[Patch, Fortran, OOP] PR 50919: Don't use vtable for NON_OVERRIDABLE TBP

[Janus Weil]

Hi all,

up to now we call all type-bound procedures in a dynamic way, i.e.
through their entry in the vtable. However, for non-overridable
procedures this is not necessary. Since they can not be overridden, a
call to those can be resolved at compile time to an ordinary function
call, without the need of a 'detour' through the vtable. This is what
the attached patch does, thereby removing unneeded overhead and
improving the possibility of optimization (inlining, etc).

The patch actually consists of two parts:
1) The resolve.c part prevents the conversion to a PPC call via the
_vptr (for functions and subroutines).
2) The class.c parts prevents adding the non-overridable TBP to the vtable.

As noted by Tobias, the second part breaks the ABI, so we might
consider deferring it until other ABI-breaking features will be
implemented (cf. http://gcc.gnu.org/wiki/LibgfortranAbiCleanup). On
the other hand, one could argue that the OOP ABI is still quite young
and hasn't really stabilized yet (it was broken already from 4.5 to
4.6), so we might as well break it again. I know that there are a
couple of real-world codes out there, which make use of gfortran's OOP
features already, but I have a hard time estimating how many such
projects exists, or how problematic an ABI breaking would be for them
(user input welcome).

So, the question is: Should I commit both parts, or only the resolve.c
one for now? The patch was regtested on x86_64-unknown-linux-gnu.

Cheers,
Janus


2011-11-06  Janus Weil  <janus@gcc.gnu.org>

	PR fortran/50919
	* class.c (add_proc_comp): Don't add non-overridable procedures to the
	vtable.
	* resolve.c (resolve_typebound_function,resolve_typebound_subroutine):
	Don't generate a dynamic _vptr call for non-overridable procedures.

2011-11-06  Janus Weil  <janus@gcc.gnu.org>

	PR fortran/50919
	* gfortran.dg/typebound_call_21.f03: New.
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