Re: [aarch64] Use dup and zip1 for interleaving elements in initializing vector

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

Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org> writes:
> Hi,
> For the following test-case:
>
> int16x8_t foo(int16_t x, int16_t y)
> {
>   return (int16x8_t) { x, y, x, y, x, y, x, y };
> }
>
> Code gen at -O3:
> foo:
>         dup    v0.8h, w0
>         ins     v0.h[1], w1
>         ins     v0.h[3], w1
>         ins     v0.h[5], w1
>         ins     v0.h[7], w1
>         ret
>
> For 16 elements, it results in 8 ins instructions which might not be
> optimal perhaps.
> I guess, the above code-gen would be equivalent to the following ?
> dup v0.8h, w0
> dup v1.8h, w1
> zip1 v0.8h, v0.8h, v1.8h
>
> I have attached patch to do the same, if number of elements >= 8,
> which should be possibly better compared to current code-gen ?
> Patch passes bootstrap+test on aarch64-linux-gnu.
> Does the patch look OK ?
>
> Thanks,
> Prathamesh
>
> diff --git a/gcc/config/aarch64/aarch64.cc b/gcc/config/aarch64/aarch64.cc
> index c91df6f5006..e5dea70e363 100644
> --- a/gcc/config/aarch64/aarch64.cc
> +++ b/gcc/config/aarch64/aarch64.cc
> @@ -22028,6 +22028,39 @@ aarch64_expand_vector_init (rtx target, rtx vals)
>        return;
>      }
>  
> +  /* Check for interleaving case.
> +     For eg if initializer is (int16x8_t) {x, y, x, y, x, y, x, y}.
> +     Generate following code:
> +     dup v0.h, x
> +     dup v1.h, y
> +     zip1 v0.h, v0.h, v1.h
> +     for "large enough" initializer.  */
> +
> +  if (n_elts >= 8)
> +    {
> +      int i;
> +      for (i = 2; i < n_elts; i++)
> +	if (!rtx_equal_p (XVECEXP (vals, 0, i), XVECEXP (vals, 0, i % 2)))
> +	  break;
> +
> +      if (i == n_elts)
> +	{
> +	  machine_mode mode = GET_MODE (target);
> +	  rtx dest[2];
> +
> +	  for (int i = 0; i < 2; i++)
> +	    {
> +	      rtx x = copy_to_mode_reg (GET_MODE_INNER (mode), XVECEXP (vals, 0, i));

Formatting nit: long line.

> +	      dest[i] = gen_reg_rtx (mode);
> +	      aarch64_emit_move (dest[i], gen_vec_duplicate (mode, x));
> +	    }

This could probably be written:

	  for (int i = 0; i < 2; i++)
	    {
	      rtx x = expand_vector_broadcast (mode, XVECEXP (vals, 0, i));
	      dest[i] = force_reg (GET_MODE_INNER (mode), x);
	    }

which avoids forcing constant elements into a register before the duplication.

OK with that change if it works.

Thanks,
Richard

> +
> +	  rtvec v = gen_rtvec (2, dest[0], dest[1]);
> +	  emit_set_insn (target, gen_rtx_UNSPEC (mode, v, UNSPEC_ZIP1));
> +	  return;
> +	}
> +    }
> +
>    enum insn_code icode = optab_handler (vec_set_optab, mode);
>    gcc_assert (icode != CODE_FOR_nothing);
>  
> diff --git a/gcc/testsuite/gcc.target/aarch64/interleave-init-1.c b/gcc/testsuite/gcc.target/aarch64/interleave-init-1.c
> new file mode 100644
> index 00000000000..ee775048589
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/aarch64/interleave-init-1.c
> @@ -0,0 +1,37 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O3" } */
> +/* { dg-final { check-function-bodies "**" "" "" } } */
> +
> +#include <arm_neon.h>
> +
> +/*
> +** foo:
> +**	...
> +**	dup	v[0-9]+\.8h, w[0-9]+
> +**	dup	v[0-9]+\.8h, w[0-9]+
> +**	zip1	v[0-9]+\.8h, v[0-9]+\.8h, v[0-9]+\.8h
> +**	...
> +**	ret
> +*/
> +
> +int16x8_t foo(int16_t x, int y)
> +{
> +  int16x8_t v = (int16x8_t) {x, y, x, y, x, y, x, y}; 
> +  return v;
> +}
> +
> +/*
> +** foo2:
> +**	...
> +**	dup	v[0-9]+\.8h, w[0-9]+
> +**	movi	v[0-9]+\.8h, 0x1
> +**	zip1	v[0-9]+\.8h, v[0-9]+\.8h, v[0-9]+\.8h
> +**	...
> +**	ret
> +*/
> +
> +int16x8_t foo2(int16_t x) 
> +{
> +  int16x8_t v = (int16x8_t) {x, 1, x, 1, x, 1, x, 1}; 
> +  return v;
> +}