Re: [PATCH] [lra] force reg update after spilling to memory [PR120424]

[Denis Chertykov]

Alexandre Oliva <ol...@adacore.com> writes:

> On Jun  6, 2025, Alexandre Oliva <ol...@adacore.com> wrote:
>
>> Now, since lra_update_fp2sp_elimination checks that
>> !elimination_fp2sp_occured_p, we *could* disable the fp2sp elimination,
>> if it's selected, right away, so that it is not applied after we've
>> disabled it, and then we don't have to worry about disabling it half-way
>> or reversing it later.
>
> [lra] inactivate disabled fp2sp elimination
>
> Even after we disable the fp2sp elimination when it is the active
> elimination for the fp, spilling might use it before
> update_reg_eliminate runs and inactivates it for good.  If it is used,
> update_reg_eliminate will fail the check that fp2sp was not used.
>
> Since we keep track of uses of this specific elimination, and
> lra_update_fp2sp_elimination checks it before disabling it, we know it
> hasn't been used, so we can inactivate it without any ill effects.
>
> This fixes the pr118591-1.c avr-none regression exposed by the
> PR120424 fix.
>
> Regstrapped on x86_64-linux-gnu.  Also testing with gcc-14 on
> arm-vx7r2.  Ok to install?
>
>
> for  gcc/ChangeLog
>
>       * lra-eliminations.cc (lra_update_fp2sp_elimination):
>       Inactivate the unused fp2sp elimination right away.

Thank you.
The patch fixes PR118591.


Denis


> ---
>  gcc/lra-eliminations.cc |   15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/gcc/lra-eliminations.cc b/gcc/lra-eliminations.cc
> index bb708b007a4ee..6c8c91086f323 100644
> --- a/gcc/lra-eliminations.cc
> +++ b/gcc/lra-eliminations.cc
> @@ -1415,6 +1415,14 @@ lra_update_fp2sp_elimination (int *spilled_pseudos)
>    if (frame_pointer_needed || !targetm.frame_pointer_required ())
>      return 0;
>    gcc_assert (!elimination_fp2sp_occured_p);
> +  ep = elimination_map[FRAME_POINTER_REGNUM];
> +  if (ep->to == STACK_POINTER_REGNUM)
> +    {
> +      elimination_map[FRAME_POINTER_REGNUM] = NULL;
> +      setup_can_eliminate (ep, false);
> +    }
> +  else
> +    ep = NULL;
>    if (lra_dump_file != NULL)
>      fprintf (lra_dump_file,
>            "     Frame pointer can not be eliminated anymore\n");
> @@ -1422,9 +1430,10 @@ lra_update_fp2sp_elimination (int *spilled_pseudos)
>    CLEAR_HARD_REG_SET (set);
>    add_to_hard_reg_set (&set, Pmode, HARD_FRAME_POINTER_REGNUM);
>    n = spill_pseudos (set, spilled_pseudos);
> -  for (ep = reg_eliminate; ep < &reg_eliminate[NUM_ELIMINABLE_REGS]; ep++)
> -    if (ep->from == FRAME_POINTER_REGNUM && ep->to == STACK_POINTER_REGNUM)
> -      setup_can_eliminate (ep, false);
> +  if (!ep)
> +    for (ep = reg_eliminate; ep < &reg_eliminate[NUM_ELIMINABLE_REGS]; ep++)
> +      if (ep->from == FRAME_POINTER_REGNUM && ep->to == STACK_POINTER_REGNUM)
> +       setup_can_eliminate (ep, false);
>    return n;
>  }