[petsc-dev] CFLAGS and COPTFLAGS

[Matthew Knepley]

On Fri, Jan 17, 2014 at 9:46 PM, Jed Brown <jed at jedbrown.org> wrote:

> Barry Smith <bsmith at mcs.anl.gov> writes:
>
> >    Yes, What if they just set a single flag having nothing to do with
> >    optimization. This will turn off PETSc’s attempt to provide any
> >    kind of optimization flag (even with —with-debugging=0). I think
> >    this is bad. *
>
> Is it really that bad?  Other packages usually don't try to do anything
> smart and I think there is a very real cost to second-guessing the user.
> Worse, the user has to wait minutes for configure to complete before
> reading the printout carefully to learn what PETSc decided to use.
>
> I think we still want warning flags because users are unlikely to guess
> decent ones.  And -fPIC needs to be automatic based on shared libs.
>
> >>>   Could we do the following?
> >>>
> >>>   Look through any provided CFLAGS (FFLAGS, CXXFLAGS) if we detect
> >>>   something that looks like optimization then act as if COPTFLAGS was
> >>>   set and do not set our own optimization default flag? We can also
> >>>   keep support for the COPTFLAGS stuff
> >>>
> >>>   We can look for -O%d , are there other things to look for?  This
> seems easy enough to add.
> >>
> >> -ftree-vectorize, -fast, -qhot -qsimd=auto
> >
> >    So if they pass -fast then we simply don’t set any optimization flags
> ourself? We could do that.
>
> My point was that there are *lots* of optimization flags, with new ones
> every release from every vendor.  We could test for a few, but can never
> be comprehensive.  Do we want to be in this business?
>

I am a fan of our current organization, which is the right mix of
flexibility and performance. I don't
think we can do much better, since we are stuck with the crap command line
interface to compilers.
I hate compilers.

   Matt

-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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