[cfe-dev] [Bug 12200] "noreturn" follows virtual method implementation (opposite of g++)

Richard Smith richard at metafoo.co.uk
Tue Jan 28 18:19:57 PST 2014

On Tue, Jan 28, 2014 at 5:41 PM, John McCall <rjmccall at apple.com> wrote:

> On Jan 28, 2014, at 4:52 PM, Richard Smith <richard at metafoo.co.uk> wrote:
> On Tue, Jan 28, 2014 at 10:34 AM, <bugzilla-daemon at llvm.org> wrote:
>> *Comment # 12 <http://llvm.org/bugs/show_bug.cgi?id=12200#c12> on bug
>> 12200 <http://llvm.org/bugs/show_bug.cgi?id=12200> from John McCall
>> <rjmccall at apple.com> *
>> (In reply to comment #10 <http://llvm.org/bugs/show_bug.cgi?id=12200#c10>)> There's also an IRgen problem: it should ignore 'noreturn' when making a
>> > (non-devirtualized) virtual call -- that's a property of the function, not
>> > of the vtable slot.
>> That's the GCC model.  Our design has always been that attribute((noreturn))
>> --- when declared in the class definition --- is a property of the signature of
>> the virtual function.
>> [Moving this discussion to cfe-dev for visibility.]
> This is a GCC attribute, so my first inclination is to say that GCC gets
> to define its semantics.
> Is there a specification for it that I've missed?

Sure, it's right there in GCC subversion ;-)

My understanding is that this is accidental fallout from a particularly
> poor representation choice that prevents them from supporting noreturn on
> member functions at all, and that GCC would consider this a (perhaps low
> priority or even unfixable) bug in their implementation.  Certainly our
> model is more consistent with the original philosophy behind making
> __attribute__((noreturn)) part of the function type.

I agree, but our model is incompatible with GCC's model, and this breaks
real code.

[For posterity, GCC's (bizarre) model is as follows... When applied to a
function, it means that function does not return. When applied to a
function pointer type or a function reference type, it affects the type.
The type of a function declared __attribute__((noreturn)) is *not* a
noreturn function type, and taking the address of a function declared
__attribute__((noreturn)) does *not* give a noreturn function pointer.]

Our model is also more expressive: among other things, it makes it
> meaningful to put __attribute__((noreturn)) on a virtual function
> declaration (including a pure one),

That's also meaningful in the GCC model: it means a devirtualized or
non-virtual call is known to not return.

> and it enables things like a member function that polymorphically throws
> its object as an exception.  In contrast, overriding a noreturn virtual
> function with a returning implementation seems basically useless -- what
> code could ever reasonably use it?

Code where the base class implements a function that always throws (maybe a
"not implemented" exception) and the derived class does not? This doesn't
seem unreasonable, and bug 12200 and its duplicate are examples reduced
from real code that did exactly this, as far as I can see.

> Do we actually have a strong case for diverging from GCC here? We have
> evidence that we're breaking existing code and damaging our portability
> story by doing so. (FWIW, C++11's [[noreturn]] behaves like GNU's
> [[noreturn]], and our implementation of _Noreturn does the same. I don't
> know how __declspec(noreturn) behaves.)
> Analogies to [[noreturn]] and _Noreturn are flawed because they are
> defined by the standards to not be part of the type.  If you want to argue
> that all of these attributes should be interchangeable, you're really
> arguing for a fundamental change to the GCC extension.

I don't want to argue these should be interchangeable -- just observing
that we are the only ones providing this
noreturn-virtual-function-implies-overrides-are-noreturn behavior (though
admittedly it's a small sample set, since _Noreturn doesn't count).

What I'm suggesting is that perhaps our GCC attribute should match GCC, and
if not, we should have a solid reason for the difference, documented
somewhere visible, explaining why we thought our semantics for a GCC
attribute were more important than compatibility with GCC, that we can
point people to when we get the next duplicate of this bug filed.
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