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84 lines
2.7 KiB
Markdown
84 lines
2.7 KiB
Markdown
# stuff
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A crate for stuffing things into a pointer.
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This crate consists of three parts:
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* The type `StuffedPtr`
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* The trait `StuffingStrategy`
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* The trait `Backend`
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`StuffedPtr` is the main type of this crate. You it's a type whose size depends on the
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choice of `Backend` (defaults to `usize`, `u64` and `u128` are also possible). It can store a
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pointer or some extra data, so you can imagine it being
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```rust
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enum StuffedPtr<T, E> {
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Ptr(*mut T),
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Extra(E),
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}
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```
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except that the extra data is bitstuffed into the pointer. You can chose any arbitrary bitstuffing
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depending on the `StuffingStrategy`, an unsafe trait that governs how the extra data
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(or the pointer itself) will be packed into the backend.
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# Example: NaN-Boxing
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Pointers are hidden in the NaN values of floats. NaN boxing often involves also hiding booleans
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or null in there, but we stay with floats and pointers (pointers to a `HashMap` that servers
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as our "object" type).
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See [crafting interpreters](https://craftinginterpreters.com/optimization.html#nan-boxing)
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for more details.
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```rust
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use std::collections::HashMap;
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use ptr_stuff::{StuffedPtr, StuffingStrategy};
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// Create a unit struct for our strategy
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struct NanBoxStrategy;
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const QNAN: u64 = 0x7ffc000000000000; // implementation detail of NaN boxing, a quiet NaN mask
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const SIGN_BIT: u64 = 0x8000000000000000; // implementation detail of NaN boxing, the sign bit of an f64
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unsafe impl StuffingStrategy<u64> for NanBoxStrategy {
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type Extra = f64;
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fn is_extra(data: u64) -> bool {
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(data & QNAN) != QNAN
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}
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fn stuff_extra(inner: Self::Extra) -> u64 {
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unsafe { std::mem::transmute(inner) } // both are 64 bit POD's
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}
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unsafe fn extract_extra(data: u64) -> Self::Extra {
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std::mem::transmute(data) // both are 64 bit POD's
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}
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fn stuff_ptr(addr: usize) -> u64 {
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// add the QNAN and SIGN_BIT
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SIGN_BIT | QNAN | u64::try_from(addr).unwrap()
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}
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fn extract_ptr(inner: u64) -> usize {
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// keep everything except for QNAN and SIGN_BIT
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(inner & !(SIGN_BIT | QNAN)).try_into().unwrap()
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}
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}
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type Object = HashMap<String, u32>; // a very, very crude representation of an object
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type Value = StuffedPtr<Object, NanBoxStrategy, u64>; // our value type
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fn main() {
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let float: Value = StuffedPtr::new_extra(123.5);
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assert_eq!(float.copy_extra(), Some(123.5));
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let object: Object = HashMap::from([("a".to_owned(), 457)]);
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let boxed = Box::new(object);
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let ptr: Value = StuffedPtr::new_ptr(Box::into_raw(boxed));
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let object = unsafe { &*ptr.get_ptr().unwrap() };
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assert_eq!(object.get("a"), Some(&457));
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drop(unsafe { Box::from_raw(ptr.get_ptr().unwrap()) });
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// `ptr` is a dangling pointer now!
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}
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```
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