Hello,
I was playing around with rust and wondered if I could use const generics for toggling debug code on and off to avoid any runtime cost while still being able to toggle the DEBUG flag during runtime. I came up with a nifty solution that requires a single dynamic dispatch which many programs have anyways. It works by rewriting the vtable. It's a zero cost bool!
Is this technique worth it?
Probably not.
It's funny though.
Repo: https://github.com/raldone01/runtime_const_generics_rs/tree/v1.0.0
Full source code below:
use std::mem::transmute;
use std::sync::atomic::AtomicU32;
use std::sync::atomic::Ordering;
use replace_with::replace_with_or_abort;
trait GameObject {
fn run(&mut self);
fn set_debug(&mut self, flag: bool) -> &mut dyn GameObject;
}
trait GameObjectBoxExt {
fn set_debug(self: Box<Self>, flag: bool) -> Box<dyn GameObject>;
}
impl GameObjectBoxExt for dyn GameObject {
fn set_debug(self: Box<Self>, flag: bool) -> Box<dyn GameObject> {
unsafe {
let selv = Box::into_raw(self);
let selv = (&mut *selv).set_debug(flag);
return Box::from_raw(selv);
}
}
}
static ID_CNT: AtomicU32 = AtomicU32::new(0);
struct Node3D<const DEBUG: bool = false> {
id: u32,
cnt: u32,
}
impl Node3D {
const TYPE_NAME: &str = "Node3D";
fn new() -> Self {
let id = ID_CNT.fetch_add(1, Ordering::Relaxed);
let selv = Self { id, cnt: 0 };
return selv;
}
}
impl<const DEBUG: bool> GameObject for Node3D<DEBUG> {
fn run(&mut self) {
println!("Hello {} from {}@{}!", self.cnt, Node3D::TYPE_NAME, self.id);
if DEBUG {
println!("Debug {} from {}@{}!", self.cnt, Node3D::TYPE_NAME, self.id);
}
self.cnt += 1;
}
fn set_debug(&mut self, flag: bool) -> &mut dyn GameObject {
unsafe {
match flag {
true => transmute::<_, &mut Node3D<true>>(self) as &mut dyn GameObject,
false => transmute::<_, &mut Node3D<false>>(self) as &mut dyn GameObject,
}
}
}
}
struct Node2D<const DEBUG: bool = false> {
id: u32,
cnt: u32,
}
impl Node2D {
const TYPE_NAME: &str = "Node2D";
fn new() -> Self {
let id = ID_CNT.fetch_add(1, Ordering::Relaxed);
let selv = Self { id, cnt: 0 };
return selv;
}
}
impl<const DEBUG: bool> GameObject for Node2D<DEBUG> {
fn run(&mut self) {
println!("Hello {} from {}@{}!", self.cnt, Node2D::TYPE_NAME, self.id);
if DEBUG {
println!("Debug {} from {}@{}!", self.cnt, Node2D::TYPE_NAME, self.id);
}
self.cnt += 1;
}
fn set_debug(&mut self, flag: bool) -> &mut dyn GameObject {
unsafe {
match flag {
true => transmute::<_, &mut Node2D<true>>(self) as &mut dyn GameObject,
false => transmute::<_, &mut Node2D<false>>(self) as &mut dyn GameObject,
}
}
}
}
fn main() {
let mut objects = Vec::new();
for _ in 0..10 {
objects.push(Box::new(Node3D::new()) as Box<dyn GameObject>);
objects.push(Box::new(Node2D::new()) as Box<dyn GameObject>);
}
for o in 0..3 {
for (i, object) in objects.iter_mut().enumerate() {
let debug = (o + i) % 2 == 0;
replace_with_or_abort(object, |object| object.set_debug(debug));
object.run();
}
}
}
Note:
If anyone gets the following to work without unsafe, maybe by using the replace_with crate I would be very happy:
impl GameObjectBoxExt for dyn GameObject {
fn set_debug(self: Box<Self>, flag: bool) -> Box<dyn GameObject> {
unsafe {
let selv = Box::into_raw(self);
let selv = (&mut *selv).set_debug(flag);
return Box::from_raw(selv);
}
}
I am curious to hear your thoughts.
I had a dl380 g6 and if a non hp certified gpu was put in it would increase the baseline Fan speed to almost full. Even though the other GPUs temperature values were correctly recognised by the ILO processor.
It was ridiculous.
Maybe its better on the g7 but I wouldn't count on it.