x86 ASM: useless conditional jump?

Question

I am looking at the following piece of x86 assembly code (Intel syntax):

movzx   eax, al
and     eax, 3
cmp     eax, 3
ja      loc_6BE9A0

In my understanding, this should equal something like this in C:

eax &= 0xFF;
eax &= 3;
if (eax > 3)
   loc_6BE9A0();

This does not seem to make much sense since this condition will never be true (because eax will never be greater than 3 if it got and-ed with 3 before). Am I missing something here or is this really just an unnecessary condition?

And also: the movzx eax, al should not be necessary either if it gets and-ed with 3 right after that, is it?

I am asking this because I am not so familiar with assembly language and so I am not entirely sure if I am missing something here.

Answer 1

You're correct: the movzx is redundant given the following and. It might have been produced by a non-optimizing compiler.

And yes, if this code executes straight through then the ja jump will never be taken. However, the cmp/ja may not be completely useless if there is code somewhere else that jumps straight to the cmp (or even to the ja).