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

[Don Hinton via llvm-dev]

I'm leaning toward Herbert's suggestion:  `nonnull_and_isa`.

What do you think?

On Thu, Apr 4, 2019 at 7:17 PM Don Hinton <hintonda at gmail.com> wrote:

> On Thu, Apr 4, 2019 at 7:10 PM Craig Topper <craig.topper at gmail.com>
> wrote:
>
>> Agreed that the new isa_or_null style is better. Just wanted mention the
>> other style so we know we should migrate those to the new one.
>>
>
> I have a checker under review that could be enhanced to do that -- though
> it currently replaces `X->foo() && isa<Y>(X->foo())` with
> `dyn_cast_or_null<Y>(X->foo())`.
>
> Please see: https://reviews.llvm.org/D59802
>
> thanks...
> don
>
>
>>
>> ~Craig
>>
>>
>> On Thu, Apr 4, 2019 at 4:37 PM Don Hinton <hintonda at gmail.com> wrote:
>>
>>> On Thu, Apr 4, 2019 at 6:29 PM Craig Topper <craig.topper at gmail.com>
>>> wrote:
>>>
>>>> There are a handful of places in LLVM that dosomething like  if
>>>> (dyn_cast_or_null<UndefValue>(P->hasConstantValue()))
>>>>
>>>
>>> Yes, I've seen those, but while working on a new checker, I was advised
>>> that replacing `X && isa<Y>(X)` with `dyn_cast_or_null<Y>(X)` was
>>> suboptimal, and it was suggested something like a `isa_or_null` style
>>> operator would better express what was actually going on, i.e., we are
>>> expecting a bool, not a pointer.
>>>
>>>
>>>>
>>>> ~Craig
>>>>
>>>>
>>>> On Thu, Apr 4, 2019 at 4:16 PM Don Hinton via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>>> I've added a patch, temporarily using the name Chris suggested.
>>>>> Please let me know what you think.
>>>>>
>>>>> https://reviews.llvm.org/D60291
>>>>>
>>>>> thanks...
>>>>> don
>>>>>
>>>>> On Thu, Apr 4, 2019 at 2:55 PM David Greene <dag at cray.com> wrote:
>>>>>
>>>>>> Don Hinton <hintonda at gmail.com> writes:
>>>>>>
>>>>>> > >  if (isa_or_null<T>(var)) {
>>>>>> > >    ...
>>>>>> > >  }
>>>>>> > >
>>>>>> > >  at least according to what "isa_or_null" conveys to me.
>>>>>> >
>>>>>> > This is the same convention used by the existing "_or_null"
>>>>>> varieties,
>>>>>> > i.e., "cast_or_null" and "dyn_cast_or_null".  They accept a null and
>>>>>> > propagate it.  In the "isa" case, it would accept a null and
>>>>>> propagate
>>>>>> > it as false.
>>>>>>
>>>>>> isa<> is very different from *cast<>.  *cast<> gives you a pointer
>>>>>> back,
>>>>>> which may be null.  isa<> is precondition check, so it "reads"
>>>>>> differently to me.  If I were to see:
>>>>>>
>>>>>> if (isa_or_null<T>(var)) {
>>>>>>   ...
>>>>>> }
>>>>>>
>>>>>> I would think, "Ok, the body is fine if var is null."
>>>>>>
>>>>>> Instead:
>>>>>>
>>>>>> if (exists_and_isa<T>(var)) {
>>>>>>   ...
>>>>>> }
>>>>>>
>>>>>> This tells me that the body expects a non-null value.
>>>>>>
>>>>>> > >  That said, I'm not sure sure we need a special API for this.  Are
>>>>>> > >  expensive calls used in the way you describe really common?
>>>>>> >
>>>>>> > I've only been looking at the ones involving method calls, but it's
>>>>>> > not too common.  Perhaps a dozen in clang/lib -- haven't run it
>>>>>> > against the rest of the code base.
>>>>>>
>>>>>> Thanks for checking.  I don't have a strong opinion about the need
>>>>>> either way, but I do care that the spelling is clear and intuitive.
>>>>>>
>>>>>>                            -David
>>>>>>
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>
>>>>
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