X can always by devided by 2, meaning the condition will always be false thus returning "a" unchanged. "xor eax, eax" is a shortcut to "mov eax, 0".

"xor r,r" is intel-ese for "set r to zero". For various reasons it's faster and smaller than the more obvious "mov r,0".

From this we can see that gcc and clang have figured out that the "if" can never be true, then they elided the loop, then optimized out the variable, ending in the equivalent of "return 0;".