Editing Basic Assembly Programming (section)

===== A Better Loop Layout =====

Because our loop has an unconditional jump at the bottom, and a conditional jump in the middle, every iteration of the loop will execute two jump instructions. For OVERTURE, this isn’t a huge deal, but for more complicated architectures this can be very expensive (jumps are typically among the slowest operations a processor can do, because processors are very optimized for “straight-line” code, as conditional jumps might or might not disrupt the flow of the program,and things like speculative execution would not be happy :/ ). What if we could make the conditional branch dual-purpose?

If a conditional jump fails, we always move on to the next instruction in sequence. If that instruction were the instruction after the loop, we wouldn’t need the second branch. Our conditional branch could jump to the top of the loop, and “falling through” to the next instruction could end the loop! The trick is that we have inverted the condition. Instead of jumping if we should ''stop'', we jump if we should ''continue''. This is called the “do-while” loop layout, and it is how almost all loops are written in assembly. It looks like this:

<pre>loadi|1
move|s0|d2
label loop
add
move|s3|d1
loadi|loop
jnz
label done</pre>
And now we have saved one instruction in the loop.

In our case, we didn’t want to check the condition before doing the first addition. But what if we did? We need the condition to be at the bottom of the loop for this to work, right? Indeed, we do. But if we use a second jump instruction to ''enter'' the loop, we don’t have to execute the second jump instruction on every iteration of the loop, and that’s still better. Such a loop might look like this:

<pre>loadi|check
j

label loop
add
move|s3|d1

label check
loadi|loop
jnz

label done</pre>

<span id="if-then-and-if-then-else"></span>