CODEWITHSUNDEEP
assemblyx86x86-64eflags
Charlotte1891
Charlotte1891

Reputation: 41

how to interpret x86-64 xor followed by jle?

If part of the assembly code is as following:

xor %ebp,%ebx
jle some address

does this jle means that it will jump when (%ebx ^ %ebp == 0) because that would set ZF to 1?

Upvotes: 4

Views: 648

Answers (1)

Peter Cordes
Peter Cordes

Reputation: 365332

That's one of the ways JLE can be true. The other is SF≠ OF, as per the manual: https://www.felixcloutier.com/x86/jcc

Since XOR always clears OF, SF != OF reduces to just SF.

jle after a boolean op will be taken if SF | ZF, i.e. if the result is <= 0.

Interesting optimization to avoid test %ebx,%ebx to compare the result against zero (AND or TEST same,same sets FLAGS identically to cmp reg,0).

Upvotes: 5

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