Re: [RFC PATCH] Optionally use -mlong-double-128 by default on {powerpc{,64},s390{,x},sparc,alpha}-linux

[Daniel Jacobowitz <drow at false dot org>]

On Fri, Jan 27, 2006 at 12:34:59PM -0500, Jakub Jelinek wrote:
> if yes, then a --with-long-double-128 compiler and then you just need
> to ensure that whenever you rebuild some package that uses long double
> in its exported ABI in interfaces to other package (e.g. long double
> related functions in libraries etc.), then you also rebuild the related
> packages together.  All I was saying is that for C++ the
> --with-long-double-128 compiled compiler only ensures libstdc++.so.6
> backward compatibility with already linked programs and shared libraries,
> but if you want to be able to use sometimes -mlong-double-64 for C++ and sometimes
> -mlong-double-128 for C++ with the same compiler, you need to add
> mlong-double-64 multilibs.

OK, so the short version is that only libraries which export long
doubles are affected (as I'd expect), and not libraries using libstdc++
(which is what I inferred from your previous post).  That's more
workable (although still a pain).

> > > But, with shared libraries we need just one library with a given SONAME
> > > that is backwards compatible, DT_RPATH hacks would just lead to extreme
> > > insanity and wouldn't really work.
> > > So, I want to do the symbol versioning part of 1) in libstdc++ too
> > > (but don't do any header redirections).
> > 
> > The ICC folks at last year's GCC summit talked about a more
> > C++-friendly way to solve this sort of problem; is that suitable here?
> > I don't remember whether it could be incrementally adopted.
> 
> I haven't been at last year's GCC summit, do you have any references
> to that?

Some details were discussed offline, but much of it is in their
presentation: "C/C++ Compatibility on Linux" in the 2005 proceedings,
which you can find on gccsummit.org.  It's around page 175.

-- 
Daniel Jacobowitz
CodeSourcery