[FFmpeg-devel] [PATCH] lavc/vc1dsp: fix R-V V vector lengths

[FFmpeg-devel] [PATCH] lavc/vc1dsp: fix R-V V vector lengths

[Rémi Denis-Courmont]

The 8x4 and 4x4 use a needlessly large multiplier (unless/until we care
about embedded 64-bit-vector hardware). This is merely suboptimal.

The 8x4 case also uses an incorrect vector length, which leads to incorrect
behaviour on future/hypothetical hardware with 256-bit or larger vectors.

Pointed-out-by: Martin Storsjö <martin@martin.st>
---
 libavcodec/riscv/vc1dsp_rvv.S | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/libavcodec/riscv/vc1dsp_rvv.S b/libavcodec/riscv/vc1dsp_rvv.S
index 1a503ecc87..4a00945ead 100644
--- a/libavcodec/riscv/vc1dsp_rvv.S
+++ b/libavcodec/riscv/vc1dsp_rvv.S
@@ -68,7 +68,7 @@ endfunc
 
 func ff_vc1_inv_trans_8x4_dc_rvv, zve64x
         lh            t2, (a2)
-        vsetivli      zero, 8, e8, mf2, ta, ma
+        vsetivli      zero, 4, e8, mf4, ta, ma
         vlse64.v      v0, (a0), a1
         sh1add        t2, t2, t2
         addi          t2, t2, 1
@@ -84,14 +84,14 @@ func ff_vc1_inv_trans_8x4_dc_rvv, zve64x
         vmax.vx       v4, v4, zero
         vsetvli       zero, zero, e8, m2, ta, ma
         vnclipu.wi    v0, v4, 0
-        vsetivli      zero, 8, e8, mf2, ta, ma
+        vsetivli      zero, 4, e8, mf4, ta, ma
         vsse64.v      v0, (a0), a1
         ret
 endfunc
 
 func ff_vc1_inv_trans_4x4_dc_rvv, zve32x
         lh            t2, (a2)
-        vsetivli      zero, 4, e8, mf2, ta, ma
+        vsetivli      zero, 4, e8, mf4, ta, ma
         vlse32.v      v0, (a0), a1
         slli          t1, t2, 4
         add           t2, t2, t1
@@ -107,7 +107,7 @@ func ff_vc1_inv_trans_4x4_dc_rvv, zve32x
         vmax.vx       v2, v2, zero
         vsetvli       zero, zero, e8, m1, ta, ma
         vnclipu.wi    v0, v2, 0
-        vsetivli      zero, 4, e8, mf2, ta, ma
+        vsetivli      zero, 4, e8, mf4, ta, ma
         vsse32.v      v0, (a0), a1
         ret
 endfunc
-- 
2.43.0

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Re: [FFmpeg-devel] [PATCH] lavc/vc1dsp: fix R-V V vector lengths

[Martin Storsjö]

On Sat, 16 Dec 2023, Rémi Denis-Courmont wrote:

> The 8x4 and 4x4 use a needlessly large multiplier (unless/until we care
> about embedded 64-bit-vector hardware). This is merely suboptimal.
>
> The 8x4 case also uses an incorrect vector length, which leads to incorrect
> behaviour on future/hypothetical hardware with 256-bit or larger vectors.
>
> Pointed-out-by: Martin Storsjö <martin@martin.st>
> ---
> libavcodec/riscv/vc1dsp_rvv.S | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/libavcodec/riscv/vc1dsp_rvv.S b/libavcodec/riscv/vc1dsp_rvv.S
> index 1a503ecc87..4a00945ead 100644
> --- a/libavcodec/riscv/vc1dsp_rvv.S
> +++ b/libavcodec/riscv/vc1dsp_rvv.S
> @@ -68,7 +68,7 @@ endfunc
> 
> func ff_vc1_inv_trans_8x4_dc_rvv, zve64x
>         lh            t2, (a2)
> -        vsetivli      zero, 8, e8, mf2, ta, ma
> +        vsetivli      zero, 4, e8, mf4, ta, ma
>         vlse64.v      v0, (a0), a1
>         sh1add        t2, t2, t2
>         addi          t2, t2, 1
> @@ -84,14 +84,14 @@ func ff_vc1_inv_trans_8x4_dc_rvv, zve64x
>         vmax.vx       v4, v4, zero
>         vsetvli       zero, zero, e8, m2, ta, ma
>         vnclipu.wi    v0, v4, 0
> -        vsetivli      zero, 8, e8, mf2, ta, ma
> +        vsetivli      zero, 4, e8, mf4, ta, ma
>         vsse64.v      v0, (a0), a1
>         ret
> endfunc
> 
> func ff_vc1_inv_trans_4x4_dc_rvv, zve32x
>         lh            t2, (a2)
> -        vsetivli      zero, 4, e8, mf2, ta, ma
> +        vsetivli      zero, 4, e8, mf4, ta, ma
>         vlse32.v      v0, (a0), a1
>         slli          t1, t2, 4
>         add           t2, t2, t1
> @@ -107,7 +107,7 @@ func ff_vc1_inv_trans_4x4_dc_rvv, zve32x
>         vmax.vx       v2, v2, zero
>         vsetvli       zero, zero, e8, m1, ta, ma
>         vnclipu.wi    v0, v2, 0
> -        vsetivli      zero, 4, e8, mf2, ta, ma
> +        vsetivli      zero, 4, e8, mf4, ta, ma
>         vsse32.v      v0, (a0), a1
>         ret
> endfunc
> -- 
> 2.43.0

The fix sounds reasonable (although I can't say I've followed the code). 
Anyway, I've tested that it does indeed fix the checkasm test, so feel 
free to push - thanks!

// Martin
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