[llvm-dev] [RFC] Should we add isa_or_null<>?

[Mehdi AMINI via llvm-dev]

I read `isa<T>(or_null(v))`  as "v is a T or nullptr", which does not match
the implementation semantics "v is a T and not null".

On Sat, Apr 6, 2019 at 9:31 PM Zachary Turner <zturner at google.com> wrote:

> Sorry, brain isn't fully working.  I meant to call the function / type
> `or_null` instead of `not_null`
>
> On Sat, Apr 6, 2019 at 11:16 AM Zachary Turner <zturner at google.com> wrote:
>
>> What about a type not_null_impl<T> and we could write:
>>
>> then you could just write bool x = isa<T>(not_null(val));
>>
>> We provide a function not_null<T> that returns a not_null_impl<T>:
>>
>> template<typename T>
>> not_null_impl<T> not_null(T *t) { return not_null_impl<T>{t}; }
>>
>> and a specialization of isa that takes a not_null_impl<T>
>>
>> template<typename T, typename U>
>> isa<T, not_null_impl<U>>(const not_null_impl<U> &u) {
>>   return u ? isa<T>(*u) : false;
>> }
>>
>> On Sat, Apr 6, 2019 at 9:45 AM Mehdi AMINI via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>>
>>>
>>> On Fri, Apr 5, 2019 at 5:15 AM Aaron Ballman via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> On Thu, Apr 4, 2019 at 12:58 PM Chris Lattner <clattner at nondot.org>
>>>> wrote:
>>>> >
>>>> >
>>>> >
>>>> > > On Apr 4, 2019, at 5:37 AM, Don Hinton via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>> > >
>>>> > > I'd like to propose adding `isa_or_null<>` to replace the following
>>>> usage pattern that's relatively common in conditionals:
>>>> > >
>>>> > >   var && isa<T>(var)  =>>  isa_or_null<T>(var)
>>>> > >
>>>> > > And in particular when `var` is a method call which might be
>>>> expensive, e.g.:
>>>> > >
>>>> > >   X->foo() && isa<T>(X->foo())  =>>  isa_or_null<T>(X->foo())
>>>> > >
>>>> > > The implementation could be a simple wrapper around isa<>, and
>>>> while the IR produced is only slightly more efficient, the elimination of
>>>> an extra call could be worthwhile.
>>>> >
>>>> > I’d love to see this, I agree with downstream comments though that
>>>> this name will be confusing.  isa_and_nonnull<>. ?
>>>>
>>>> tbh, I don't think the proposed name will be all that confusing --
>>>>
>>>
>>> I am with David on this, this sounds like misleading naming to me, I
>>> would expect true on null value when reading : if (isa_or_null<T>(var))
>>>
>>> we're used to _or_null() returning "the right thing" when given null.
>>>>
>>>
>>> I think we're used to have "the right thing" because the name matches
>>> the semantic: the "_or_null()" suffix matches the semantics a conversion
>>> operator that returns nullptr on failure.
>>> It does not translate with isa<> IMO.
>>>
>>>
>>>
>>>> isa_and_nonnull<> is a bit of a weird name for me, but I could
>>>> probably live with it. We could spell it nonnull_and_isa<> to reflect
>>>> the order of the operations, but that sort of hides the important part
>>>> of the API (the "isa" bit).
>>>>
>>>
>>> isa_nonnulll works fine for me, isa_and_nonnull is a bit verbose but
>>> seems OK as well.
>>>
>>> For nonnull_and_isa<T>(val) ; it starts to look strangely close to the
>>> pattern !val && isa<T>(val) ; and I'm not sure it is really such a
>>> readability improvement anymore?
>>>
>>> --
>>> Mehdi
>>>
>>>
>>>
>>>>
>>>> ~Aaron
>>>>
>>>> >
>>>> > -Chris
>>>> >
>>>> _______________________________________________
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>>>> llvm-dev at lists.llvm.org
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>>>>
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>>
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