According to Agner the Skylake puts the top-half of the 256-bit AVX execution units to sleep when they're not in use, and during the warm-up period when you fir

According to Agner the Skylake puts the top-half of the 256-bit AVX execution units to sleep when they're not in use, and during the warm-up period when you first start using them again, 256-bit operations are done by using the bottom 128-bits of the XUs twice. http://www.agner.org/optimize/blog/read.php?i=415 http://www.agner.org/optimize/blog/read.php?i=415