RE: [ARM] PR98435: Missed optimization in expanding vector constructor

[Kyrylo Tkachov <Kyrylo.Tkachov@arm.com>]

> -----Original Message-----
> From: Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org>
> Sent: 06 July 2021 08:06
> To: Christophe LYON <christophe.lyon@foss.st.com>
> Cc: Kyrylo Tkachov <Kyrylo.Tkachov@arm.com>; gcc Patches <gcc-
> patches@gcc.gnu.org>
> Subject: Re: [ARM] PR98435: Missed optimization in expanding vector
> constructor
> 
> On Thu, 1 Jul 2021 at 16:26, Prathamesh Kulkarni
> <prathamesh.kulkarni@linaro.org> wrote:
> >
> > On Wed, 30 Jun 2021 at 20:51, Christophe LYON
> > <christophe.lyon@foss.st.com> wrote:
> > >
> > >
> > > On 29/06/2021 12:46, Prathamesh Kulkarni wrote:
> > > > On Mon, 28 Jun 2021 at 14:48, Christophe LYON
> > > > <christophe.lyon@foss.st.com> wrote:
> > > >>
> > > >> On 28/06/2021 10:40, Kyrylo Tkachov via Gcc-patches wrote:
> > > >>>> -----Original Message-----
> > > >>>> From: Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org>
> > > >>>> Sent: 28 June 2021 09:38
> > > >>>> To: Kyrylo Tkachov <Kyrylo.Tkachov@arm.com>
> > > >>>> Cc: Christophe Lyon <christophe.lyon@linaro.org>; gcc Patches
> <gcc-
> > > >>>> patches@gcc.gnu.org>
> > > >>>> Subject: Re: [ARM] PR98435: Missed optimization in expanding
> vector
> > > >>>> constructor
> > > >>>>
> > > >>>> On Thu, 24 Jun 2021 at 22:01, Kyrylo Tkachov
> <Kyrylo.Tkachov@arm.com>
> > > >>>> wrote:
> > > >>>>>
> > > >>>>>> -----Original Message-----
> > > >>>>>> From: Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org>
> > > >>>>>> Sent: 14 June 2021 09:02
> > > >>>>>> To: Christophe Lyon <christophe.lyon@linaro.org>
> > > >>>>>> Cc: gcc Patches <gcc-patches@gcc.gnu.org>; Kyrylo Tkachov
> > > >>>>>> <Kyrylo.Tkachov@arm.com>
> > > >>>>>> Subject: Re: [ARM] PR98435: Missed optimization in expanding
> vector
> > > >>>>>> constructor
> > > >>>>>>
> > > >>>>>> On Wed, 9 Jun 2021 at 15:58, Prathamesh Kulkarni
> > > >>>>>> <prathamesh.kulkarni@linaro.org> wrote:
> > > >>>>>>> On Fri, 4 Jun 2021 at 13:15, Christophe Lyon
> > > >>>> <christophe.lyon@linaro.org>
> > > >>>>>> wrote:
> > > >>>>>>>> On Fri, 4 Jun 2021 at 09:27, Prathamesh Kulkarni via Gcc-
> patches
> > > >>>>>>>> <gcc-patches@gcc.gnu.org> wrote:
> > > >>>>>>>>> Hi,
> > > >>>>>>>>> As mentioned in PR, for the following test-case:
> > > >>>>>>>>>
> > > >>>>>>>>> #include <arm_neon.h>
> > > >>>>>>>>>
> > > >>>>>>>>> bfloat16x4_t f1 (bfloat16_t a)
> > > >>>>>>>>> {
> > > >>>>>>>>>     return vdup_n_bf16 (a);
> > > >>>>>>>>> }
> > > >>>>>>>>>
> > > >>>>>>>>> bfloat16x4_t f2 (bfloat16_t a)
> > > >>>>>>>>> {
> > > >>>>>>>>>     return (bfloat16x4_t) {a, a, a, a};
> > > >>>>>>>>> }
> > > >>>>>>>>>
> > > >>>>>>>>> Compiling with arm-linux-gnueabi -O3 -mfpu=neon -mfloat-
> > > >>>> abi=softfp
> > > >>>>>>>>> -march=armv8.2-a+bf16+fp16 results in f2 not being
> vectorized:
> > > >>>>>>>>>
> > > >>>>>>>>> f1:
> > > >>>>>>>>>           vdup.16 d16, r0
> > > >>>>>>>>>           vmov    r0, r1, d16  @ v4bf
> > > >>>>>>>>>           bx      lr
> > > >>>>>>>>>
> > > >>>>>>>>> f2:
> > > >>>>>>>>>           mov     r3, r0  @ __bf16
> > > >>>>>>>>>           adr     r1, .L4
> > > >>>>>>>>>           ldrd    r0, [r1]
> > > >>>>>>>>>           mov     r2, r3  @ __bf16
> > > >>>>>>>>>           mov     ip, r3  @ __bf16
> > > >>>>>>>>>           bfi     r1, r2, #0, #16
> > > >>>>>>>>>           bfi     r0, ip, #0, #16
> > > >>>>>>>>>           bfi     r1, r3, #16, #16
> > > >>>>>>>>>           bfi     r0, r2, #16, #16
> > > >>>>>>>>>           bx      lr
> > > >>>>>>>>>
> > > >>>>>>>>> This seems to happen because vec_init pattern in neon.md
> has VDQ
> > > >>>>>> mode
> > > >>>>>>>>> iterator, which doesn't include V4BF. In attached patch, I
> changed
> > > >>>>>>>>> mode
> > > >>>>>>>>> to VDQX which seems to work for the test-case, and the
> compiler
> > > >>>> now
> > > >>>>>> generates:
> > > >>>>>>>>> f2:
> > > >>>>>>>>>           vdup.16 d16, r0
> > > >>>>>>>>>           vmov    r0, r1, d16  @ v4bf
> > > >>>>>>>>>           bx      lr
> > > >>>>>>>>>
> > > >>>>>>>>> However, the pattern is also gated on TARGET_HAVE_MVE
> and I am
> > > >>>>>> not
> > > >>>>>>>>> sure if either VDQ or VDQX are correct modes for MVE since
> MVE
> > > >>>> has
> > > >>>>>>>>> only 128-bit vectors ?
> > > >>>>>>>>>
> > > >>>>>>>> I think patterns common to both Neon and MVE should be
> moved to
> > > >>>>>>>> vec-common.md, I don't know why such patterns were left in
> > > >>>> neon.md.
> > > >>>>>>> Since we end up calling neon_expand_vector_init for both
> NEON and
> > > >>>> MVE,
> > > >>>>>>> I am not sure if we should separate the pattern ?
> > > >>>>>>> Would it make sense to FAIL if the mode size isn't 16 bytes for
> MVE as
> > > >>>>>>> in attached patch so
> > > >>>>>>> it will call neon_expand_vector_init only for 128-bit vectors ?
> > > >>>>>>> Altho hard-coding 16 in the pattern doesn't seem a good idea to
> me
> > > >>>> either.
> > > >>>>>> ping https://gcc.gnu.org/pipermail/gcc-patches/2021-
> June/572342.html
> > > >>>>>> (attaching patch as text).
> > > >>>>>>
> > > >>>>> --- a/gcc/config/arm/neon.md
> > > >>>>> +++ b/gcc/config/arm/neon.md
> > > >>>>> @@ -459,10 +459,12 @@
> > > >>>>>    )
> > > >>>>>
> > > >>>>>    (define_expand "vec_init<mode><V_elem_l>"
> > > >>>>> -  [(match_operand:VDQ 0 "s_register_operand")
> > > >>>>> +  [(match_operand:VDQX 0 "s_register_operand")
> > > >>>>>       (match_operand 1 "" "")]
> > > >>>>>      "TARGET_NEON || TARGET_HAVE_MVE"
> > > >>>>>    {
> > > >>>>> +  if (TARGET_HAVE_MVE && GET_MODE_SIZE (GET_MODE
> > > >>>> (operands[0])) != 16)
> > > >>>>> +    FAIL;
> > > >>>>>      neon_expand_vector_init (operands[0], operands[1]);
> > > >>>>>      DONE;
> > > >>>>>    })
> > > >>>>>
> > > >>>>> I think we should move this to vec-common.md like Christophe
> said.
> > > >>>>> Perhaps rather than making it FAIL for non-16 MVE sizes we just
> disable it in
> > > >>>> the expander condition?
> > > >>>>> "TARGET_NEON || (TARGET_HAVE_MVE && GET_MODE_SIZE (<
> > > >>>> VDQ>mode) != 16)"
> > > >>>> Is it OK to use <MODE>mode ? Because using <VDQ>mode resulted
> in lot
> > > >>>> of build errors.
> > > >>>> Also, I think the comparison should be inverted, ie, GET_MODE_SIZE
> > > >>>> (<MODE>mode) == 16 since
> > > >>>> we want to make the pattern pass if target is MVE and vector size is
> 16 bytes ?
> > > >>>> Do these changes in attached patch look OK ?
> > > >>> Yes, you're right.
> > > >>
> > > >> Can't this be ARM_HAVE_<MODE>_ARITH like in most expanders in
> vec-common.md?
> > > >>
> > > >> (maybe with a && !TARGET_REALLY_IWMMXT if needed)
> > > > I wonder if this should be ARM_HAVE_<MODE>_LDST instead since
> we're
> > > > initializing the vector ?
> > >
> > >
> > > Well, it really depends on which modes you want to enable.
> > >
> > >
> > > Looks like your move VDQ -> VDQ adds V4BF, V8BF and DI.
> > >
> > > Are they all OK for Neon?
> > >
> > > They are not OK for MVE.
> > >
> > > Ideally you could add testcases to cover to the supported and
> > > unsupported modes for both Neon and MVE.\
> > >
> > > Before your patch, the expander is enabled for MVE for 64 bit modes
> > > (V8QI, V4HI, V2SI): what happens in this case? Does the compiler crash
> > > or is there something else preventing the match?
> > Hi,
> > Apparently there is VALID_MVE_MODE macro, so is it better to use:
> > TARGET_NEON || (TARGET_HAVE_MVE &&
> VALID_MVE_MODE(<MODE>mode))
> > as in the attached patch ?

The change is ok. I would like to see some testcases like Christophe suggested, but this patch just moves the expander around rather than introducing new functionality.
Thanks,
Kyrill

> ping https://gcc.gnu.org/pipermail/gcc-patches/2021-July/574206.html
> 
> Thanks,
> Prathamesh
> >
> > Thanks,
> > Prathamesh
> > >
> > >
> > > Thanks,
> > >
> > >
> > > Christophe
> > >
> > >
> > > > Thanks,
> > > > Prathamesh
> > > >>
> > > >> Christophe
> > > >>
> > > >>
> > > >>> Ok.
> > > >>> Thanks,
> > > >>> Kyrill
> > > >>>
> > > >>>
> > > >>>> Thanks,
> > > >>>> Prathamesh
> > > >>>>> Thanks,
> > > >>>>> Kyrill
> > > >>>>>
> > > >>>>>> Thanks,
> > > >>>>>> Prathamesh
> > > >>>>>>> Thanks,
> > > >>>>>>> Prathamesh
> > > >>>>>>>> That being said, I suggest you look at other similar patterns in
> > > >>>>>>>> vec-common.md, most of which are gated on
> > > >>>>>>>> ARM_HAVE_<MODE>_ARITH
> > > >>>>>>>> and possibly beware of issues with iwmmxt :-)
> > > >>>>>>>>
> > > >>>>>>>> Christophe
> > > >>>>>>>>
> > > >>>>>>>>> Thanks,
> > > >>>>>>>>> Prathamesh