[RFC] .debug_frame adjustments for mips-irix

[Olivier Hainque <hainque at adacore dot com>]

Eric Christopher wrote:
> Please do.
> 
> thanks.

 The first issue was .debug_frame sections not emitted without -g for
 mips-sgi-irix6.5 configured with --with-gnu-as, despite those bits in
 iris6.h:

   /* Force the generation of dwarf .debug_frame sections even if not
      compiling -g.  This guarantees that we can unwind the stack.  */
   #define DWARF2_FRAME_INFO !TARGET_SGI_O32_AS

 and what the documentation says about this macro.

 We reach dwar2fout_frame_finish thanks to DWARF2_UNWIND_INFO defined and
 dwarf2out_do_frame returning 1, but frame_finish, unlike do_frame, has no
 check to honor the DWARF2_FRAME_INFO setting above.

 The first patch below (GCC 3.4 context) addresses that by adding to
 frame_finish the same check already in do_frame. 

 --

 The second issue is a bit more involved. GCC currently uses the hard return
 reg column (31) as the dwarf return address column. This badly confuses
 the system unwinder on IRIX (experimented a couple of days ago after
 adjusting the CIE data alignment factor, suggested by looking at the SGI's
 compiler output, confirmed by a member of the SGI team and by experiments
 with different approaches to use a virtual column number in GCC).

 The second patch below (also GCC 3.4 based), addresses that by overriding
 DWARF_FRAME_RETURN_COLUMN on IRIX and by adjusting 'mips_frame_set' to note
 an additional dwarf ra column change when r31 is saved and the dwarf ra
 column number differs.

 --

 As previously mentioned, both patches have been locally tested in various
 ways, although not yet using the full GCC testuite due to lack of CPU
 resources up to now.

 Thanks in advance for your feedback,

 Olivier

------------------------------------------------------------------------------
2005-03-15  Olivier Hainque  <hainque@adacore.com>

	* dwarf2out.c (dwarf2out_frame_finish): Honor DWARF2_FRAME_INFO
	defined and non-zero.

*** gcc/dwarf2out.c.ori	Wed Mar  9 03:54:06 2005
--- gcc/dwarf2out.c	Wed Mar  9 04:07:36 2005
*************** void
*** 2308,2314 ****
  dwarf2out_frame_finish (void)
  {
    /* Output call frame information.  */
!   if (write_symbols == DWARF2_DEBUG || write_symbols == VMS_AND_DWARF2_DEBUG)
      output_call_frame_info (0);
  
    if (! USING_SJLJ_EXCEPTIONS && (flag_unwind_tables || flag_exceptions))
--- 2308,2319 ----
  dwarf2out_frame_finish (void)
  {
    /* Output call frame information.  */
!   if (write_symbols == DWARF2_DEBUG
!       || write_symbols == VMS_AND_DWARF2_DEBUG
! #ifdef DWARF2_FRAME_INFO
!       || DWARF2_FRAME_INFO
! #endif
!       )
      output_call_frame_info (0);
  
    if (! USING_SJLJ_EXCEPTIONS && (flag_unwind_tables || flag_exceptions))
------------------------------------------------------------------------------
2005-03-15  Olivier Hainque  <hainque@adacore.com>

	* config/mips/iris6.h (DWARF_FRAME_RETURN_COLUMN): Redefine to
	match what the system unwinder expects.
	* config/mips/mips.c (mips_frame_set): If we're saving the return
	address register and the dwarf return address column number differs
	from the hard register number, note the dwarf return address save
	too.

*** gcc/config/mips/iris6.h.ori	Tue Mar 15 06:39:49 2005
--- gcc/config/mips/iris6.h	Tue Mar 15 08:30:43 2005
*************** Boston, MA 02111-1307, USA.  */
*** 165,170 ****
--- 165,175 ----
     compiling -g.  This guarantees that we can unwind the stack.  */
  #define DWARF2_FRAME_INFO !TARGET_SGI_O32_AS
  
+ /* The system unwinder in libexc requires a specific dwarf return address
+    column to work.  */
+ #undef  DWARF_FRAME_RETURN_COLUMN
+ #define DWARF_FRAME_RETURN_COLUMN (FP_REG_LAST + 1)
+ 
  /* The size in bytes of a DWARF field indicating an offset or length
     relative to a debug info section, specified to be 4 bytes in the DWARF-2
     specification.  The SGI/MIPS ABI defines it to be the same as PTR_SIZE.  */
*** gcc/config/mips/mips.c.ori	Tue Mar 15 06:16:18 2005
--- gcc/config/mips/mips.c	Tue Mar 15 04:51:53 2005
*************** mips_frame_set (rtx mem, rtx reg)
*** 6785,6790 ****
--- 6785,6806 ----
  {
    rtx set = gen_rtx_SET (VOIDmode, mem, reg);
    RTX_FRAME_RELATED_P (set) = 1;
+ 
+   /* If we're saving the return address register and the dwarf return
+      address column number differs from the hard register number, note
+      the dwarf return address save too.  */
+   if (REGNO (reg) == GP_REG_FIRST + 31
+       && DWARF_FRAME_RETURN_COLUMN != GP_REG_FIRST + 31)
+     {
+       rtx ra_column_set
+ 	= gen_rtx_SET (VOIDmode,
+ 		       mem,
+ 		       gen_rtx_REG (word_mode, DWARF_FRAME_RETURN_COLUMN));
+       RTX_FRAME_RELATED_P (ra_column_set) = 1;
+ 
+       set = gen_rtx_PARALLEL (VOIDmode, gen_rtvec (2, set, ra_column_set));
+     }
+ 
    return set;
  }