lavc/h264dsp: unroll R-V V weight16

As VLSE128.V does not exist, we have no other way to deal with latency.

T-Head C908:
h264_weight16_8_c:                                     989.4 ( 1.00x)
h264_weight16_8_rvv_i32:                               193.2 ( 5.12x)

SpacemiT X60:
h264_weight16_8_c:                                     874.1 ( 1.00x)
h264_weight16_8_rvv_i32:                               196.9 ( 4.44x)

libavcodec/riscv/h264dsp_init.c

@@ -97,7 +97,7 @@ av_cold void ff_h264dsp_init_riscv(H264DSPContext *dsp, const int bit_depth,
         const bool zvl128b = ff_rv_vlen_least(128);
 
         if (bit_depth == 8) {
-            if (zvl128b)
+            if (zvl128b && (flags & AV_CPU_FLAG_RVB))
                 dsp->weight_h264_pixels_tab[0] =
                     ff_h264_weight_funcs_8_rvv[0].weight;
             if (flags & AV_CPU_FLAG_RVV_I64)

libavcodec/riscv/h264dsp_rvv.S

@@ -29,20 +29,29 @@
 #include "libavutil/riscv/asm.S"
 
         .variant_cc ff_h264_weight_pixels_simple_8_rvv
-func ff_h264_weight_pixels_simple_8_rvv, zve32x
+func ff_h264_weight_pixels_simple_8_rvv, zve32x, b
         csrwi   vxrm, 0
         sll     a5, a5, a3
 1:
         vsetvli     zero, t6, e16, m2, ta, ma
-        vle8.v  v8, (a0)
-        addi    a2, a2, -1
+        add     t0, a0, a1
+        vle8.v      v8, (a0)
+        addi    a2, a2, -2
+        vle8.v      v9, (t0)
         vzext.vf2   v24, v8
+        vzext.vf2   v26, v9
         vmul.vx     v16, v24, a4
+        vmul.vx     v18, v26, a4
         vsadd.vx    v16, v16, a5
-        vmax.vx v16, v16, zero
-        vsetvli zero, zero, e8, m1, ta, ma
+        vsadd.vx    v18, v18, a5
+        vmax.vx     v16, v16, zero
+        vmax.vx     v18, v18, zero
+        vsetvli     zero, zero, e8, m1, ta, ma
         vnclipu.wx  v8, v16, a3
-        vse8.v  v8, (a0)
+        vnclipu.wx  v9, v18, a3
+        vse8.v      v8, (a0)
+        vse8.v      v9, (t0)
+        sh1add  a0, a1, a0
         add     a0, a0, a1
         bnez    a2, 1b