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C++ ABI RFC [was Re: C++/libiberty PATCH for many mangling fixes (6057, 48051, 50855, 51322, etc)]

> bkoz pointed out that I forgot to update invoke.texi about 
> -fabi-version=6.  Applying to trunk

I've been thinking about this.

As it turns out, the mangling changes don't really impact the explicit
instantiations compiled in to So, it seems possible to say

1. compile with -fabi-version=6, and run the testsuites
with the default abi (2) and have everything work. 

2. compile with the default abi and run the testsuites
with -fabi-version=6. (Haven't tested this one completely yet.)

Nothing tricky seems to be instantiated. That gives us a wider field of
options for dealing with ABI issues...

Here's my favorite idea at the moment for dealing with what we should
be trying to encourage, ABI experimentation as part of GCC's mission
for both technical excellence and liberty in computing. 

I think a compelling case could be made to ship 4.7 with a
configure-time flag that sets the default C++ dialect to C++11.

So, I would propose

--with-std = dialect


--with-std-version=c dialect default, c++ dialect default

be added to the top-level gcc configure and documented here:


    Specify a string that identifies the -std=dialect defaults for the
    built compiler and any associated runtimes. Default values are
    gnu99, g++98. For example, -std=gnu11,gnu++11 would build a
    compiler whose default language standards for "C" and C++ would be
    the GNU extensions to the C11 and C++11 standards.

Then, I believe for C++ targets defaulting to C++11, then the language
would be set to -fabi-version=6 and any other changes, and for the
library, we'd use .so.7 and namespace versioning, and do the
basic_string swap to __gnu_cxx::__versa_string (or alias it? hmm....)
and any other changes.


Why do it this way?

Because I think this would be an easier way for maintainers and other
interested parties to experiment with changes. If we are happy and
things look promising, then these people can lobby the rest of the
community (with their collected experimental evidence) to change the
language defaults. Or not, since people will now be able to just set it
themselves if they feel strongly about the issue.

I suspect that the standard practice in the wider communities and/or
sub-communities will become clear with a mechanism like this. At least,
we'll have a better idea of which sub-communities care and which don't.
That's still better than what we've got now... it's been a mighty long
time to be at -fabi-version=2 and

Lemme know what you think. 


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