neg rax
sbb rax,rax
neg rax
See explanation
Double negation is a common practice in C to force non-zero numbers to be always 1.
It is typically used when doing boolean operations with integers, where it is
desirable to have 0 to mean false, and 1 to mean true, in case of direct
comparison to possible defines of TRUE and FALSE. Whether this is a good
practice or not is out of scope here.
The double negation looks like this:
!!x;
If x is 0, it will stay 0. If x is 1, 2, 3 or any other non-zero number, it
will be come 1.
This assembly snippet does exactly that. How?
NEG calculates the
two's complement of its operand,
and sets the carry flag (CF) to 0 if the operand is 0, or 1 otherwise.
For example, NEG of 0 is 0, and CF is set to 0. NEG of 1 is 0xffffffff
(assuming 32 bits) and CF is 1. NEG of 2 is 0xfffffffe and CF is 1. And so
on.
Then SBB subtracts the
operands with borrow: the carry flag from the previous operation is added to the
source operand (the rightmost in this example, which is rax anyway), then it
is subtracted from the destination operand (the leftmost, rax again).
At this point, after NEG and SBB:
- 0 became 0
- 1 became 0xffffffff (assuming 32 bits)
- 2 became 0xfffffffe (assuming 32 bits)
- etc (just keep decrementing)
Last but not least, NEG is applied again as at the first step. At the end of
the process:
- 0 becomes 0
- 1 becomes 1
- 2 becomes 1
- 3 becomes 1
and so on.