"suffix or operands invalid for 'shr'"

Question

I'm trying to get rid of the lower bits (before doing a popcnt) in a register, but can't seem to get the syntax for the instruction right. I want to clear the lower %rdx bits from the value in %r8d. Searching told me the count argument should be an 8-bit register, so I'm currently at:

shrl %dl, %r8d

GAS complains about this, and I've really no clue what exactly the arguments should be if those are wrong.

Answer 1

According intel PRM, you should use %cl instead of %dl