"Mixing AVX and SSE

Besides 3-operand support, the primary difference between the AVX and SSE encodings of an SSE instruction is that the AVX version clears the unused portion of the ymm register (the top 128 bits), while the SSE version does not modify it. Intel strongly advises against mixing SSE 128-bit instructions and AVX 256-bit instructions, as this "mode-switching" can cost upwards of 70 clock cycles. However, mixing SSE 128-bit and AVX 128-bit is okay, as is mixing AVX 128-bit and AVX 256-bit.

In order to safely switch modes, Intel recommends using vzeroupper after using 256-bit AVX instructions and before using 128-bit SSE instructions, if the two are being used in the same program."