fix remaining ia64 compat failures

[Richard Henderson <rth at redhat dot com>]

Sometimes I really really hate gcc extensions.  The remaining 
failures are passing structures of the form

	struct S { float f[0]; };

That is, a zero sized structure containing a zero-sized member
that would otherwise cause the structure to be considered an
homogenous floating-point aggregate.

The immediate problem is that we'd create a PARALLEL for this
with no elements.  I considered several alternatives for fixing
this, but the most straight-forward seems to be to just not 
consider such a beast for HFA processing.

Fixes

FAIL: tmpdir-gcc.dg-struct-layout-1/t027 c_compat_x_tst.o compile
FAIL: tmpdir-gcc.dg-struct-layout-1/t027 c_compat_y_tst.o compile
FAIL: tmpdir-gcc.dg-struct-layout-1/t028 c_compat_x_tst.o compile
FAIL: tmpdir-gcc.dg-struct-layout-1/t028 c_compat_y_tst.o compile


r~


        * config/ia64/ia64.c (hfa_element_mode): Return false for
        zero-sized top-level aggregates.

Index: config/ia64/ia64.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/config/ia64/ia64.c,v
retrieving revision 1.337
diff -c -p -d -r1.337 ia64.c
*** config/ia64/ia64.c	30 Dec 2004 08:59:15 -0000	1.337
--- config/ia64/ia64.c	30 Dec 2004 20:41:19 -0000
*************** static rtx gen_movdi_x (rtx, rtx, rtx);
*** 186,192 ****
  static rtx gen_fr_spill_x (rtx, rtx, rtx);
  static rtx gen_fr_restore_x (rtx, rtx, rtx);
  
! static enum machine_mode hfa_element_mode (tree, int);
  static void ia64_setup_incoming_varargs (CUMULATIVE_ARGS *, enum machine_mode,
  					 tree, int *, int);
  static bool ia64_pass_by_reference (CUMULATIVE_ARGS *, enum machine_mode,
--- 186,192 ----
  static rtx gen_fr_spill_x (rtx, rtx, rtx);
  static rtx gen_fr_restore_x (rtx, rtx, rtx);
  
! static enum machine_mode hfa_element_mode (tree, bool);
  static void ia64_setup_incoming_varargs (CUMULATIVE_ARGS *, enum machine_mode,
  					 tree, int *, int);
  static bool ia64_pass_by_reference (CUMULATIVE_ARGS *, enum machine_mode,
*************** ia64_setup_incoming_varargs (CUMULATIVE_
*** 2968,2977 ****
  
     An aggregate is a homogeneous floating point aggregate is if all
     fields/elements in it have the same floating point type (e.g,
!    SFmode).  128-bit quad-precision floats are excluded.  */
  
  static enum machine_mode
! hfa_element_mode (tree type, int nested)
  {
    enum machine_mode element_mode = VOIDmode;
    enum machine_mode mode;
--- 2968,2981 ----
  
     An aggregate is a homogeneous floating point aggregate is if all
     fields/elements in it have the same floating point type (e.g,
!    SFmode).  128-bit quad-precision floats are excluded.
! 
!    Variable sized aggregates should never arrive here, since we should
!    have already decided to pass them by reference.  Top-level zero-sized
!    aggregates are excluded because our parallels crash the middle-end.  */
  
  static enum machine_mode
! hfa_element_mode (tree type, bool nested)
  {
    enum machine_mode element_mode = VOIDmode;
    enum machine_mode mode;
*************** hfa_element_mode (tree type, int nested)
*** 2979,2984 ****
--- 2983,2991 ----
    int know_element_mode = 0;
    tree t;
  
+   if (!nested && (!TYPE_SIZE (type) || integer_zerop (TYPE_SIZE (type))))
+     return VOIDmode;
+ 
    switch (code)
      {
      case VOID_TYPE:	case INTEGER_TYPE:	case ENUMERAL_TYPE: