Optimized MIPS instructions from C returning immediately?

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?

There are some processors that will always execute the instruction after a branch instruction. MIPS may be one of those. — user3386109, Aug 09 '19 at 21:42
Here's a possible duplicate: https://stackoverflow.com/questions/4115847/strange-jump-in-mips-assembly — user3386109, Aug 09 '19 at 21:47
[MIPS jump instruction delay slot](https://stackoverflow.com/q/47423368/995714), [Weird MIPS assembler behavior with jump (and link) instruction](https://stackoverflow.com/q/3807480/995714), [Example with MIPS, Pipelining and Branch Delay Slot](https://stackoverflow.com/q/48650320/995714) — phuclv, Aug 10 '19 at 01:17

score 3 · Accepted Answer · answered Aug 09 '19 at 21:43

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.

Optimized MIPS instructions from C returning immediately?

1 Answers1