compiler hints

[Dirk Herrmann]

Hello folks,

in error.h, the macro SCM_NORETURN is defined.  When compiling on gcc,
this macro expands to a compiler hint telling the compiler that the
corresponding function will never return.  This is a nice option offered
by gcc, since it eliminates a lot of compiler warnings without need to add
some dummy return statement at different points of the code.  Further, it
is usefull for a developer, since it offers additional information about
the code.

I figured out that when guile is compiled with the -W option in addition
to -Wall, gcc complains about a lot of unused function parameters.  A few
probes showed, that these are no bugs - some function just provide some
extra parameters for the sake of a clean interface, even if not all of
them are used.  However, the compiler still warns about such places.  For
this situation, gcc offers the possibility to mark a variable, function or
function parameter as potentially unused.

Thus, I suggest:

* Add a section {Compiler hints} to __scm.h
* Move the definition of SCM_NORETURN from error.h to __scm.h
* Add a definition SCM_UNUSED to __scm.h
* Add SCM_UNUSED to intentionally unused parameters.

There are further compiler hints available with gcc.  For example, the gcc
function attribute 'const' declares a function to be sideeffect free,
which potentially allows for more compiler optimizations.  Thus, it may
make sense to use a common naming scheme for compiler hints like
SCM_HINT_NORETURN or something similar instead of plain SCM_NORETURN or
SCM_UNUSED.  Hints for other compilers than gcc could be added in the same
way.

Best regards,
Dirk Herrmann