Check if a number is even

Question

I\'m working my way through low level bit hacks, and would like to write an assembly program for each. Here is what I have for checking if a number is even or not:

Answer 1

I can't get it down to two instructions, but I can golf it a little shorter.

Your current version is 12 bytes including the ret. You can shave off two bytes with test $1, %dil instead, since the high bytes of the input are irrelevant, thus trading the 4-byte immediate for a 1-byte immediate and a prefix byte. That gets it down to 10.

You can shave off two more bytes by taking advantage of the somewhat obscure fact that the shift instructions shift into the carry flag, and doing

is_even: // 8 bytes
    xor %eax, %eax
    shr $1, %edi
    setnc %al
    ret

gcc and clang both do

is_even: // 8 bytes
    mov %edi, %eax
    not %eax
    and $1, %eax
    ret

For one less byte, there's

is_even: // 7 bytes
    shr $1, %edi
    sbb %eax, %eax
    inc %eax
    ret

sbb is "subtract with borrow", which subtracts one register from another, then subtracts 1 more if the carry flag was set. This leaves us with 0 if the input was even, and -1 if it was odd. Then adding 1 gets us where we want to be. This might be slower because I'm not sure if the CPU knows that the result does not depend on the previous value of %eax.

I don't see a way to get down to two instructions, though. It's an annoying feature of the conditional setcc instructions that they only set the low byte and leave the rest of the register alone, forcing you to zero it yourself in the common case that you want your boolean in the full register. And we have to get the input and output in two different registers, which is awkward because of x86's model where the output register is always one of the inputs.