[CorvusCrypto]

was going to say this. It probably only does SSE and not SSE2. Therefore vectorization only happens for 32 bit ints.

[user2994cb]

Seems to need -mavx2 to really go to town with 64 bit: https://godbolt.org/g/6EFYeY

[jdcarter]

Thank you both, I appreciate the insight!