lavc/vp8dsp: add R-V V vp7_idct_dc_add4y

As with idct_dc_add, most of the code is shared with, and replaces, the
previous VP8 function. To improve performance, we break down the 16x4
matrix into 4 rows, rather than 4 squares. Thus strided loads and
stores are avoided, and the 4 DC calculations are vectored.
Unfortunately this requires a vector gather to splat the DC values, but
overall this is still a win for performance:

T-Head C908:
vp7_idct_dc_add4y_c:       7.2
vp7_idct_dc_add4y_rvv_i32: 2.2
vp8_idct_dc_add4y_c:       6.2
vp8_idct_dc_add4y_rvv_i32: 2.2 (before)
vp8_idct_dc_add4y_rvv_i32: 1.7

SpacemiT X60:
vp7_idct_dc_add4y_c:       6.2
vp7_idct_dc_add4y_rvv_i32: 2.0
vp8_idct_dc_add4y_c:       5.5
vp8_idct_dc_add4y_rvv_i32: 2.5 (before)
vp8_idct_dc_add4y_rvv_i32: 1.7

I also tried to provision the DC values using indexed loads. It ends up
slower overall, especially for VP7, as we then have to compute 16 DC's
instead of just 4.

3 files changed, 54 insertions, 10 deletions

diff --git a/libavcodec/riscv/vp7dsp_init.c b/libavcodec/riscv/vp7dsp_init.c
index 491874483f..fa5fb9d2ae 100644
--- a/libavcodec/riscv/vp7dsp_init.c
+++ b/libavcodec/riscv/vp7dsp_init.c
@@ -28,6 +28,7 @@
 void ff_vp7_luma_dc_wht_rvv(int16_t block[4][4][16], int16_t dc[16]);
 void ff_vp7_idct_add_rvv(uint8_t *dst, int16_t block[16], ptrdiff_t stride);
 void ff_vp78_idct_dc_add_rvv(uint8_t *, int16_t block[16], ptrdiff_t, int dc);
+void ff_vp7_idct_dc_add4y_rvv(uint8_t *dst, int16_t block[4][16], ptrdiff_t);
 
 static void ff_vp7_idct_dc_add_rvv(uint8_t *dst, int16_t block[16],
                                    ptrdiff_t stride)
@@ -49,6 +50,7 @@ av_cold void ff_vp7dsp_init_riscv(VP8DSPContext *c)
         c->vp8_idct_add = ff_vp7_idct_add_rvv;
 #endif
         c->vp8_idct_dc_add = ff_vp7_idct_dc_add_rvv;
+        c->vp8_idct_dc_add4y  = ff_vp7_idct_dc_add4y_rvv;
     }
 #endif
 }
diff --git a/libavcodec/riscv/vp7dsp_rvv.S b/libavcodec/riscv/vp7dsp_rvv.S
index 2a4c404bbb..39b23c2e79 100644
--- a/libavcodec/riscv/vp7dsp_rvv.S
+++ b/libavcodec/riscv/vp7dsp_rvv.S
@@ -127,3 +127,19 @@ func ff_vp7_idct_add_rvv, zve32x
         ret
 endfunc
 #endif
+
+func ff_vp7_idct_dc_add4y_rvv, zve32x
+        li       t0, 32
+        vsetivli zero, 4, e16, mf2, ta, ma
+        li       t1, 23170
+        vlse16.v v8, (a1), t0 # block[0..3][0]
+        vwmul.vx v0, v8, t1
+        li       t2, 0x20000
+        vsetvli  zero, zero, e32, m1, ta, ma
+        vsra.vi  v0, v0, 14
+        vmul.vx  v0, v0, t1
+        vadd.vx  v0, v0, t2
+        vsetvli  zero, zero, e16, mf2, ta, ma
+        vnsra.wi v8, v0, 18   # 4x DC
+        tail     ff_vp78_idct_dc_add4y_rvv
+endfunc
diff --git a/libavcodec/riscv/vp8dsp_rvv.S b/libavcodec/riscv/vp8dsp_rvv.S
index 7a3ab576e9..8ea0a0c9bd 100644
--- a/libavcodec/riscv/vp8dsp_rvv.S
+++ b/libavcodec/riscv/vp8dsp_rvv.S
@@ -105,6 +105,7 @@ func ff_vp8_idct_dc_add_rvv, zve32x
         # fall through
 endfunc
 
+# a3 = DC
 func ff_vp78_idct_dc_add_rvv, zve32x
         csrwi      vxrm, 0
         vsetivli   zero, 4, e8, mf4, ta, ma
@@ -121,6 +122,41 @@ func ff_vp78_idct_dc_add_rvv, zve32x
         ret
 endfunc
 
+func ff_vp8_idct_dc_add4y_rvv, zve32x
+        li       t0, 32
+        vsetivli zero, 4, e16, mf2, ta, ma
+        vlse16.v v8, (a1), t0
+        vadd.vi  v8, v8, 4
+        vsra.vi  v8, v8, 3
+        # fall through
+endfunc
+
+        .variant_cc ff_vp78_idct_dc_add4y_rvv
+# v8 = [dc0, dc1, dc2, dc3]
+func ff_vp78_idct_dc_add4y_rvv, zve32x
+        vsetivli    zero, 16, e16, m2, ta, ma
+        vid.v       v4
+        vsrl.vi     v4, v4, 2
+        vrgather.vv v0, v8, v4 # replicate each DC four times
+        vsetvli     zero, zero, e8, m1, ta, ma
+        li          a4, 4
+1:
+        vle8.v      v8, (a0)
+        addi        a4, a4, -1
+        vwaddu.wv   v16, v0, v8
+        sh          zero, (a1)
+        vsetvli     zero, zero, e16, m2, ta, ma
+        vmax.vx     v16, v16, zero
+        addi        a1, a1, 32
+        vsetvli     zero, zero, e8, m1, ta, ma
+        vnclipu.wi  v8, v16, 0
+        vse8.v      v8, (a0)
+        add         a0, a0, a2
+        bnez        a4, 1b
+
+        ret
+endfunc
+
 .macro vp8_idct_dc_add
         vlse32.v      v0, (a0), a2
         lh            a5, 0(a1)
@@ -143,16 +179,6 @@ endfunc
         addi          a1, a1, 32
 .endm
 
-func ff_vp8_idct_dc_add4y_rvv, zve32x
-        vsetivli      zero, 4, e8, mf4, ta, ma
-        .rept 3
-        vp8_idct_dc_addy
-        .endr
-        vp8_idct_dc_add
-
-        ret
-endfunc
-
 func ff_vp8_idct_dc_add4uv_rvv, zve32x
         vsetivli      zero, 4, e8, mf4, ta, ma
         vp8_idct_dc_addy