Re: gfortran and -mlong-double-128

[Jakub Jelinek <jakub@redhat.com>]

On Thu, Feb 16, 2006 at 01:57:39PM +0100, Fran?ois-Xavier Coudert wrote:
> > I guess libgfortran has configury to figure out if kind=16 is available?
> 
> Yes.
> 
> > If so then libgfortran should be built with -mlong-double-128, as this
> > should only add extra symbols that do not conflict with kind=4 and kind=8
> > ones.
> 
> Hum, that has to be done early in the configury (before all
> autodetection). Do you think it's better suited at the beginning of
> libgfortran/configure.ac (a special test, to see if -mlong-double-128
> is available, and if it is add it to CFLAGS), or should it be done in
> an upper level (and here is the limit of my understanding of the build
> mechanism)?

That will not work.  It is not enough if GCC supports -mlong-double-128
switch, you also need runtime library support for that.
So the check needs to be something like
http://gcc.gnu.org/ml/gcc-patches/2006-02/msg01075.html
(still not reviewed :( ).  If you configure gcc with --with-long-double-128
(what I'm using e.g. on redhat/gcc-4_1-branch now for
{ppc*,s390*,sparc,alpha}-*-linux*), then libgfortran is automatically built
with -mlong-double-128 and can cope with both -mlong-double-128 and
-mlong-double-64 code.  libstdc++.so will only support both
-mlong-double-{64,128} if it is built with -mlong-double-128, so it is in
the same boat as libgfortran.
Not sure if it makes any sense to detect the presence of glibc 2.4+ and
build libstdc++-v3/libgfortran with -mlong-double-128 even if configured
with --without-long-double-128.

	Jakub