tuple move constructor

[Marc Glisse]

On Thu, 26 May 2016, Jonathan Wakely wrote:

> On 25/05/16 14:54 +0100, Jonathan Wakely wrote:
>> On 23/05/16 20:39 +0200, Marc Glisse wrote:
>>> Ping
>>> 
>>> (re-attaching, I just added a one-line comment before the tag class as 
>>> asked by Ville)
>> 
>> This is OK for trunk - thanks.
>
> On second thoughts - does this change the passing conventions for
> std::tuple if it gets a trivial move ctor?

Note that this part of the ABI is ill-defined
http://sourcerytools.com/pipermail/cxx-abi-dev/2016-February/002884.html

but yes, good catch, it does change the passing convention (by value), and 
not just for weirdo types, it even changes for tuple<int>. It is clearly a 
change in the right direction, not passing tuple<int> in a register is 
weird, but yeah, compatibility :-(

I don't even want to think of trying to fix this issue in C++11 while 
artificially preserving the non-triviality of tuple, the headache is not 
worth it. I guess I'll open an entry in bugzilla with the ABI tag and let 
it rot there...

Maybe we could

#if __cpp_concepts >= 201500
the alternative discussed with Ville
#endif

but that won't fix the fact that tuple<int> should be trivially move 
constructible...

We could add __attribute__(non_trivial_for_purpose_of_passing_convention), 
but I think abi_tag has already stretched enough the idea that gcc is 
following the itanium abi.

Bah, forget this patch. Thanks for noticing early, that spares me the 
trouble of reverting later.

-- 
Marc Glisse