Comment by jxors

> Some instructions require VEX.L or VEX.W to be 0 or 1, and some encodings result in completely different instructions if you change VEX.L.

There is even an instruction where AMD got this wrong! VPERMQ requires VEX.W=1, but some AMD CPUs also happily execute it when VEX.W=0 even though that is supposed to raise an exception.