[cxx-abi-dev] manglings for exception specifications in function types

[Gabriel Dos Reis]

FWIW, I don't approve of making contracts part of the function type -- and
have said so in unambiguous terms at the last (Oulu) meeting.

On Wed, Oct 12, 2016 at 2:35 PM, David Vandevoorde <daveed at edg.com> wrote:

>
> On Oct 12, 2016, at 4:51 PM, John McCall <rjmccall at apple.com> wrote:
>
> […]
>
> I'll admit that I don't attend committee meetings, but I thought that
> implementation experience was expected *prior* to standardization, not
> something that gets done months after voting the thing in concurrently with
> the committee finalizing the language in a draft for next year's release.
>
>
> That’s cute ;-)
>
>
> You will note that I have omitted the necessary specializations for
>>> "transaction_safe", as well as the incredibly common extension of
>>> specialized calling conventions.
>>>
>>> This also breaks source compatibility for template matching, and
>>> basically every function template in the standard library is going to
>>> change manglings (and become *much* larger) due to noexcept expressions now
>>> being mangled.
>>>
>>
>> It's a problem, but I don't think it's as bad as you claim. The mangling
>> of a function still wouldn't include its exception specification; this
>> would only affect mangling in cases where a parameter or return type or
>> template argument involves a function type with an exception-specification
>> -- a lot less common than every function template in the standard library,
>> but this still does change manglings for existing code.
>>
>>
>> Okay, so it only triggers SFINAE failures in nested function types, and
>> you can't overload templates by it?  I agree that that helps a lot.
>>
>> And the entire proposal seems to have forgotten about
>>> reference-to-function types.
>>>
>>
>> The change to [dcl.init.ref]p4 allows a reference to non-noexcept
>> function to bind to a noexcept function, and this indirectly allows the
>> same during overload resolution, casts, and so on. What additional
>> considerations were missed?
>>
>>
>> I hadn't realized that the expression logic was so consistent about
>> defining e.g. the behavior of the conditional operator on l-values in terms
>> of reference binding.  I apologize.
>>
>> ...I see that this adds a new special case to exception handling.
>>
>
> Yes; I'd forgotten to mention this side of the ABI change.
>
> We'll also need a new flag on type_info objects to model this. In line
> with the transaction_safe changes that Jason proposed, I suggest adding a
> __noexcept_mask = 0x40 to __pbase_type_info, and representing a pointer to
> noexcept function as a pointer with __noexcept_mask bit set to the
> corresponding *non-noexcept* function pointer type.
>
>
> I strongly disagree; we should take this opportunity to revisit that
> decision.  The floodgates are open, and this set of function type
> attributes is clearly going to grow over time.  (It's actually
> transaction_safe that really drives this point home; noexcept is at least a
> longstanding part of the core language in various forms.)  We also have a
> lot of vendor-specific function type attributes that are part of the type
> but just aren't standardized and can't be represented in type_info.  I
> don't think it makes sense to indefinitely keep hacking these things into
> the pointer type flags; we should just bite the bullet and create a new
> function_type_info subclass.
>
>
> I agree with you (John).
>
> (That said, I’ll keep resisting adding standard bifurcations to function
> types. “contracts” and one alternative for coroutines are currently trying
> to add to that list.)
>
> Daveed
>
>
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