C的优化MIPS指令立即返回？

Question

I've been playing around with Godbolt a bit to see how the compiler optimizes instructions, and something I've noticed is how the optimization of simple C programs seems to return immediately without actually doing the computation. Say I have a very simple program in Godbolt ( here ):

int square(int num) {
    return num * num;
}

int main(int argc, const char* argv[]){
    return square(argc);
}

The MIPS instruction output is:

square:
        j       $31
        mul     $2,$4,$4

main:
        j       $31
        mul     $2,$4,$4

From my recollection of MIPS, isn't this just jumping to the $ra register immediately, effectively not doing anything at all? I thought that once we j $31 or jump to the return address (and not doing a jal ), we're effectively returning from the function at that point. So how is this working if it's returning before doing the multiply?

Answer 1

I'm totally not an assembler guy, but I did read about this a little while ago. The answer: branch delay slots . Read more here: https://devblogs.microsoft.com/oldnewthing/20180411-00/?p=98485

When you perform a branch instruction, the instruction after the branch instruction is executed, even if the branch is taken. The branch itself is delayed by one instruction.