Re: [AArch64][PR65375] Fix RTX cost for vector SET

[Kyrill Tkachov <kyrylo dot tkachov at arm dot com>]

On 16/03/15 05:36, Kugan wrote:

Hi Kugan,

> AArch64 RTX cost for vector SET is causing PR65375. Lower subreg is
> using this rtx_cost to compute the cost of moves, and splitting anything
> larger than word size, 64-bits in this case. The aarch64 rtx_costs is
> returning 2 * COST_N_INSNS(1) for vector moves, so they get split.
> Attach patch fixes this.
>
> With the patch the testcase in the PR:
>
> #include <arm_neon.h>
> void hello_vst2(float* fout, float *fin)
> {
>    float32x4x2_t a;
>    a = vld2q_f32 (fin);
>    vst2q_f32 (fout, a);
> }
>
> Changes to:
>
>   hello_vst2:
> -	ld2	{v0.4s - v1.4s}, [x1]
> -	sub	sp, sp, #32
> -	umov	x1, v0.d[0]
> -	umov	x2, v0.d[1]
> -	str	q1, [sp, 16]
> -	mov	x5, x1
> -	stp	x5, x2, [sp]
> -	ld1	{v0.16b - v1.16b}, [sp]
> +	ld2	{v2.4s - v3.4s}, [x1]
> +	orr	v0.16b, v2.16b, v2.16b
> +	orr	v1.16b, v3.16b, v3.16b
>   	st2	{v0.4s - v1.4s}, [x0]
> -	add	sp, sp, 32
>   	ret
>
>
> lower-subreg.c:compute_costs() only cares about the cost of a (set (reg)
> (const_int )) move but I think the intention, at least for now, is to
> return extra_cost->vect.alu for all the vector operations.

Almost, what we want at the moment is COSTS_N_INSNS (1) + 
extra_cost->vect.alu
> Regression tested on aarch64-linux-gnu with no new regression. Is this
> OK for trunk?

Are you sure it's a (set (reg) (const_int)) that's being costed here? I 
thought for moves into vecto registers it would be a (set (reg) 
(const_vector)) which we don't handle in our rtx costs currently. I 
think the correct approach would be to extend the aarch64_rtx_costs 
switch statement to handle the CONST_VECT case. I believe you can use 
aarch64_simd_valid_immediate to check whether x is a valid immediate for 
a simd instruction and give it a cost of extra_cost->vect.alu. The logic 
should be similar to the CONST_INT case.

Thanks,
Kyrill
> Thanks,
> Kugan
>
>
> gcc/ChangeLog:
>
> 2015-03-16  Kugan Vivekanandarajah  <kuganv@linaro.org>
>              Jim Wilson  <jim.wilson@linaro.org>
>
> 	PR target/65375
> 	* config/aarch64/aarch64.c (aarch64_rtx_costs): Return
> 	extra_cost->vect.alu for SET.