Re: [FFmpeg-devel] [PATCH] Revert "lavc/sbrdsp: R-V V neg_odd_64"

["Rémi Denis-Courmont" <remi@remlab.net>]

Le 13 mai 2024 20:06:34 GMT+03:00, Lynne via ffmpeg-devel <ffmpeg-devel@ffmpeg.org> a écrit :
>On 13/05/2024 18:43, Rémi Denis-Courmont wrote:
>> While this function can easily be written with vectors, it just fails to
>> get any performance improvement.
>> 
>> For reference, this is a simpler loop-free implementation that does get
>> better performance than the current one depending on hardware, but still
>> more or less the same metrics as the C code:
>> 
>>   func ff_sbr_neg_odd_64_rvv, zve64x
>>           li      a1, 32
>>           addi    a0, a0, 7
>>           li      t0, 8
>>           vsetvli zero, a1, e8, m2, ta, ma
>>           li      t1, 0x80
>>           vlse8.v v8, (a0), t0
>>           vxor.vx v8, v8, t1
>>           vsse8.v v8, (a0), t0
>>           ret
>>   endfunc
>> 
>> This reverts commit d06fd18f8f4c6a81ef94cbb600620d83ad51269d.
>> ---
>>   libavcodec/riscv/sbrdsp_init.c |  5 -----
>>   libavcodec/riscv/sbrdsp_rvv.S  | 17 -----------------
>>   2 files changed, 22 deletions(-)
>> 
>> diff --git a/libavcodec/riscv/sbrdsp_init.c b/libavcodec/riscv/sbrdsp_init.c
>> index f937c47e22..d3bafa961e 100644
>> --- a/libavcodec/riscv/sbrdsp_init.c
>> +++ b/libavcodec/riscv/sbrdsp_init.c
>> @@ -26,7 +26,6 @@
>>     void ff_sbr_sum64x5_rvv(float *z);
>>   float ff_sbr_sum_square_rvv(float (*x)[2], int n);
>> -void ff_sbr_neg_odd_64_rvv(float *x);
>>   void ff_sbr_autocorrelate_rvv(const float x[40][2], float phi[3][2][2]);
>>   void ff_sbr_hf_gen_rvv(float (*X_high)[2], const float (*X_low)[2],
>>                          const float alpha0[2], const float alpha1[2],
>> @@ -64,9 +63,5 @@ av_cold void ff_sbrdsp_init_riscv(SBRDSPContext *c)
>>           }
>>           c->autocorrelate = ff_sbr_autocorrelate_rvv;
>>       }
>> -#if __riscv_xlen >= 64
>> -    if ((flags & AV_CPU_FLAG_RVV_I64) && (flags & AV_CPU_FLAG_RVB_ADDR))
>> -        c->neg_odd_64 = ff_sbr_neg_odd_64_rvv;
>> -#endif
>>   #endif
>>   }
>> diff --git a/libavcodec/riscv/sbrdsp_rvv.S b/libavcodec/riscv/sbrdsp_rvv.S
>> index 918c37882f..aba9a28108 100644
>> --- a/libavcodec/riscv/sbrdsp_rvv.S
>> +++ b/libavcodec/riscv/sbrdsp_rvv.S
>> @@ -68,23 +68,6 @@ NOHWF   fmv.x.w  a0, fa0
>>           ret
>>   endfunc
>>   -#if __riscv_xlen >= 64
>> -func ff_sbr_neg_odd_64_rvv, zve64x
>> -        li      a1, 32
>> -        li      t1, 1 << 63
>> -1:
>> -        vsetvli t0, a1, e64, m8, ta, ma
>> -        vle64.v v8, (a0)
>> -        sub     a1, a1, t0
>> -        vxor.vx v8, v8, t1
>> -        vse64.v v8, (a0)
>> -        sh3add  a0, t0, a0
>> -        bnez    t0, 1b
>> -
>> -        ret
>> -endfunc
>> -#endif
>> -
>>   func ff_sbr_autocorrelate_rvv, zve32f
>>           vsetvli t0, zero, e32, m4, ta, ma
>>           vmv.v.x v0, zero
>
>Do you think a 256bit implementation be able to overcome the overhead and end up being faster?

Based on Sunyeuchi's tests, it gets worse with larger vectors.

I guess the C code is just bound by memory bandwidth, since the "calculations" are so trivial, and thus we can't really beat C here.
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