[petsc-dev] Internal and external symbols

[Jed Brown]

If you configure --with-visibility, PETSc will compile with
-fvisibility=hidden so that any symbols that are _not_ declared with extern
or PETSC_EXTERN will become local to the file. Unfortunately, when
configure --with-clanguage=C++ --with-c-support, these internal symbols
will have their names mangled and will expect function pointers to _not_ be
extern "C" (unless using an extern "C" typedef). This is a relatively minor
issue, but I would like to be more precise about what symbols we wish to be
accessible. Note that anything that we want to be directly referenced by a
plugin must be PETSC_EXTERN. This means that most functions in
include/petsc-private should be PETSC_EXTERN, even though we don't expect
normal users to call them.

I just pushed a couple patches that add PETSC_INTERN and PETSC_INTERN_C (to
be used when you always need an extern "C" interface) and applied the rule
to a bunch of the src/**/*impl.h declarations. This reduces the number of
exported symbols by a factor of 2, speeding up dispatch (no more calling
through the PLT), allowing more shared-library code to be shared between
multiple processes (important when running many MPI processes on
small-memory nodes), and reducing linking and loading time.

There are still more places that we could tighten up, but we probably need
to refine our concept of which symbols are meant to be directly addressable
from outside PETSc. Anyway, the initial guidelines from
https://bitbucket.org/petsc/petsc-dev/commits/8413f6e91807356123f19f648513e17d0ebc6e4eare

PETSC_INTERN is for symbols that are shared between files, but only used
within one shared library. Note that PETSC_EXTERN must be used for symbols
that are accessed from other shared libraries (e.g., a symbol in
petscmat.so that is accessed from petscksp.so).

PETSC_INTERN_C is for functions with internal visibility that must use the
`extern "C"` calling convention. Normally this is any symbol passed to a
dynamic registration function.
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