf[3..<6]
> >>>    slice[3] —> slice[slice.startIndex + 0] // fixit would somehow need
> to know this is 0 not 3
> >>>    slice[i] —> slice[slice.startIndex + ??] // or even need to
> >>> know this is, erm, I haven’t had enough coffee this morning
> >>>
> >>> The other downside is it would thwart speculatively switching an Array
> >>> to an UnsafeBuffer to see if that was a bottleneck, then switching
> >>> back.
> >>>
> >>>> On Dec 1, 2016, at 11:33 PM, Nate Cook via swift-evolution <
> swift-evolution at swift.org> wrote:
> >>>>
> >>>> 1) Switch to using Slice as a wrapper for UnsafeRawBufferPointer.
> >>>>
> >>>
> >>> Based on the above, it seems like this is the least bad option, and we
> >>> need to do this ASAP as currently UnsafeRawBufferPointer is
> >>> non-compliant with the requirements of slicing and needs changing
> >>> before it’s more widely adopted.
> >>
> >> Or we could say that UnsafeRawBufferPointer isn't a Collection.  Making
> >> it a Collection in the first place has always seemed suspect to me.
> >>
> >
> > If this is considered a viable option, it's the one I want. Passing
> > types without bounds checks into generic "safe" code shouldn't be this
> > easy.
>
> I don't think I agree, but that's a separate argument, about
> UnsafeBufferPointer.  This is about passing *untyped* memory without
> bounds checks.
>
> > You should need to explicitly wrap it up in something safe. And I
> > really don't want the known-to-be-error-prone indexing model in
> > concrete unsafe code.
>
>
> --
> -Dave
>
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