lavc/sbrdsp: R-V V neg_odd_64

With 128-bit vectors, this is mostly pointless but also harmless.
Performance gains should be more noticeable with larger vector sizes.

neg_odd_64_c:       76.2
neg_odd_64_rvv_i64: 74.7

libavcodec/riscv/sbrdsp_init.c

@@ -25,6 +25,7 @@
 
 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);
 
 av_cold void ff_sbrdsp_init_riscv(SBRDSPContext *c)
 {
@@ -35,5 +36,9 @@ av_cold void ff_sbrdsp_init_riscv(SBRDSPContext *c)
         c->sum64x5 = ff_sbr_sum64x5_rvv;
         c->sum_square = ff_sbr_sum_square_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
 }

libavcodec/riscv/sbrdsp_rvv.S

@@ -67,3 +67,20 @@ func ff_sbr_sum_square_rvv, zve32f
 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