Q40

[Anding]

label:
neg rax
jl label
; here rax holds its absolute value

jl responds to SF != OF. js seems to work as well and is a "logical" choice.

Is jl preferred for any reason?

[sayon]

Thank you for a wonderful question!

• jl jumps if: SF <> OF
• js jumps if: SF = 1
• neg changes CF (if operand = 0, CF = 0) , OF, SF, ZF, AF, and PF.

Usually, js will do the trick except for one case: if rax is equal to the smallest signed integer. It is because the range of 64-bit integers is asymmetric: -2^64 ... 2^64 - 1. When rax is the smallest negative integer, its 2 complement (computed by neg) is the same and as the result we get stuck in a loop. Let's study these two examples:

Normal case: rax = -10

label:      ; rax = -10
neg rax  ; rax = 10, CF, PF, AF set
js label   ;  OK, no jump

Corner case: rax = -2^64 = 0x80 00 00 00 00 00 00 00

label:      ; rax = -2^64
neg rax  ; rax = -2^64, CF, PF, SF, OF  are set
js label   ;  `rax` is still negative and won't change its sign. 

Now we are stuck performing neg (which has no effect) and js till the end of the world. However, notice, that both SF and OF are set in this case. This is the exact situation when jl will prevent us from being stuck in a loop.