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    <p>If you keep u &amp; v interleaved in one register, and use 'pmaddwd' and precomputed constants instead of your shift-and-add approach, you can compress the conversion code to about a third, and get rid of most stalls at the same time:</p> <pre><code>; xmm0 = y y y y y y y y ; xmm3 = u v u v u v u v psubsw xmm3, [Const128] psubsw xmm0, [Const16] movdqa xmm4, xmm3 movdqa xmm5, xmm3 pmaddwd xmm3, [const_1] pmaddwd xmm4, [const_2] pmaddwd xmm5, [const_3] psrad xmm3, 14 psrad xmm4, 14 psrad xmm5, 14 pshufb xmm3, xmm3, [const_4] ; or pshuflw &amp; pshufhw pshufb xmm4, xmm4, [const_4] pshufb xmm5, xmm5, [const_4] paddsw xmm3, xmm0 paddsw xmm4, xmm0 paddsw xmm5, xmm0 </code></pre> <p>If you want it to work even faster, playing with PMADDUBSW should allow you to work on 16 pixels at a time with a small increase in complexity.</p> <p>Most processors (particularly non-Intels, notorious for not having a well-working hardware prefetcher, but, to a lesser extent, Intels too) will benefit from a prefetchnta [esi+256] thrown inside the loop. </p> <p>EDIT: the code that uses PMADDUBSW could look like this (correctness not guaranteed):</p> <pre><code>const a: times 4 db 1,3 times 4 db 5,7 const b: times 4 db 9,11 times 4 db 13,15 const_c: times 8 dw 0x00ff const_d: times 4 dd 0x00ffffff const_uv_to_rgb_mul: ... const_uv_to_rgb_add: ... movdqa xmm4, [esi] movdqa xmm0, xmm4 movdqa xmm1, xmm4 pshufb xmm0, [const_a] pshufb xmm1, [const_b] pand xmm4, [const_c] ; xmm0: uv0 uv0 uv0 uv0 uv2 uv2 uv2 uv2 uv2 ; xmm1: uv4 uv4 uv4 uv4 ... ; xmm4: y0 0 y1 0 y2 0 y3 0 y4 0 y5 0 y6 0 y7 0 pmaddubsw xmm0, [const_uv_to_rgb_mul] pmaddubsw xmm1, [const_uv_to_rgb_mul] paddsw xmm0, [const_uv_to_rgb_add] paddsw xmm1, [const_uv_to_rgb_add] psraw xmm0, 6 psraw xmm1, 6 ; r01 g01 b01 0 r23 g23 b23 0 pshufd xmm2, xmm0, 2+3*4+2*16+3*64 pshufd xmm0, xmm0, 0+1*4+0+16+1*64 pshufd xmm3, xmm1, 2+3*4+2*16+3*64 pshufd xmm1, xmm1, 0+1*4+0+16+1*64 ; xmm0: r01 g01 b01 0 r01 g01 b01 0 ; xmm2: r23 g23 b23 0 r23 g23 b23 0 ; xmm1: r45 g45 b45 0 r45 g45 b45 0 paddsw xmm0, xmm4 ; add y paddsw xmm1, xmm4 paddsw xmm2, xmm4 paddsw xmm3, xmm4 packuswb xmm0, xmm2 ; pack with saturation into 0-255 range packuswb xmm1, xmm3 pand xmm0, [const_d] ; zero out the alpha byte pand xmm1, [const_d] movntdq [edi], xmm0 movntdq [edi+16], xmm1 </code></pre>
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    1. COOk this is interesting. So you think all the calculations can be done in 8 bit without overflow.
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    2. COpmaddwd works in 32 bit. pmaddubsw works in 16 bit.
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    3. COOk this looks good. I will do some work on this and see how I go. Thanks for your help.
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