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

[Don Hinton via llvm-dev]

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
>>>>
>>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190404/bd42749b/attachment.html>