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

[Zachary Turner via llvm-dev]

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
>> >
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190406/ad5183d0/attachment.html>