r218141 - In the Itanium ABI, move stuff to the comdat of variables with static init.

[David Majnemer]

On Thu, Dec 18, 2014 at 5:46 PM, Richard Smith <richard at metafoo.co.uk>
wrote:
>
> On Thu, Dec 18, 2014 at 2:30 PM, David Majnemer <david.majnemer at gmail.com>
> wrote:
>>
>> On Wednesday, December 17, 2014, Richard Smith <richard at metafoo.co.uk>
>> wrote:
>>
>>> On Wed, Dec 17, 2014 at 3:52 PM, Rafael Espíndola <
>>> rafael.espindola at gmail.com> wrote:
>>>>
>>>> > Based on talking to David, we think the frontend should be
>>>> responsible for
>>>> > picking whether things are ODR or not, and then LLVM optimizations can
>>>> > continue to be mostly ignorant of comdats. For example, the D5
>>>> destructor
>>>> > comdat group functions should all be inlinable.
>>>>
>>>> +1 :-)
>>>>
>>>
>>> I think that makes a lot of sense. Does this imply that globals within a
>>> comdat would need to have some kind of weak linkage?
>>>
>>
>> The only non-weak linkage we allow in COMDATs is external.  The closest
>> analog is weak_odr but it has subtly different semantics wrt duplicate
>> symbol definitions.
>>
>> I'll try to dig up a concrete example as soon as I can get to my laptop.
>>
>
> I think Rafael's right here: external symbols in a comdat are meaningful
> (and presumably they mean that you can't have that symbol defined in any
> other object file you link in, so you either get your definition or no
> definition), and I can imagine (rare) cases when they might be what you
> want. (And if not, don't do that...)
>

I think it means something different from what you are suggesting.
STB_WEAK is orthogonal to COMDAT-ness.  Clang when targeting MSVC uses
COMDATs with external linkage to mean:
- This symbol is in some COMDAT group
- It is not weak, don't replace my symbol with some non-weak definition.
- This symbol cannot be discarded.

I don't think there is another combination of linkage and comdat-ness which
gets the same effects.
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