On Thu, Sep 10, 2020 at 4:22 PM Aaron Sawdey <acsaw...@linux.ibm.com> wrote:
>
> If it feels like a hack, that would because it is a hack.
>
> What I’d really like to discuss is how to accomplish the real goal: keep
> anything from trying to do other operations (zero/sign extend for one) to
> POImode.
>
> Is there an existing mechanism for this?
Not that I know, but somehow x86 gets away with OImode and XImode without
providing too many patterns for those.
Richard.
> Thanks,
> Aaron
>
> Aaron Sawdey, Ph.D. saw...@linux.ibm.com
> IBM Linux on POWER Toolchain
>
>
> > On Sep 10, 2020, at 4:36 AM, Richard Biener <richard.guent...@gmail.com>
> > wrote:
> >
> > 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
> >>
>
