[ARM] PR98435: Missed optimization in expanding vector constructor

Prathamesh Kulkarni prathamesh.kulkarni@linaro.org
Tue Jun 29 10:46:12 GMT 2021


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 ?

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


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