[RFC] ABI change for variable sized types

[Richard Henderson <rth at redhat dot com>]

So I've been working for about 2 weeks trying to come up with a design
to break the tree/rtl phase ordering problem in assign_parms.  I've 
implemented and discarded as flawed three variations on that theme.

Having spent Sunday night scratching out a design for a fourth try, I
was cranky Monday morning when I logged on to the gcc irc channel and
offered a contract killing on whomever first thought variable-sized
pass-by-value was a good idea.

Then David Edelsohn asked me why we didn't just change the ABI.

Somehow, it had never occurred to me that this was an option.  One
I've grown more and more fond of as I've thought about it over the 
past two days.

It would affect, as far as I can tell, only Ada, and only in cases
in which the size of the type is variable but whose size is not
computed from the object itself (CONTAINS_PLACEHOLDER_P).

The cases of variable sized structures as GCC extensions in C are,
as far as I can tell, only valid in the context of nested functions.
Which means that the functions are private to the translation unit,
and thus there is no ABI change.

Further, it would affect only a subset of targets:

	alpha	avr	c4x	h8	i386
	i860	ip2k	m32r	m68hc11	m68k
	ns32k	pdp11	stormy16	vax
	xtensa

It would not affect

	arc	arm	cris	fr30	frv
	x86-64	ia64	iq2000	m32r	mcore
	mips	mmix	mn10300	pa	rs6000
	s390	sh	sparc	v850

as these targets already pass all variable sized types by reference.

If it weren't for i386, the "no one cares" factor would probably be
high enough to make this change unilaterally.

I believe I've now cleaned up all of the backends such that making
this change is the simplest of patches, as appended.

If this proposal is rejected, I'll go back to my fourth design.
I do have some hope of it actually working, but it'll be complex.

Thoughts?



r~



	* function.c (pass_by_reference): True for any variable sized type.

Index: function.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/function.c,v
retrieving revision 1.557
diff -c -p -d -r1.557 function.c
*** function.c	14 Jul 2004 07:30:19 -0000	1.557
--- function.c	14 Jul 2004 21:24:24 -0000
*************** pass_by_reference (CUMULATIVE_ARGS *ca, 
*** 2039,2047 ****
        if (TREE_ADDRESSABLE (type))
  	return true;
  
!       /* If an object's size is dependent on itself, there's no way
! 	 to *not* pass by reference.  */
!       if (CONTAINS_PLACEHOLDER_P (TYPE_SIZE (type)))
  	return true;
      }
  
--- 2039,2046 ----
        if (TREE_ADDRESSABLE (type))
  	return true;
  
!       /* GCC post 3.4 passes *all* variable sized types by reference.  */
!       if (!TYPE_SIZE (type) || TREE_CODE (TYPE_SIZE (type)) != INTEGER_CST)
  	return true;
      }