Re: [RFC] Update gmp/mpfr/mpc minimum versions

[Martin Sebor <msebor at gmail dot com>]

On 04/27/2016 09:47 AM, Bernd Edlinger wrote:
> Am 27.04.2016 um 17:37 schrieb Rainer Orth:
> > Bernd Edlinger <bernd.edlinger@hotmail.de> writes:
> >
> > > On 26.04.2016 22:14, Joseph Myers wrote:
> > > > On Tue, 26 Apr 2016, Bernd Edlinger wrote:
> > > >
> > > > > Hi,
> > > > >
> > > > > as we all know, it's high time now to adjust the minimum supported
> > > > > gmp/mpfr/mpc versions for gcc-7.
> > > >
> > > > I think updating the minimum versions (when using previously built
> > > > libraries, not in-tree) is only appropriate when it allows some cleanup in
> > > > GCC, such as removing conditionals on whether a more recently added
> > > > function is available, adding functionality that depends on a newer
> > > > interface, or using newer interfaces instead of older ones that are now
> > > > deprecated.
> > > >
> > > > For example, you could justify a move to requiring MPFR 3.0.0 or later
> > > > with cleanups to use MPFR_RND* instead of the older GMP_RND*, and
> > > > similarly mpfr_rnd_t instead of the older mp_rnd_t and likewise mpfr_exp_t
> > > > and mpfr_prec_t in fortran/.  You could justify a move to requiring MPC
> > > > 1.0.0 (or 1.0.2) by optimizing clog10 using mpc_log10.  I don't know what
> > > > if any newer GMP interfaces would be beneficial in GCC.  And as always in
> > > > such cases, it's a good idea to look at e.g. how widespread the newer
> > > > versions are in GNU/Linux distributions, which indicates how many people
> > > > might be affected by an increase in the version requirement.
> > >
> > > Yes I see.
> > >
> > > I would justify it this way: gmp-6.0.0 is the first version that does
> > > not invoke undefined behavior in gmp.h, once we update to gmp-6.0.0
> > > we could emit at least a warning in cstddef for this invalid code.
> > >
> > > Once we have gmp-6.0.0, the earliest mpfr version that compiles at all
> > > is mpfr-3.1.1 and the earliest mpc version that compiles at all is
> > > mpc-0.9.  This would be the supported installed versions.
> > >
> > > In-tree gmp-6.0.0 does _not_ work for ARM.  But gmp-6.1.0 does (with a
> > > little quirk).  All supported mpfr and mpc versions are working in-tree
> > > too, even for the ARM target.
> > >
> > > When we have at least mpfr-3.1.1, it is straight forward to remove the
> > > pre-3.1.0 compatibility code from gcc/fortran/simplify.c for instance.
> > >
> > > So I would propose this updated patch for gcc-7.
> >
> > would this version combo (gmp 6.0.0, mpfr 3.1.1, mpc 0.9) also work on
> > the active release branches (gcc-5 and gcc-6, gcc-4.9 is on it's way
> > out)?  Having to install two different sets of the libraries for trunk
> > and branch work would be extremely tedious.
> >
> > 	Rainer
>
> Yes, when they are pre-installed there should be no problem.
> Also newer versions than these seem to work.
>
> In-tree only the versions that download_prerequisite picks are
> tested and guaranteed to work.

I was made aware today that my recent patch for pr49905 broke
bootstrap with MPFR 2.4:

  https://gcc.gnu.org/ml/gcc-patches/2016-09/msg01507.html

In light of this risk and given that the recommended MPFR version
is still 2.4 I wonder if the download_prerequisites script shouldn't
instead download the minimum supported version.  That way those of
us working with MPFR but not intimately familiar with its version
specific details would have an easier way of avoiding such breakage.

Alternatively, perhaps the script could be extended to make it
possible to choose between the most recent and the recommended
versions of the prerequisites that GCC is intended to work with,
and people who make use of either in their code encouraged to
test with both.

Martin