[LLVMdev] Addressing const reference in ArrayRef

[David Blaikie]

On Fri, Aug 22, 2014 at 2:34 PM, Duncan P. N. Exon Smith
<dexonsmith at apple.com> wrote:
>
>> On 2014-Aug-22, at 11:37, David Blaikie <dblaikie at gmail.com> wrote:
>>
>> On Fri, Aug 22, 2014 at 11:22 AM, Duncan P. N. Exon Smith
>> <dexonsmith at apple.com> wrote:
>>>
>>>> On 2014-Aug-22, at 10:43, David Blaikie <dblaikie at gmail.com> wrote:
>>>>
>>>> Yep - the convenience of one-element->ArrayRef is "cute" at best, I
>>>> think. Having to wrap it doesn't seem detrimental. Would have to look
>>>> at some numbers, though (if we could easily gather the number of
>>>> instances of this - I'm not sure of the easiest way to do that, given
>>>> the build system likes to stop after it sees an error).
>>>
>>> diff --git a/include/llvm/ADT/ArrayRef.h b/include/llvm/ADT/ArrayRef.h
>>> index 0351cf5..79f14da 100644
>>> --- a/include/llvm/ADT/ArrayRef.h
>>> +++ b/include/llvm/ADT/ArrayRef.h
>>> @@ -68,7 +68,7 @@ namespace llvm {
>>>     /*implicit*/ ArrayRef(NoneType) : Data(nullptr), Length(0) {}
>>>
>>>     /// Construct an ArrayRef from a single element.
>>> -    /*implicit*/ ArrayRef(const T &OneElt)
>>> +    __attribute__((deprecated)) /*implicit*/ ArrayRef(const T &OneElt)
>>>       : Data(&OneElt), Length(1) {}
>>>
>>>     /// Construct an ArrayRef from a pointer and length.
>>>
>>> and rebuilding LLVM and Clang gives me:
>>>
>>> $ ninja 2>&1 | grep -e Wdeprecated-declarations | wc -l
>>>    6083
>>
>> Uniqued? I imagine a few are in headers.
>
> Good point.
>
> $ grep Wdeprecated-declarations warnings.log | sort -u | wc -l
>      644

Thanks!

I'll try to sum up my thoughts on this:

That seems like many enough that I'd rather not force all those
callers to use an explicit local array.
The utility function doesn't actually save us - we still have to be
diligent about not using it for named ArrayRefs.
  But it might be enough...
A clang-tidy check would be great (instead of any API change) - if we
had a decent developer integration, clang-tidy must-clean shortlist
that was on a buildbot, etc.

All that being said - I don't have any great alternative suggestions,
nor do I feel personally compelled to do any of the work listed above,
so if anyone feels like doing any of those things (even the ones I
don't much like) I don't think I'll stand in the way of progress :)

- David