The mmxext implementation is slower than the C version in x86_64. m32 m64 rgb24toyv12_16_200_c: 24942.7 14812.6 rgb24toyv12_16_200_mmxext: 17857.2 ( 1.40x) 17400.4 ( 0.85x) rgb24toyv12_128_60_c: 56892.9 35616.9 rgb24toyv12_128_60_mmxext: 40730.9 ( 1.40x) 39610.4 ( 0.90x) rgb24toyv12_512_16_c: 58402.7 37209.4 rgb24toyv12_512_16_mmxext: 44842.4 ( 1.30x) 41136.2 ( 0.90x) rgb24toyv12_1920_4_c: 54827.4 34737.4 rgb24toyv12_1920_4_mmxext: 51169.9 ( 1.07x) 34818.9 ( 1.00x)