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

[David Greene via llvm-dev]

Don Hinton via llvm-dev <llvm-dev at lists.llvm.org> writes:

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

I don't think I would know what that means when I see it in code.

                       -David

>  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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