alt Hacker NewsI wonder if compilers do multiple passes on the intermediate code in order to optimize / simplify it. For example, during each pass the optimizer searches some known harcoded patterns and replaces them with something else and repeats until no possible improvement is found.
Also optimizers have a limit, they can't reason as abstractly as humans, for example:
bool is_divisible_by_6(int x) {
return x % 2 == 0 && x % 3 == 0;
}
bool is_divisible_by_6_optimal(int x) {
return x % 6 == 0;
}
The morale is that you still have to optimize algorithms (O notation) and math operations / expressions.
Replies
I don't know enough about ASM. Are u saying the first one is more optimal because it is faster or because it uses less instructions? Would this reflect a real world use case? Do any other compilers (e.g. V8) optimize modulo's into something else?
They do, and the order of the passes matter. Sometimes, optimizations are missed because they require a certain order of passes that is different from the one your compiler uses.
On higher optimization levels, many passes occur multiple times. However, as far as I know, compilers don't repeatedly run passes until they've reached an optimum. Instead, they run a fixed series of passes. I don't know why, maybe someone can chime in.
Those aren't isomorphic. The C spec says `is_divisible_by_6` short-circuits. You don't want the compiler optimising away null checks.
https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf
6.5.13, semantics