xorl %eax - Instruction set architecture in IA-32

Question

I am experiencing some difficulties interpreting this exercise;

What does exactly xorl does in this assembly snippet?

C Code:

int i = 0;
if (i>=55)
    i++;
else
    i--;

Assembly

xorl ____ , %ebx
cmpl ____ , %ebx
Jel  .L2
____ %ebx
.L2:
____ %ebx
.L3:

What's happening on the assembly part?

Solution 1

It's probably:

xorl %ebx, %ebx

This is a common idiom for zeroing a register on x86. This would correspond with i = 0 in the C code.

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If you are curious "but why ?" the short answer is that the xor instruction is fewer bytes than mov $0, %ebx. The long answer includes other subtle reasons.

I am leaving out the rest of the exercise since there's nothing idiosyncratic left.

Solution 2

This is the completed and commented assembly equivalent to your C code:

xorl %ebx , %ebx    ; i = 0
cmpl $54, %ebx
jle  .L2            ; if (i <= 54) jump to .L2, otherwise continue with the next instruction (so if i>54... which equals >=55 like in your C code)
addl $2, %ebx         ; >54 (or: >=55)
.L2:
decl %ebx            ; <=54 (or <55, the else-branch of your if) Note: This code also gets executed if i >= 55, hence why we need +2 above so we only get +1 total
.L3:

So, these are the (arithmetic) instructions that get executed for all numbers >=55:

addl $2, %ebx
decl %ebx

So for numbers >=55, this is equal to incrementing. The following (arithmetic) instructions get executed for numbers <55:

decl %ebx

We jump over the addl $2, %ebx instruction, so for numbers <55 this is equal to decrementing.

In case you're not allowed to type addl $2, (since it's not just the instruction but also an argument) into a single blank there's probably an error in the asm code you've been given (missing a jump between line 4 and 5 to .L3).

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Also note that jel is clearly a typo for jle in the question.