How an if becomes assembly

Until now every function ran straight down the page. This chapter is about the ones that don't — and the handful of reading habits that make branchy MIPS legible. Here's the C we'll dissect:

s32 pick(s32 x) {
    if (x != 0) {
        return 5;
    }
    return 2;
}

and what IDO makes of it:

 0:  beqz  a0, 0x10       # x == 0? jump to the second return
 4:  addiu v0, zero, 2    # (delay slot) v0 = 2 — runs EITHER WAY
 8:  jr    ra             # return…
 c:  addiu v0, zero, 5    # (delay slot) …with v0 = 5
10:  jr    ra             # the branch lands here — v0 still holds 2
14:  nop

Four habits, one per paragraph.

The branch tests the OPPOSITE of the C

The C says if (x != 0); the assembly says beqz — branch if equal to zero. That's not a mistake, it's the standard compilation: the branch's job is to skip the "then" body, so it fires when the condition fails. Whenever you decode a branch back into an if, flip the condition. A bnez in the target usually means == 0 in the C, a beqz means != 0.

The address operand is where it lands

beqz a0, 0x10 names its destination — address 0x10, the last jr ra. Those addresses down the left gutter finally earn their keep: to follow a branch, find the line whose address matches. Code between the branch and its target is the code being jumped over.

The delay slot runs either way

The line directly under the branch — addiu v0, zero, 2 — executes whether or not the branch is taken. You met this rule at jr ra; it's true of every branch. Here the compiler uses it shrewdly: it loads the return 2 value before knowing if it's needed. If the branch is taken, we arrive at 0x10 with v0 already holding 2 — the work is done. If not, the 5 overwrites it and no harm done. Reading a branch means reading two lines: the condition, and the slot beneath it.

Count the returns

One C function, two jr ras. IDO happily gives each return its own exit, and each exit's delay slot carries its return value. A quick scan for jr ra tells you how many ways out a function has — often the fastest way to guess its shape before reading a single condition.

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That's the whole toolkit: flip the condition, follow the address, read the slot, count the exits. Every lesson in this chapter is one of these habits getting sharper. Time to write your first branch.