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?
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
>>
