lavc/vc1dsp: fix R-V V avg_mspel_pixels

The 8x8 pixel arrays are not necessarily aligned to 64 bits, so the
current code leads to Bus error on real hardware. This reproducible
with FATE's vc1_ilaced_twomv test case.

The new "pessimist" code can trivially be shared for 16x16 pixel
arrays so we also do that. FWIW, this also nominally reduces the
hardware requirement from Zve64x to Zve32x.

T-Head C908:
vc1dsp.avg_vc1_mspel_pixels_tab[0][0]_c:      14.7
vc1dsp.avg_vc1_mspel_pixels_tab[0][0]_rvv_i32: 3.5
vc1dsp.avg_vc1_mspel_pixels_tab[1][0]_c:       3.7
vc1dsp.avg_vc1_mspel_pixels_tab[1][0]_rvv_i32: 1.5

SpacemiT X60:
vc1dsp.avg_vc1_mspel_pixels_tab[0][0]_c:      13.0
vc1dsp.avg_vc1_mspel_pixels_tab[0][0]_rvv_i32: 3.0
vc1dsp.avg_vc1_mspel_pixels_tab[1][0]_c:       3.2
vc1dsp.avg_vc1_mspel_pixels_tab[1][0]_rvv_i32: 1.2

libavcodec/riscv/vc1dsp_init.c

@@ -56,10 +56,10 @@ av_cold void ff_vc1dsp_init_riscv(VC1DSPContext *dsp)
             dsp->vc1_inv_trans_4x8_dc = ff_vc1_inv_trans_4x8_dc_rvv;
             dsp->vc1_inv_trans_4x4_dc = ff_vc1_inv_trans_4x4_dc_rvv;
             dsp->avg_vc1_mspel_pixels_tab[0][0] = ff_avg_pixels16x16_rvv;
+            dsp->avg_vc1_mspel_pixels_tab[1][0] = ff_avg_pixels8x8_rvv;
             if (flags & AV_CPU_FLAG_RVV_I64) {
                 dsp->vc1_inv_trans_8x8_dc = ff_vc1_inv_trans_8x8_dc_rvv;
                 dsp->vc1_inv_trans_8x4_dc = ff_vc1_inv_trans_8x4_dc_rvv;
-                dsp->avg_vc1_mspel_pixels_tab[1][0] = ff_avg_pixels8x8_rvv;
             }
         }
         dsp->startcode_find_candidate = ff_startcode_find_candidate_rvv;

libavcodec/riscv/vc1dsp_rvv.S

@@ -132,31 +132,25 @@ endfunc
 .endm
 
 func ff_avg_pixels16x16_rvv, zve32x
-        csrwi         vxrm, 0
-        vsetivli      zero, 16, e8, m1, ta, ma
-        mspel_op_all  l a1 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
-        mspel_op_all  l a0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
-        vsetvli       t0, zero, e8, m8, ta, ma
-        sub           a0, a0, a2
-        vaaddu.vv     v0, v0, v16
-        neg           a2, a2
-        vaaddu.vv     v8, v8, v24
-        vsetivli      zero, 16, e8, m1, ta, ma
-        mspel_op_all  s a0 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
-
-        ret
+        li       t0, 16
+        vsetivli zero, 16, e8, m1, ta, ma
+        j        1f
 endfunc
 
-func ff_avg_pixels8x8_rvv, zve64x
-        csrwi         vxrm, 0
-        li            t0, 64
-        vsetivli      zero, 8, e8, mf2, ta, ma
-        vlse64.v      v16, (a1), a2
-        vlse64.v      v8, (a0), a2
-        vsetvli       zero, t0, e8, m4, ta, ma
-        vaaddu.vv     v16, v16, v8
-        vsetivli      zero, 8, e8, mf2, ta, ma
-        vsse64.v      v16, (a0), a2
+func ff_avg_pixels8x8_rvv, zve32x
+        li        t0, 8
+        vsetivli  zero, 8, e8, mf2, ta, ma
+1:
+        csrwi     vxrm, 0
+2:
+        vle8.v    v16, (a1)
+        addi      t0, t0, -1
+        vle8.v    v8, (a0)
+        add       a1, a1, a2
+        vaaddu.vv v16, v16, v8
+        vse8.v    v16, (a0)
+        add       a0, a0, a2
+        bnez      t0, 2b
 
         ret
 endfunc