Re: [FFmpeg-devel] [PATCH] libavcodec/hevc: add hevc idct_4x4_neon of aarch64

["Martin Storsjö" <martin@martin.st>]

On Tue, 14 Feb 2023, xufuji456 wrote:

> ---
> libavcodec/aarch64/hevcdsp_idct_neon.S    | 51 +++++++++++++++++++++++
> libavcodec/aarch64/hevcdsp_init_aarch64.c |  4 ++
> 2 files changed, 55 insertions(+)

This looks mostly fine, but there's a couple issues that breaks it when 
built with toolchains other than what you've been using. (I presume you've 
built it with Clang on macOS, since that's the only toolchain that built 
it successfully and ran without failures.)

>
> diff --git a/libavcodec/aarch64/hevcdsp_idct_neon.S b/libavcodec/aarch64/hevcdsp_idct_neon.S
> index 124c50998a..fe8baf1348 100644
> --- a/libavcodec/aarch64/hevcdsp_idct_neon.S
> +++ b/libavcodec/aarch64/hevcdsp_idct_neon.S
> @@ -245,6 +245,54 @@ function hevc_add_residual_32x32_16_neon, export=0
>         ret
> endfunc
>
> +.macro tr_4x4 in0, in1, in2, in3, out0, out1, out2, out3, shift, tmp0, tmp1, tmp2, tmp3, tmp4
> +         sshll           \tmp0, \in0, #6

Nitpick: the indentation of \tmp0 is off here

> +         smull          \tmp2, \in1, v4.h[1]
> +         mov            \tmp1, \tmp0

Both binutils and MS armasm64.exe refuse to assemble an instruction like 
"mov v0.4s, v1.4s" - for the mov instruction, they only accept .8b or 
.16b, not .4s - even if it doesn't make any functional difference for what 
the instruction does.

Due to how these macros are used here, it's not very easy to come up with 
a neat solution. One possible solution is to just repeat "sshll \tmp1, 
\in0, #6"; that does the same shift operation twice, but reduces the chain 
depth for setting tmp1, so it's possible that it doesn't make any big 
difference in the end. (On Cortex A53/A72/A73, I didn't measure any 
significant difference here.)

> +         smull          \tmp3, \in1, v4.h[3]
> +         smlal          \tmp0, \in2, v4.h[0] //e0
> +         smlsl          \tmp1, \in2, v4.h[0] //e1
> +         smlal          \tmp2, \in3, v4.h[3] //o0
> +         smlsl          \tmp3, \in3, v4.h[1] //o1
> +
> +         add            \tmp4, \tmp0, \tmp2
> +         sub            \tmp0, \tmp0, \tmp2
> +         add            \tmp2, \tmp1, \tmp3
> +         sub            \tmp1, \tmp1, \tmp3
> +         sqrshrn        \out0, \tmp4, #\shift
> +         sqrshrn        \out3, \tmp0, #\shift
> +         sqrshrn        \out1, \tmp2, #\shift
> +         sqrshrn        \out2, \tmp1, #\shift
> +.endm
> +
> +.macro transpose_4x4 r0, r1, r2, r3
> +        trn1            v22.8h, \r0\().8h, \r1\().8h
> +        trn2            v23.8h, \r0\().8h, \r1\().8h
> +        trn1            v24.8h, \r2\().8h, \r3\().8h
> +        trn2            v25.8h, \r2\().8h, \r3\().8h
> +        trn1            \r0\().4s, v22.4s, v24.4s
> +        trn2            \r2\().4s, v22.4s, v24.4s
> +        trn1            \r1\().4s, v23.4s, v25.4s
> +        trn2            \r3\().4s, v23.4s, v25.4s
> +.endm

neon.S already has got a bunch of similar matching transposes. I see that 
this file already had some similar custom transposes; I sent 
https://patchwork.ffmpeg.org/project/ffmpeg/patch/20230223083826.3149827-1-martin@martin.st/ 
to fix that. Here, you can use transpose_4x8H from neon.S too.

> +
> +.macro idct_4x4 bitdepth
> +function ff_hevc_idct_4x4_\bitdepth\()_neon, export=1
> +        ld1             {v0.4h-v3.4h}, [x0]
> +
> +        movrel          x1, trans
> +        ld1             {v4.4h}, [x1]
> +
> +        tr_4x4          v0.4h, v1.4h, v2.4h, v3.4h, v16.4h, v17.4h, v18.4h, v19.4h, 7, v10.4s, v11.4s, v12.4s, v13.4s, v15.4s

You're not allowed to clobber v8-v15 without backing them up/restoring 
them. Checkasm on macOS doesn't detect this, but it does on other OSes.

Here it should be trivial to use other temporary registers instead, 
without needing to backup/restore anything.

// Martin

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