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

[Don Hinton via llvm-dev]

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
>>>
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>
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