Ping: [PATCH] PR rtl-optimization/96791 Check precision of partial modes

[Aaron Sawdey]

Not exactly a patch ping, but I was hoping we could re-engage the discussion on this and figure out how we can make POImode work for powerpc.

How does x86 solve this? There was some suggestion that it has some similar situations? 

Thanks,
   

Aaron Sawdey, Ph.D. sawdey@linux.ibm.com
IBM Linux on POWER Toolchain
 

> On Sep 9, 2020, at 1:27 PM, Aaron Sawdey <acsawdey@linux.ibm.com> wrote:
> 
> Now that the documentation for partial modes says they have a known
> number of bits of precision, would it make sense for extract_low_bits to
> check this before attempting to extract the bits?
> 
> This would solve the problem we have been having with POImode and
> extract_low_bits -- DSE tries to use it to extract part of a POImode
> register used in a previous store. We do not want to supply any patterns
> to make POImode (or OImode) used like a regular integer mode.
> 
> This patch adds such a check, and sets the precision of POImode to one
> bit, which resolves the problems of PR/96791 for ppc64 target.
> 
> Bootstrap passes on ppc64le and x86_64.
> 
> Thanks,
>   Aaron
> 
> gcc/ChangeLog:
> 
> 	* config/rs6000/rs6000-modes.def (POImode): Change precision.
> 	* expmed.c (extract_low_bits): Check precision.
> ---
> gcc/config/rs6000/rs6000-modes.def | 2 +-
> gcc/expmed.c                       | 3 +++
> 2 files changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/gcc/config/rs6000/rs6000-modes.def b/gcc/config/rs6000/rs6000-modes.def
> index ddb218b3fba..aa7d60dd835 100644
> --- a/gcc/config/rs6000/rs6000-modes.def
> +++ b/gcc/config/rs6000/rs6000-modes.def
> @@ -90,5 +90,5 @@ INT_MODE (OI, 32);
> INT_MODE (XI, 64);
> 
> /* Modes used by __vector_pair and __vector_quad.  */
> -PARTIAL_INT_MODE (OI, 256, POI);	/* __vector_pair.  */
> +PARTIAL_INT_MODE (OI, 1, POI);	/* __vector_pair.  */
> PARTIAL_INT_MODE (XI, 512, PXI);	/* __vector_quad.  */
> diff --git a/gcc/expmed.c b/gcc/expmed.c
> index d34f0fb0b54..23ca181afa6 100644
> --- a/gcc/expmed.c
> +++ b/gcc/expmed.c
> @@ -2396,6 +2396,9 @@ extract_low_bits (machine_mode mode, machine_mode src_mode, rtx src)
>   if (GET_MODE_CLASS (mode) == MODE_CC || GET_MODE_CLASS (src_mode) == MODE_CC)
>     return NULL_RTX;
> 
> +  if (known_lt (GET_MODE_PRECISION (src_mode), GET_MODE_BITSIZE (mode)))
> +    return NULL_RTX;
> +
>   if (known_eq (GET_MODE_BITSIZE (mode), GET_MODE_BITSIZE (src_mode))
>       && targetm.modes_tieable_p (mode, src_mode))
>     {
> -- 
> 2.17.1
>