What is the easiest way to check an integer's

问题:

For example:

int x = 35;
if( x%2==1) { //do something }

I just want to check the modulus value without assigning the result to x.

Assume that value is in eax, so I can use DIV instruction, then put back the original value to eax etc.. but that seems inefficient. Can you suggest a better way?

回答1:

To branch according to the modulo 2 of a value in al/ax/eax/rax:

    test al,1
    jnz is_odd

is_even:
    ; do something for even numbers.

is_odd:
    ; do something for odd numbers.

However, if all you want is the modulo value, you don't need any branches.

test al,1
setnz bl   ; modulo 2 of al/ax/eax/rax is now in bl.

回答2:

If the low bit is on, it is not divisible by two:

test   x, 1
jne    somewhere_when_odd