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Re: CAN_ELIMINATE question
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