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

[Craig Topper via llvm-dev]

There are a handful of places in LLVM that dosomething like  if
(dyn_cast_or_null<UndefValue>(P->hasConstantValue()))

~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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