[cfe-dev] Proposal: Managing ABI changes in libc++

[Marshall Clow]

> On Dec 8, 2014, at 8:40 AM, Renato Golin <renato.golin at linaro.org> wrote:
> 
> Hi Marshall,
> 
> I agree this is a good idea...
> 
> 
> On 8 December 2014 at 15:42, Marshall Clow <mclow.lists at gmail.com> wrote:
>> a) There are the people who ship libc++ in production systems, who say: Whoa! Don’t do that! Ever! (or at least “let us decide when”).
>> b) There are the people who use libc++ internally, who say: Is it faster? Does it work better? Do it!
> 
> And there are those that accept each one on different occasions. :)
> 
> 
>> Concrete steps:
>> 1) Give each ABI-breaking change its own "enabling macro”, that starts with “_LIBCPP_ABI_”
> 
> This sounds reasonable.
> 
> 
>> 2) Create a global macro “_LIBCPP_ABI_UNSTABLE” which, when defined, turns on ALL of the _LIBCPP_ABI_* changes.
> 
> While this can be problematic. What if group A wants ABI-breaking
> changes X and Y, while group B want Z? In the beginning, we could all
> all of them under the same umbrella, but as requests come in, we could
> end up with a set of sets, unions and intersections of macros, which
> are never pretty. Brainstorming a bit, to avoid intersections and
> separate sets, we could make it into a severity scale (like unstable
> -> broken -> napalm). But I'm over-engineering, as usual.
> 
> Having said that, I think the simple all or nothing will be perfect
> for 99% of the cases, so I think this could work very well in
> practice. We just have to make sure we test both variations as often
> as possible.

My thought here is that if someone wants a subset of the ABI-breaking changes (but not all of them), 
they can define the _LIBCPP_ABI_* flags individually in their build system.

Suppose we have:  in <__config>

#ifdef _LIBCPP_ABI_UNSTABLE
#define _LIBCPP_ABI_FEATURE1
#define _LIBCPP_ABI_FEATURE2
#define _LIBCPP_ABI_FEATURE3
#define _LIBCPP_ABI_FEATURE4
#endif

If someone wanted just FEATURE2 and FEATURE3, they could do:
	cmake <blah, blah> -D_LIBCPP_ABI_FEATURE2 -D_LIBCPP_ABI_FEATURE3

— Marshall