Re: [PATCH] or1k: Fix ICE in libgcc caused by recent validate_subreg changes

[Stafford Horne <shorne@gmail.com>]

On Mon, Jun 09, 2025 at 07:59:46PM +0100, Stafford Horne wrote:
> After commit eb2ea476db2 ("emit-rtl: Allow extra checks for
> paradoxical subregs [PR119966]") paradoxical subregs or the OpenRISC
> condition flag register (reg:BI sr_f) are no longer allowed.
> 
> This causes and ICE in the ce1 pass which tries to get the or1k flag
> register into an SI register, which is no longer possible.
> 
> Adjust or1k_can_change_mode_class to allow changing the or1k flag reg to
> SI mode which in turn allows paradoxical subregs to bre generated again.
> 
> gcc/ChangeLog:
> 
> 	PR or1k/120587
> 	* config/or1k/or1k.cc (or1k_can_change_mode_class): Allow
> 	changing flags mode from BI to SI to allow for paradoxical
> 	subregs.
> ---
> Sending again to correct mailing list.
> 
>  gcc/config/or1k/or1k.cc | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/gcc/config/or1k/or1k.cc b/gcc/config/or1k/or1k.cc
> index 62e2168e0ee..f1c92c6bf6c 100644
> --- a/gcc/config/or1k/or1k.cc
> +++ b/gcc/config/or1k/or1k.cc
> @@ -1408,8 +1408,9 @@ static bool
>  or1k_can_change_mode_class (machine_mode from, machine_mode to,
>  			    reg_class_t rclass)
>  {
> +  /* Allow cnoverting special flags to SI mode subregs.  */
>    if (rclass == FLAG_REGS)
> -    return from == to;
> +    return from == to || (from == BImode && to == SImode);
>    return true;
>  }

I have pushed this now, it restores the or1k port to be able to compile again by
providing the behavior with subreg's that we had before.

However, this did uncover an issue with how the (reg:SI SR_F_REGNUM) SR[F]
register is treated in OpenRISC.  I think the backend is producing sub optimal
code that's ok because it gets discarded, but I think its preventing CE passes
from producing optimal cmov sequences.  I am working to fix this by either:

 1. Working withing the current contraints of treating SR[F] as BImode
 2. Change SR[F] to SImode and added proper splits/etc to make it work (like sh
    does)
 3. Harder, convert SR[F] handing to use CC_MODE (this might work the best?)

-Stafford