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

[via llvm-dev]

I have to say `not_null(v)` reads more like an assertion than a predicate, in which case `isa<T>(not_null(v))` reads like it has the exact same semantics that `isa<T>(v)` has currently—asserts that `v` is not null.
I don't dispute that you can *make* it have the desired semantics, it just won't *look* that way.

maybe `isaT_or_null<Foo>(v)` ? Still looks awkward but maybe less naively misleading.


From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Zachary Turner via llvm-dev
Sent: Saturday, April 06, 2019 10:15 PM
To: Mehdi AMINI
Cc: LLVM Development List; Aaron Ballman
Subject: Re: [llvm-dev] [RFC] Should we add isa_or_null<>?

In that case my original suggestion of isa<T>(not_null(v)) matches the semantics right?

On Sat, Apr 6, 2019 at 5:03 PM Mehdi AMINI <joker.eph at gmail.com<mailto:joker.eph at gmail.com>> wrote:
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<mailto: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<mailto: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<mailto: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<mailto:llvm-dev at lists.llvm.org>> wrote:
On Thu, Apr 4, 2019 at 12:58 PM Chris Lattner <clattner at nondot.org<mailto:clattner at nondot.org>> wrote:
>
>
>
> > On Apr 4, 2019, at 5:37 AM, Don Hinton via llvm-dev <llvm-dev at lists.llvm.org<mailto: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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