Re: [siddhesh/is_in_module] Fix SHLIB_COMPAT -Wundef warning

[Roland McGrath <roland at hack dot frob dot com>]

I think the general approach is sound.  I was going to call the main
macro just IS_IN, and I still like that a bit better than IS_IN_MODULE.

Rather than the last patch, just make the headers test _LIBC and make it
such that IS_IN (*) is always false #ifndef _LIBC.  Likewise, every
place you touch that is "#if defined _LIBC && IS_IN..." can be simplified.

libc-modules.h badly needs comments explaining the scheme (plus the
usual header).  I don't really like maintaining that big list and
picking numbers by hand.  IMHO it would be better to generate a header
from a simple list of names.  Most of the elements of that list can be
culled from shlib-versions files by script, and only the few
non-libraries need to be manually maintained in a list.

I'm not entirely sure that IN_MODULE should be defaulted that way.  It
seems more right that we just ensure that it's always defined for every
compilation command we run.  Part of doing that (and also generally
cleaner) could be to have a dedicated makefile variable for it, rather
than just adding it to CPPFLAGS and the like.  I think we should avoid
any scheme where we wind up producing -Dfoo=blah -Ufoo -Dfoo=bar.

I don't think I fully understand what's going on with MODULE_libs.  At
any rate, I don't think that distinction should be made by having the
rhs of -D do arithmetic.  With a generate header file, it would be
straightforward to just encode the "category" distinctions by ordering
the numerical values and then test with < and >.

The #elif chain to define MODULE_NAME is particularly bletcherous.  It
would be unnecessary if -DMODULE_NAME=foo is what's passed and
PASTE(MODULE_, MODULE_NAME) is what's used to define IN_MODULE.  Of
course, with a generated header you can do it in a variety of ways that
are all equally fine because there is just as little to be maintained by
hand.

-DRUNTIME_LINKER belongs in CPPFLAGS-interp.c rather than
CFLAGS-interp.c just as much as IN_*/NOT_* do.  An unrelated cleanup is
that that RUNTIME_LINKER really should not be set via -D at all.  There
should be a file generated from the makefile (and using move-if-change)
so that dependencies track it.

What about adding an IS_IN_SHARED(foo) and IS_IN_STATIC(foo) that just
combine IS_IN(foo) with {!,}defined(SHARED) so that we can simplify the
boilerplate for those cases and also get closer to the day when we can
use typo-proof #if for SHARED as well?


Thanks,
Roland