Difference between clang and gcc [closed]

Answer 1

Yes. And no.

This is like asking whether an Audi car has an advantage over a Mercedes car. Like them, the two compilers are two different projects aiming to do the same thing. In some cases, gcc will emit better code, in others it will be clang.

When you need to know, you have to compile your code with both and then measure it.

There is an argument here and somewhat less related here.

Answer 2

In this case the Clang output is better, because it does not branch; instead it loads the value of num % 2 == 1 to al the code generated by gcc uses jumps. If num is expected to be even/odd with 50 % chances, and with no repeating patterns, the code generated by GCC will be susceptible to branch prediction failure.

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However you can make the code well-behaved on GCC as well by doing

int foo(int num) {
    return num * num + (num % 2 != 1);
}

Even more so, as it seems that your algorithm is really defined for unsigned numbers only, you should use unsigned int (they're different for negative numbers) - actually you get a major speedup by using unsigned int for the argument, as now GCC/Clang can optimize num % 2 to num & 1:

unsigned int foo(unsigned int num) {
    return num * num + (num % 2 != 1);
}

The resulting code generated by gcc -O2

movl    %edi, %edx
imull   %edi, %edi
andl    $1, %edx
xorl    $1, %edx
leal    (%rdi,%rdx), %eax
ret

is much better than the code for your original function generated by either compiler. Thus a compiler does not matter as much as does a programmer who knows what he's doing.