eh frame debug size improvement

[Richard Henderson <rth at cygnus dot com>]

With the following minimal patch, I get a 5 to 6 fold reduction in
.eh_frame on i686.  It is beginning to approach acceptible.

I have one idea to get things a bit smaller using only the tools
at hand: we need one CFA instruction to describe something akin
to m68k movem, in which a bit mask of registers is saved at 
consecutive locations.  The fact that x86 has no such insn is
irrelevant.  If the prologue is in one block (some relaxation of
the rule is possible), then we can consider all of the CFA insns
to occur at the same point.  This is important considering the
rather large penalty we incur with the current tools by moving
the CFA loc.

Judging from the sparc port, which does effectively this via its
window_save CFA op, we can potentially get another 2 fold reduction.

Unfortunately, the only way to get beyond this is adding smarts
to gnu ld to combine CIE's from across object files, and to recode
CFA instruction blocks now that the absolute distances are known.


r~
Sizes of text and frame unwind sizes and ratios for several
configurations.  Following proper statistical practice, the programs
listed were of course chosen at random and can therefore be expected
to be representative of their class.  ;-)


i686, base egcs

    genattr
	.text          10860     
	.eh_frame       5500     50.6%
    cpp
	.text          61852     
	.eh_frame      25428     41.1%
    f771
	.text        1532420     
	.eh_frame     944176     61.6%

sparc, base egcs

    genattr
	.text          12616       
	.eh_frame       1092      8.65%
    cpp
	.text          74820     
	.eh_frame       3960      5.29%
    f771
	.text        2025500     
	.eh_frame     110756      5.47%


i686, with patch

    genattr
	.text          10860     
	.eh_frame       1988     18.3%
    cpp
	.text          61852     
	.eh_frame       7060     11.4%
    f771
	.text        1532476     
	.eh_frame     191700     12.5%
Wed Oct 15 02:09:44 1997  Richard Henderson  <rth@cygnus.com>

	* dwarf2out.c (dwarf2out_stack_adjust): Skip if frame_pointer_needed.

Index: dwarf2out.c
===================================================================
RCS file: /cvs/cvsfiles/egcs/gcc/dwarf2out.c,v
retrieving revision 1.13
diff -u -p -d -r1.13 dwarf2out.c
--- dwarf2out.c	1997/10/07 21:35:54	1.13
+++ dwarf2out.c	1997/10/15 09:00:27
@@ -992,6 +992,14 @@ dwarf2out_stack_adjust (insn)
   long offset;
   char *label;
 
+  /* If this function uses a frame pointer, we know a-priori the frame
+     size does not vary.  If we don't have EH regions, we can unwind
+     the function with just the normal prologue info; if we do, we can
+     either reinit the stack pointer off of the frame pointer, or leave
+     it alone if EXIT_IGNORE_STACK.  */
+  if (frame_pointer_needed)
+    return;
+
   if (GET_CODE (insn) == BARRIER)
     {
       /* When we see a BARRIER, we know to reset args_size to 0.  Usually