On Jan 5, 2010, at 11:46 AM, Chris Lattner wrote:
>
> On Jan 5, 2010, at 11:06 AM, Douglas Gregor wrote:
>
>> Author: dgregor
>> Date: Tue Jan 5 13:06:31 2010
>> New Revision: 92753
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=92753&view=rev
>> Log:
>> Improve key-function computation for templates. In particular:
>> - All classes can have a key function; templates don't change that.
>> non-template classes when computing the key function.
>> - We always mark all of the virtual member functions of class
>> template instantiations.
>> - The vtable for an instantiation of a class template has weak
>> linkage.
>>
>> We could probably use available_externally linkage for vtables of
>> classes instantiated by explicit instantiation declarations (extern
>> templates), but GCC doesn't do this and I'm not 100% that the ABI
>> permits it.
>
> GCC is missing several optimizations in this area that prevent
> devirtualization etc. available_externally can be used any time we
> know the contents of the object and we know it will be emitted to
> some other translation unit. It just gives the optimizer the
> ability to see into the data without emitting it.
Yes, I know. The question is whether the ABI requires us to emit a
(weak) vtable when there is an explicit instantiation declaration,
since the ABI document just refers to "template instantiations" as if
they were all the same.
- Doug
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