c - why logical"NOT" the condition in if statement in assembly?

Question

I'm learning assembly and a textbook shows an example of condition control:

I don't understand why x>=y is used in line 3, why not just follow the logic and use x<y (same as original C code)? is any particular reason to !(not) the condition in if statement?

Answer 1

An if() statement means "skip this if the condition is not true".

If the condition is true, you want execution to fall-through into the if body, and if it's false you want execution to jump over the if body.

Thus, the obvious / literal way to compile an if is with a jcc on the inverse condition, like jnl. (Optimized code could certainly do much better for those highly-related if/else blocks, like subtract and conditional negate.)

If you wanted to use jl, you have to put the if-body out-of-line, maybe after the ret at the end of the function, and then jump back from it.