[FFmpeg-devel] [PATCH] libswscale/riscv: Fix syntax of vsetvli

[Rémi Denis-Courmont]

Hi,

The diff is ok. I even have a slight incline in favour thereof for the sake of consistency. Yet, I may be vainly pedantic but I do have problems with the description.

Le 3 juillet 2023 05:44:12 GMT+03:00, Khem Raj <raj.khem at gmail.com> a écrit :
>Add missing operand

The spec does explicitly make the mask and tail policy mandatory, but I can't see any such requirements for the group multiplier. To the contrary, there are several examples in the spec _without_ explicit multiplier.

In reality, `vsetvli` (and `vsetivli`) merely transfers an immediate value to the vector configuration register. Missing fields are to be left at zero for forward compatibility, and `m1` happens to encode as 0b000.

> Which clang complains about but gcc assumes it to be
>'m1' if not specifiied.

The technical term for the RISC-V support in LLVM AS version 15 and earlier is "useless junk". If you want to compile FFmpeg on RISC-V with Clang, you *must* disable the integrated AS and use binutils GNU/as instead.

The Linux RISC-V kernel altogether gave up on LLVM entirely, requiring GCC for RVV, so they're even stricter (and without kernel support, FFmpeg support is obviously useless).

*Hopefully* LLVM gets their act together by release 16, and ship a usable assembler, rather than tell us to use automatic RVV vectorisation (which *is* a release 16 feature, though it was half-baked last time I tried).

>
>Fixes building with clang

More like bug-compatible work-around than fix, AFAIU.

>| src/libswscale/riscv/rgb2rgb_rvv.S:88:25: error: operand must be e[8|16|32|64|128|256|512|1024],m[1|2|4|8|f2|f4|f8],[ta|tu],[ma|mu]

Do you have a reference to the Github RVV spec to validate this, that I overlooked, or it's just misled and misleading spew from LLVM?

>|         vsetvli t4, t3, e8, ta, ma
>|                         ^
>
>Signed-off-by: Khem Raj <raj.khem at gmail.com>
>---
> libswscale/riscv/rgb2rgb_rvv.S | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/libswscale/riscv/rgb2rgb_rvv.S b/libswscale/riscv/rgb2rgb_rvv.S
>index 5626d906eb..bbdfdbebbc 100644
>--- a/libswscale/riscv/rgb2rgb_rvv.S
>+++ b/libswscale/riscv/rgb2rgb_rvv.S
>@@ -85,7 +85,7 @@ func ff_interleave_bytes_rvv, zve32x
>         mv      t3, a3
>         addi    a4, a4, -1
> 2:
>-        vsetvli    t4, t3, e8, ta, ma
>+        vsetvli    t4, t3, e8, m1, ta, ma
>         sub        t3, t3, t4
>         vle8.v     v8, (t0)
>         add        t0, t4, t0