On Jul 26, 2013, at 2:09 PM, Jonathan Schleifer <[email protected]> wrote:
> thanks for reviewing it.
>
>> This isn’t really a language extension; it’s an ABI feature that fixes a
>> language
>> feature. I don’t think it rises to the level of being worth documenting in
>> the
>> language-extensions section.
>
> I agree here, a lot. However, Eli asked me to document it there as all other
> __has_feature are documented there.
>
>> I’m also skeptical that this deserves to be exposed to the user with
>> __has_feature.
>> Do you have a use case for that, or are you just doing it for generic
>> reasons?
>
> Yes! What I do is check that there is an assembly implementation for
> of_forward_stret the architecture and that
> __has_feature(objc_msg_lookup_stret) is true. If that is the case, I set a
> define so that it's possible to check if forwarding is available with
> structs. This is important in e.g. the automatic tests: Without such a
> define, it would just always try to forward for stret - and crash.
>
>> If it is important to expose this with __has_feature, you should really
>> abstract this
>> into ObjCRuntime, like what we do with hasTerminate() (which I’ll note also
>> does
>> not have a __has_feature check).
>
> Yes, __has_feature is important as it's the only way to see if Clang supports
> it. After all, the runtime has to provide of_forward_stret and
> objc_msg_lookup_stret, but both are useless if Clang does not emit calls to
> objc_msg_lookup_stret. Thus the __has_feature to check if all three are
> given. It's the only way to make it possible to let code check if that
> feature is available instead of just hoping it is and crash.
I feel like making your runtime's unit tests more portable across compiler
versions isn’t really a compelling reason to expose something like this that’s
essentially a fix to an existing language feature. We don’t get to remove
__has_feature checks ever, so we’ve generally tried to avoid them for
improvements like this, because otherwise the logic produces a plausible
argument for basically every change to the compiler to get an independent
__has_feature check.
John.
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