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Re: [patch] Use abi_tag attribute on std::list


On 03/10/14 16:25 +0200, Marc Glisse wrote:
On Fri, 3 Oct 2014, Jonathan Wakely wrote:

This is the patch I intend to commit to make std::list::size() O(1) as
required by C++11.

This is an ABI change, so std::list will get tagged with
abi_tag("cxx11") so that it mangles differently.

Assuming a future where we have both _GLIBCXX_ABI_TAG_CXX11 and _GLIBCXX_ABI_TAG_CXX17, I don't really see how _GLIBCXX_DEFAULT_ABI_TAG is supposed to work. We don't want to define _GLIBCXX_DEFAULT_ABI_TAG to _GLIBCXX_ABI_TAG_CXX17 and suddenly have std::list change mangling.

True.

Should it be called _GLIBCXX_DEFAULT_ABI_TAG_CXX11, meaning _GLIBCXX_ABI_TAG_CXX11_IF_ENABLED_AND_NOTHING_OTHERWISE?

I suppose so ... or _GLIBCXX_MAYBE_ABI_TAG_CXX11 ?

Defining a dummy _M_distance in the old abi case is a bit strange (we could protect the single use with #if _GLIBCXX_USE_CXX11_ABI), but why not...

Yeah, I was trying to minimise the preprocessor conditionals, but
maybe that's one step too far.

Do you mind if I move (in a future patch once yours is committed) _M_size into _M_impl::_M_node as suggested in PR 61347?

Gah, that's where I had it until earlier this week, and I looked at it
and wondered why it was in the _List_impl class (because you only need
one member in there to benefit from the empty base-class
optimisation).

I will move it back there, since I already have that code on another
branch, so there's no point making you change the code to match
something I've already got!

Thanks for your useful comments (as always).


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