SHL EAX,1 and ADD EAX,EAX are the same, time wise. The only difference is that the A flag (auxiliary carry [1]) is undefined for SHL and defined for ADD. SHL EAX,n may be faster than n repeats of ADD EAX,EAX, but there are other ways to do quick multiplication by powers of 2 though.
[1] The auxiliary carry reflects a borrow from bit 4 to bit 3, or a carry from bit 3 to bit 4. It's used for BCD [2] arithmetic, and there is is no direct way to test for it.
[1] The auxiliary carry reflects a borrow from bit 4 to bit 3, or a carry from bit 3 to bit 4. It's used for BCD [2] arithmetic, and there is is no direct way to test for it.
[2] Binary Coded Decimal