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

[Don Hinton via llvm-dev]

On Sat, Apr 6, 2019 at 11:18 AM Zachary Turner via llvm-dev <
llvm-dev at lists.llvm.org> 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;
> }
>

I like the way you're thinking, but how about something a bit simpler:

template <class X, bool Check, class Y,
          typename std::enable_if<Check, X>::type * = nullptr>
LLVM_NODISCARD inline bool isa(const Y *Val) {
  return Val && isa<X>(Val);
}

template <class X, bool Check, class Y,
          typename std::enable_if<!Check, X>::type * = nullptr>
LLVM_NODISCARD inline bool isa(const Y *Val) {
  return isa<X>(Val);
}

Used like this:  isa<T,1>(v) or isa<T, true>(v)


>
> 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
>>>
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