Re: CAN_ELIMINATE question

[Denis Chertykov <denisc@overta.ru>]

Eric Christopher <echristo@apple.com> writes:

> On Feb 13, 2006, at 7:43 AM, Denis Chertykov wrote:
> 
> > Hi All.
> >
> > While I have debugging AVR target bug I found that something wrong in
> > port code or in reload.
> >
> > Is it right to use of get_frame_size() inside CAN_ELIMINATE macro
> > valid ?
> > If yes then reload have a bug.
> > If no then AVR and probably MIPS ports have invalid definitions of
> > CAN_ELIMINATE.
> 
> Can you point to where you think reload is getting this wrong?

Yes.

Code fragment from reload1.c: reload()
----------------------------------------------------
      if (caller_save_needed)
	setup_save_areas ();

      /* If we allocated another stack slot, redo elimination bookkeeping.  */
      if (starting_frame_size != get_frame_size ())
	continue;

      if (caller_save_needed)
	{
	  save_call_clobbered_regs ();
	  /* That might have allocated new insn_chain structures.  */
	  reload_firstobj = obstack_alloc (&reload_obstack, 0);
	}

      calculate_needs_all_insns (global);
----------------------------------------------------

Call to setup_save_areas () can change frame size but only offsets on
eliminable registers will be changed before call to calculate_needs_all_insns.
calculate_needs_all_insns will calculate wrong needs for elimination.

Example for AVR:

avr target can eliminate fp -> sp only if get_frame_size () == 0.

Before call to setup_save_areas() frame size was 0 (CAN_ELIMINATE (FP,SP) != 0)

setup_save_areas() increase frame size.
set_initial_elim_offsets() correct offsets but can_eliminate isn't changed.

save_call_clobbered_regs () emit save insn
(insn 659 161 162 16 (set (mem/c:HI (plus:HI (reg/f:HI 28 r28) # it's FP
                (const_int 1 [0x1])) [29 S2 A8])
        (reg:HI 24 r24)) 12 {*movhi} (nil)
    (nil))

calculate_needs_all_insns() try to eliminate (reg/f:HI 28 r28) to SP.
It's wrong because get_frame_size () != 0 and CAN_ELIMINATE (FP,SP) == 0




I think that better to call update_eliminables() somewhere after
setup_save_areas()


Denis.