On Wed, Sep 9, 2020 at 8:28 PM Aaron Sawdey via Gcc-patches
<gcc-patches@gcc.gnu.org> 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.
How many bits are you actually storing in POImode? If you say it's
precision is 1 then the middle-end might be tempted to ignore any
changes to the upper bits. You now probably say "but we don't have
any such interesting operation done on POImode" but still ... it feels
like a hack.
Richard.
> 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
>
