[petsc-dev] Quad precision and external libraries

[Patrick Sanan]

It seems to be possible to configure and build PETSc with
quad/__float128 precision only to find that your code won't run
because the external package you were planning on using doesn't
support quad precision (as it usually won't).

It seems that configure will not prevent you from building with
external packages which don't support quad precision; instead, the
make process will simply skip some things, as controlled by
"#requiresprecision double" in local makefiles.  This means that you
will eventually notice the problem when you get a link error during
"make test", like

Undefined symbols for architecture x86_64:
  "_MatSolverPackageRegister_SuiteSparse", referenced from:
      import-atom in libpetsc.dylib

It seems like this behavior includes a lot of the popular external
packages (including several sparse direct solvers, which is how this
was noticed), based on grepping the src/ tree for "requiresprecision".

There is also at least one package (SuperLU) that is missing this
flag. For instance, I can configure, build, and test with no visible
problems with these configure options with master (on my OS X laptop
with gcc from macports):
--with-precision=__float128 --download-superlu --with-cc=gcc
--with-cxx=g++ --with-fc=gfortran --download-mpich
--download-f2cblaslapack

I then only see failure when I try to use the external package. e.g.
for KSP ex1:

$ ./ex1 -pc_type lu -pc_factor_mat_solver_package superlu -ksp_converged_reason
Linear solve did not converge due to DIVERGED_PCSETUP_FAILED iterations 0
               PCSETUP_FAILED due to FACTOR_NUMERIC_ZEROPIVOT
...

So, if I haven't made any false assumptions yet,
1. it'd be easy to add any missing "#requiresprecision double" lines
to more makefiles (though I'm not sure what to do re single precision,
for packages like SuperLU that support single and double).
2.  It also might be worth going further to modify the BuildSystem to
allow packages to specify precision-dependency there. (And if that's
the way to go, note that in /lib/petsc/conf/rules there are also
procedures for real/complex and C/C++ conditional compilation, which
would likely affect external packages in the same way).