[cfe-dev] Could we emit inline virtual member functions as available_externally when there's a key function (same as vtable and debug info)?

[David Blaikie via cfe-dev]

I'm curious - how did it come up as a problem in devirtualization?

On Sat, Jun 4, 2016 at 7:33 AM, Piotr Padlewski via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

>
> 2016-06-04 4:07 GMT+02:00 John McCall via cfe-dev <cfe-dev at lists.llvm.org>
> :
>
>> > On Jun 3, 2016, at 7:05 PM, John McCall via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>> >> On Jun 3, 2016, at 8:43 AM, David Blaikie via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>> >> I assume there's a good reason we don't do this - but I can't quite
>> figure it out off the top of my head.
>> >>
>> >> Any thoughts?
>> >
>> > The translation unit that emits the key function is required to emit
>> the v-table symbol with as a strong definition with the type's visibility.
>> It is not required to emit visible symbols for any of the inline virtual
>> functions.  It is, of course, required to emit those functions as part of
>> emitting the v-table; but it doesn't have to provide symbols for them at
>> all, because even if it were possible to take the address of a virtual
>> member function in a comparable way, there is no way to take that address
>> *via the v-table* and so the v-table's reference may be to its own
>> implementation.  This is good because it allows virtual methods to be given
>> hidden visibility by default; it also permits v-tables to contain
>> specialized method implementations, e.g. methods that use a known
>> most-derived class.
>>
>> To be clear, your idea would be a reasonable guarantee to make in a
>> revised C++ ABI; it's just that it's not guaranteed to work under the
>> current one.
>>
>> John.
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>>
>
> This problem was pretty annoying when I was working on devirtualization.
> It would be cool to fix ABI to support that.
>
>
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