Re: [aarch64] Code-gen for vector initialization involving constants

[Richard Sandiford <richard.sandiford@arm.com>]

Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org> writes:
> On Tue, 25 Apr 2023 at 16:29, Richard Sandiford
> <richard.sandiford@arm.com> wrote:
>>
>> Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org> writes:
>> > Hi Richard,
>> > While digging thru aarch64_expand_vector_init, I noticed it gives
>> > priority to loading a constant first:
>> >  /* Initialise a vector which is part-variable.  We want to first try
>> >      to build those lanes which are constant in the most efficient way we
>> >      can.  */
>> >
>> > which results in suboptimal code-gen for following case:
>> > int16x8_t f_s16(int16_t x)
>> > {
>> >   return (int16x8_t) { x, x, x, x, x, x, x, 1 };
>> > }
>> >
>> > code-gen trunk:
>> > f_s16:
>> >         movi    v0.8h, 0x1
>> >         ins     v0.h[0], w0
>> >         ins     v0.h[1], w0
>> >         ins     v0.h[2], w0
>> >         ins     v0.h[3], w0
>> >         ins     v0.h[4], w0
>> >         ins     v0.h[5], w0
>> >         ins     v0.h[6], w0
>> >         ret
>> >
>> > The attached patch tweaks the following condition:
>> > if (n_var == n_elts && n_elts <= 16)
>> >   {
>> >     ...
>> >   }
>> >
>> > to pass if maxv >= 80% of n_elts, with 80% being an
>> > arbitrary "high enough" threshold. The intent is to dup
>> > the most repeating variable if it it's repetition
>> > is "high enough" and insert constants which should be "better" than
>> > loading constant first and inserting variables like in the above case.
>>
>> I'm not too keen on the 80%.  Like you say, it seems a bit arbitrary.
>>
>> The case above can also be handled by relaxing n_var == n_elts to
>> n_var >= n_elts - 1, so that if there's just one constant element,
>> we look for duplicated variable elements.  If there are none
>> (maxv == 1), but there is a constant element, we can duplicate
>> the constant element into a register.
>>
>> The case when there's more than one constant element needs more thought
>> (and testcases :-)).  E.g. after a certain point, it would probably be
>> better to load the variable and constant parts separately and blend them
>> using TBL.  It also matters whether the constants are equal or not.
>>
>> There are also cases that could be handled using EXT.
>>
>> Plus, if we're inserting many variable elements that are already
>> in GPRs, we can probably do better by coalescing them into bigger
>> GPR values and inserting them as wider elements.
>>
>> Because of things like that, I think we should stick to the
>> single-constant case for now.
> Hi Richard,
> Thanks for the suggestions. The attached patch only handles the single
> constant case.
> Bootstrap+test in progress on aarch64-linux-gnu.
> Does it look OK ?
>
> Thanks,
> Prathamesh
>>
>> Thanks,
>> Richard
>
> [aarch64] Improve code-gen for vector initialization with single constant element.
>
> gcc/ChangeLog:
> 	* config/aarch64/aarc64.cc (aarch64_expand_vector_init): Tweak condition
> 	if (n_var == n_elts && n_elts <= 16) to allow a single constant,
> 	and if maxv == 1, use constant element for duplicating into register.
>
> gcc/testsuite/ChangeLog:
> 	* gcc.target/aarch64/vec-init-single-const.c: New test.
>
> diff --git a/gcc/config/aarch64/aarch64.cc b/gcc/config/aarch64/aarch64.cc
> index 2b0de7ca038..f46750133a6 100644
> --- a/gcc/config/aarch64/aarch64.cc
> +++ b/gcc/config/aarch64/aarch64.cc
> @@ -22167,7 +22167,7 @@ aarch64_expand_vector_init (rtx target, rtx vals)
>       and matches[X][1] with the count of duplicate elements (if X is the
>       earliest element which has duplicates).  */
>  
> -  if (n_var == n_elts && n_elts <= 16)
> +  if ((n_var >= n_elts - 1) && n_elts <= 16)
>      {
>        int matches[16][2] = {0};
>        for (int i = 0; i < n_elts; i++)
> @@ -22227,6 +22227,18 @@ aarch64_expand_vector_init (rtx target, rtx vals)
>  	     vector register.  For big-endian we want that position to hold
>  	     the last element of VALS.  */
>  	  maxelement = BYTES_BIG_ENDIAN ? n_elts - 1 : 0;
> +
> +	  /* If we have a single constant element, use that for duplicating
> +	     instead.  */
> +	  if (n_var == n_elts - 1)
> +	    for (int i = 0; i < n_elts; i++)
> +	      if (CONST_INT_P (XVECEXP (vals, 0, i))
> +		  || CONST_DOUBLE_P (XVECEXP (vals, 0, i)))
> +		{
> +		  maxelement = i;
> +		  break;
> +		}
> +
>  	  rtx x = force_reg (inner_mode, XVECEXP (vals, 0, maxelement));
>  	  aarch64_emit_move (target, lowpart_subreg (mode, x, inner_mode));

We don't want to force the constant into a register though.

>  	}
> diff --git a/gcc/testsuite/gcc.target/aarch64/vec-init-single-const.c b/gcc/testsuite/gcc.target/aarch64/vec-init-single-const.c
> new file mode 100644
> index 00000000000..517f47b13ec
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/aarch64/vec-init-single-const.c
> @@ -0,0 +1,66 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2" } */
> +/* { dg-final { check-function-bodies "**" "" "" } } */
> +
> +#include <arm_neon.h>
> +
> +/*
> +** f_s8:
> +**	...
> +**	dup	v[0-9]+\.16b, w[0-9]+
> +**	movi	v[0-9]+\.8b, 0x1
> +**	ins	v[0-9]+\.b\[15\], v[0-9]+\.b\[0\]
> +**	...
> +**	ret
> +*/
> +
> +int8x16_t f_s8(int8_t x)
> +{
> +  return (int8x16_t) { x, x, x, x, x, x, x, x,
> +                       x, x, x, x, x, x, x, 1 };
> +}
> +
> +/*
> +** f_s16:
> +**	...
> +**	dup	v[0-9]+\.8h, w[0-9]+
> +**	movi	v[0-9]+\.4h, 0x1
> +**	ins	v[0-9]+\.h\[7\], v[0-9]+\.h\[0\]
> +**	...
> +**	ret
> +*/
> +
> +int16x8_t f_s16(int16_t x)
> +{
> +  return (int16x8_t) { x, x, x, x, x, x, x, 1 };
> +}
> +
> +/*
> +** f_s32:
> +**	...
> +**	movi	v[0-9]\.2s, 0x1
> +**	dup	v[0-9]\.4s, w[0-9]+
> +**	ins	v[0-9]+\.s\[3\], v[0-9]+\.s\[0\]
> +**	...
> +**	ret
> +*/
> +
> +int32x4_t f_s32(int32_t x)
> +{
> +  return (int32x4_t) { x, x, x, 1 };
> +}
> +
> +/*
> +** f_s64:
> +**	...
> +**	fmov	d[0-9]+, x[0-9]+
> +**	mov	x[0-9]+, 1
> +**	ins	v[0-9]+\.d\[1\], x[0-9]+
> +**	...
> +**	ret
> +*/
> +
> +int64x2_t f_s64(int64_t x)
> +{
> +  return (int64x2_t) { x, 1 };
> +}