Re: CAN_ELIMINATE question

[Ian Lance Taylor <ian@airs.com>]

Denis Chertykov <denisc@overta.ru> writes:

> 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

But then we'll call update_eliminables(), notice that something
changed, and go around the loop again.

There may be a bug here, but it needs more explanation.

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

Exactly.  We do that.  About 15 lines after the lines you quoted
above.

What am I missing?

Ian