Re: [PATCH] loading float member of parameter stored via int registers

[Segher Boessenkool <segher@kernel.crashing.org>]

Hi!

On Fri, Dec 30, 2022 at 10:22:31AM +0800, Jiufu Guo wrote:
> Considering the limitations of CSE, I try to find other places
> to handle this issue, and notice DSE can optimize below code:
> "[sfp:DI]=x:DI ; y:SI=[sfp:DI]" to "y:SI=x:DI#0".
> 
> So, I drafted a patch to update DSE to handle DI->DF/SF.
> The patch updates "extract_low_bits" to get mode change 
> with subreg.
> 
> diff --git a/gcc/expmed.cc b/gcc/expmed.cc
> index b12b0e000c2..5e36331082c 100644
> --- a/gcc/expmed.cc
> +++ b/gcc/expmed.cc
> @@ -2439,7 +2439,10 @@ extract_low_bits (machine_mode mode, machine_mode src_mode, rtx src)
>  
>    if (!targetm.modes_tieable_p (src_int_mode, src_mode))
>      return NULL_RTX;
> -  if (!targetm.modes_tieable_p (int_mode, mode))
> +  if (!targetm.modes_tieable_p (int_mode, mode)
> +      && !(known_le (GET_MODE_BITSIZE (mode), GET_MODE_BITSIZE (src_mode))
> +	   && GET_MODE_CLASS (mode) == MODE_FLOAT
> +	   && GET_MODE_CLASS (src_mode) == MODE_INT))
>      return NULL_RTX;
>  
>    src = gen_lowpart (src_int_mode, src);

Ah!  This simply shows rs6000_modes_tieable_p is decidedly non-optimal:
it does not allow tying a scalar float to anything else.  No such thing
is required, or good apparently.  I wonder why we have such restrictions
at all in rs6000; is it just unfortunate history, was it good at one
point in time?


Segher