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

[David Majnemer]

On Thu, Dec 18, 2014 at 5:56 PM, Richard Smith <richard at metafoo.co.uk>
wrote:
>
> On Thu, Dec 18, 2014 at 5:52 PM, David Majnemer <david.majnemer at gmail.com>
> wrote:
>>
>> 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.
>>
>
> ... in that case, all our interprocedural optimizations *do* need to know
> about comdats, and being in a comdat needs to make an external symbol
> effectively weak for optimization purposes (you can't propagate information
> out of it, because it might be replaced by a different definition). This
> violates Reid's desire that optimizations can be mostly ignorant of comdats.
>

An external symbol in a COMDAT should be identical in all COMDAT groups
that it could be in.  If you can't find issue with it, I'll add it to the
langref.
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