> This instruction is used in some bignum code

Could you be more specific? I think for that to work one would also need the upper half of 64x64 multiplication and `vpmullq` provides only the lower half. You could break one 64x64 multiplication into four 32x32 multiplications (i.e. emulate the full 64x64 = 128 bits multiplication) but I was under the impression that this was slow.