[FFmpeg-devel] [PATCH v2 2/2] swscale: Neon rgb_to_yuv_half process 32 pixels at a time

[Dmitriy Kovalenko <dmtr.kovalenko@outlook.com>]

This patch integrates so called double bufferring when we are loading
2 batch of elements at a time and then processing them in parallel. On the
moden arm processors especially Apple Silicon it gives a visible
benefit, for subsampled pixel processing it is especially nice because
it allows to read elements w/ 2 instructions and write with a single one
(especially visible on a platforms with slower memory like ios).

Including the previous patch in a stack on macbook pro m4 max rgb_to_yuv_half
in checkasm goes up 2x of the c version
---
 libswscale/aarch64/input.S | 105 ++++++++++++++++++++++++++++---------
 1 file changed, 80 insertions(+), 25 deletions(-)

diff --git a/libswscale/aarch64/input.S b/libswscale/aarch64/input.S
index d61f8b3dba..00aab26ce1 100644
--- a/libswscale/aarch64/input.S
+++ b/libswscale/aarch64/input.S
@@ -197,40 +197,94 @@ function ff_\fmt_rgb\()ToUV_half_neon, export=1
         ldp             w12, w13, [x6, #20]     // w12: bu, w13: rv
         ldp             w14, w15, [x6, #28]     // w14: gv, w15: bv
 4:
-        cmp             w5, #8
         rgb_set_uv_coeff half=1
-        b.lt            2f
-1:  // load 16 pixels and prefetch memory for the next block
+        
+        cmp             w5, #16
+        b.lt            2f                      // Go directly to scalar if < 16
+
+1:
     .if \element == 3
-        ld3             { v16.16b, v17.16b, v18.16b }, [x3], #48
-        prfm            pldl1strm, [x3, #48]
+        ld3             { v16.16b, v17.16b, v18.16b }, [x3], #48  // First 16 pixels
+        ld3             { v26.16b, v27.16b, v28.16b }, [x3], #48  // Second 16 pixels
+        prfm            pldl1keep, [x3, #96]
     .else
-        ld4             { v16.16b, v17.16b, v18.16b, v19.16b }, [x3], #64
-        prfm            pldl1strm, [x3, #64]
+        ld4             { v16.16b, v17.16b, v18.16b, v19.16b }, [x3], #64  // First 16 pixels
+        ld4             { v26.16b, v27.16b, v28.16b, v29.16b }, [x3], #64  // Second 16 pixels
+        prfm            pldl1keep, [x3, #128]
     .endif
-
+    
+    // **Sum adjacent pixel pairs**
     .if \alpha_first
-        uaddlp          v21.8h, v19.16b         // v21: summed b pairs
-        uaddlp          v20.8h, v18.16b         // v20: summed g pairs
-        uaddlp          v19.8h, v17.16b         // v19: summed r pairs
+        uaddlp          v21.8h, v19.16b         // Block 1: B sums
+        uaddlp          v20.8h, v18.16b         // Block 1: G sums  
+        uaddlp          v19.8h, v17.16b         // Block 1: R sums
+        uaddlp          v31.8h, v29.16b         // Block 2: B sums
+        uaddlp          v30.8h, v28.16b         // Block 2: G sums
+        uaddlp          v29.8h, v27.16b         // Block 2: R sums
     .else
-        uaddlp          v19.8h, v16.16b         // v19: summed r pairs
-        uaddlp          v20.8h, v17.16b         // v20: summed g pairs
-        uaddlp          v21.8h, v18.16b         // v21: summed b pairs
+        uaddlp          v19.8h, v16.16b         // Block 1: R sums
+        uaddlp          v20.8h, v17.16b         // Block 1: G sums
+        uaddlp          v21.8h, v18.16b         // Block 1: B sums
+        uaddlp          v29.8h, v26.16b         // Block 2: R sums
+        uaddlp          v30.8h, v27.16b         // Block 2: G sums
+        uaddlp          v31.8h, v28.16b         // Block 2: B sums
     .endif
 
-        mov             v22.16b, v6.16b         // U first half
-        mov             v23.16b, v6.16b         // U second half
-        mov             v24.16b, v6.16b         // V first half
-        mov             v25.16b, v6.16b         // V second half
+        // init accumulatos for both blocks
+        mov             v7.16b, v6.16b          //  U_low 
+        mov             v8.16b, v6.16b          //  U_high
+        mov             v9.16b, v6.16b          //  V_low
+        mov             v10.16b, v6.16b         //  V_high
+        mov             v11.16b, v6.16b         //  U_low
+        mov             v12.16b, v6.16b         //  U_high  
+        mov             v13.16b, v6.16b         //  V_low
+        mov             v14.16b, v6.16b         //  V_high
+
+        smlal           v7.4s, v0.4h, v19.4h    // U += ru * r (0-3)
+        smlal           v9.4s, v3.4h, v19.4h    // V += rv * r (0-3) 
+        smlal           v11.4s, v0.4h, v29.4h   // U += ru * r (0-3)
+        smlal           v13.4s, v3.4h, v29.4h   // V += rv * r (0-3)
+
+        smlal2          v8.4s, v0.8h, v19.8h    // U += ru * r (4-7)
+        smlal2          v10.4s, v3.8h, v19.8h   // V += rv * r (4-7)
+        smlal2          v12.4s, v0.8h, v29.8h   // U += ru * r (4-7)
+        smlal2          v14.4s, v3.8h, v29.8h   // V += rv * r (4-7)
+
+        smlal           v7.4s, v1.4h, v20.4h    // U += gu * g (0-3)
+        smlal           v9.4s, v4.4h, v20.4h    // V += gv * g (0-3)
+        smlal           v11.4s, v1.4h, v30.4h   // U += gu * g (0-3)
+        smlal           v13.4s, v4.4h, v30.4h   // V += gv * g (0-3)
+
+        smlal2          v8.4s, v1.8h, v20.8h    // U += gu * g (4-7)
+        smlal2          v10.4s, v4.8h, v20.8h   // V += gv * g (4-7)
+        smlal2          v12.4s, v1.8h, v30.8h   // U += gu * g (4-7)
+        smlal2          v14.4s, v4.8h, v30.8h   // V += gv * g (4-7)
+
+        smlal           v7.4s, v2.4h, v21.4h    // U += bu * b (0-3)
+        smlal           v9.4s, v5.4h, v21.4h    // V += bv * b (0-3)
+        smlal           v11.4s, v2.4h, v31.4h   // U += bu * b (0-3)
+        smlal           v13.4s, v5.4h, v31.4h   // V += bv * b (0-3)
+
+        smlal2          v8.4s, v2.8h, v21.8h    // U += bu * b (4-7)
+        smlal2          v10.4s, v5.8h, v21.8h   // V += bv * b (4-7)
+        smlal2          v12.4s, v2.8h, v31.8h   // U += bu * b (4-7)
+        smlal2          v14.4s, v5.8h, v31.8h   // V += bv * b (4-7)
+
+        sqshrn          v16.4h, v7.4s, #10      // U (0-3)
+        sqshrn          v17.4h, v9.4s, #10      // V (0-3)
+        sqshrn          v22.4h, v11.4s, #10     // U (0-3)
+        sqshrn          v23.4h, v13.4s, #10     // V (0-3)
+
+        sqshrn2         v16.8h, v8.4s, #10      // U (0-7)
+        sqshrn2         v17.8h, v10.4s, #10     // V (0-7)
+        sqshrn2         v22.8h, v12.4s, #10     // U (0-7)
+        sqshrn2         v23.8h, v14.4s, #10     // V (0-7)
+
+        stp             q16, q22, [x0], #32     // Store all 16 U values
+        stp             q17, q23, [x1], #32     // Store all 16 V values
 
-        rgb_to_uv_interleaved_product v19, v20, v21, v0, v1, v2, v3, v4, v5, v22, v23, v24, v25, v16, v17, #10
-
-        str             q16, [x0], #16          // store dst_u
-        str             q17, [x1], #16          // store dst_v
-
-        sub             w5, w5, #8              // width -= 8
-        cmp             w5, #8                  // width >= 8 ?
+        sub             w5, w5, #16             // width -= 16
+        cmp             w5, #16                 // width >= 16 ?
         b.ge            1b
         cbz             w5, 3f                  // No pixels left? Exit
 
@@ -444,3 +498,4 @@ endfunc
 
 DISABLE_DOTPROD
 #endif
+
-- 
2.49.0

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