Re: [FFmpeg-devel] [PATCH 1/2] aarch64/hevcdsp_idct_neon: Optimize idct dc

["Martin Storsjö" <martin@martin.st>]

On Thu, 20 Feb 2025, Zhao Zhili wrote:

> From: Zhao Zhili <zhilizhao@tencent.com>
>
> clang does better than the assembly code before the patch, especially
> for small size:
>
> hevc_idct_4x4_dc_8_c:                                   11.2 ( 1.00x)
> hevc_idct_4x4_dc_8_neon:                                15.5 ( 0.73x)
> hevc_idct_4x4_dc_10_c:                                  12.0 ( 1.00x)
> hevc_idct_4x4_dc_10_neon:                               15.2 ( 0.79x)
> hevc_idct_8x8_dc_8_c:                                   13.2 ( 1.00x)
> hevc_idct_8x8_dc_8_neon:                                18.2 ( 0.73x)
> hevc_idct_8x8_dc_10_c:                                  13.5 ( 1.00x)
> hevc_idct_8x8_dc_10_neon:                               17.2 ( 0.78x)
> hevc_idct_16x16_dc_8_c:                                 41.8 ( 1.00x)
> hevc_idct_16x16_dc_8_neon:                              37.8 ( 1.11x)
> hevc_idct_16x16_dc_10_c:                                41.8 ( 1.00x)
> hevc_idct_16x16_dc_10_neon:                             37.8 ( 1.11x)
> hevc_idct_32x32_dc_8_c:                                130.2 ( 1.00x)
> hevc_idct_32x32_dc_8_neon:                             132.2 ( 0.98x)
> hevc_idct_32x32_dc_10_c:                               130.2 ( 1.00x)
> hevc_idct_32x32_dc_10_neon:                            132.2 ( 0.98x)
>
> This patch basically clone what the compiler does, so the performance
> is the same.
> ---
> libavcodec/aarch64/hevcdsp_idct_neon.S | 59 ++++++++++++++------------
> 1 file changed, 33 insertions(+), 26 deletions(-)
>
> diff --git a/libavcodec/aarch64/hevcdsp_idct_neon.S b/libavcodec/aarch64/hevcdsp_idct_neon.S
> index 3cac6e6db9..4543ab6b07 100644
> --- a/libavcodec/aarch64/hevcdsp_idct_neon.S
> +++ b/libavcodec/aarch64/hevcdsp_idct_neon.S
> @@ -888,38 +888,45 @@ function ff_hevc_transform_luma_4x4_neon_8, export=1
>         ret
> endfunc
>
> +.macro idct_8x8_dc_store offset
> +.irp    i, 0x0, 0x20, 0x40, 0x60
> +        stp             q0, q0, [x0, #(\offset + \i)]
> +.endr
> +.endm
> +
> +.macro idct_16x16_dc_store
> +.irp    index, 0x0, 0x80, 0x100, 0x180
> +        idct_8x8_dc_store offset=\index
> +.endr
> +.endm
> +
> // void ff_hevc_idct_NxN_dc_DEPTH_neon(int16_t *coeffs)
> .macro idct_dc size, bitdepth
> function ff_hevc_idct_\size\()x\size\()_dc_\bitdepth\()_neon, export=1
> -        ld1r            {v4.8h}, [x0]
> -        srshr           v4.8h,  v4.8h,  #1
> -        srshr           v0.8h,  v4.8h,  #(14 - \bitdepth)
> -        srshr           v1.8h,  v4.8h,  #(14 - \bitdepth)
> -.if \size > 4
> -        srshr           v2.8h,  v4.8h,  #(14 - \bitdepth)
> -        srshr           v3.8h,  v4.8h,  #(14 - \bitdepth)
> -.if \size > 16 /* dc 32x32 */
> -        mov             x2,  #4
> +        ldrsh           w1, [x0]
> +        add             w1, w1, #1
> +        asr             w1, w1, #1
> +        add             w1, w1, #(1 << (13 - \bitdepth))
> +        asr             w1, w1, #(14 - \bitdepth)
> +        dup             v0.8h, w1
> +
> +.if \size < 8
> +        stp             q0, q0, [x0]
> +.else
> +.if \size < 16

You can use .elseif instead of .else and a nested .if.

Other than that, this patch looks good to me, thanks!

// Martin

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