Div and idiv (x86)

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Flesh out, include more than just caveats

Caveats

Both of these instructions are notoriously slow. If we do [ilmath]a%b[/ilmath] where [ilmath]0\le a<b[/ilmath] and [ilmath]b>0[/ilmath] then the result will be [ilmath]0[/ilmath] for the divisor and [ilmath]a[/ilmath] for the remainder. This is the trivial case, yet it's still really slow.

It is faster to do:

//for a,b unsigned integers
if(a < b) {
    return a;
}
return a % b;

and similarly in the signed case, if the "return a" happens 80% of the time (this is from memory on Sandy Bridge arch, confirmed on Haswell.