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nora 2022-10-09 19:45:11 +02:00 • committed by nils
parent ed67aa4a9b
commit cd7d924318
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@ -36,8 +36,10 @@ for more details.
```rust ```rust
use std::collections::HashMap; use std::collections::HashMap;
use std::convert::{TryFrom, TryInto};
use std::mem::ManuallyDrop;
use stuff::{StuffedPtr, StuffingStrategy}; use stuff::{StuffedPtr, StuffingStrategy, Unstuffed};
// Create a unit struct for our strategy // Create a unit struct for our strategy
struct NanBoxStrategy; struct NanBoxStrategy;
@ -47,30 +49,22 @@ const QNAN: u64 = 0x7ffc000000000000;
// implementation detail of NaN boxing, the sign bit of an f64 // implementation detail of NaN boxing, the sign bit of an f64
const SIGN_BIT: u64 = 0x8000000000000000; const SIGN_BIT: u64 = 0x8000000000000000;
unsafe impl StuffingStrategy<u64> for NanBoxStrategy { impl StuffingStrategy<u64> for NanBoxStrategy {
type Other = f64; type Other = f64;
fn is_other(data: u64) -> bool {
(data & QNAN) != QNAN
}
fn stuff_other(inner: Self::Other) -> u64 { fn stuff_other(inner: Self::Other) -> u64 {
unsafe { std::mem::transmute(inner) } // both are 64 bit POD's unsafe { std::mem::transmute(inner) } // both are 64 bit POD's
} }
fn extract(data: u64) -> Unstuffed<usize, f64> {
unsafe fn extract_other(data: u64) -> Self::Other { if (data & QNAN) != QNAN {
std::mem::transmute(data) // both are 64 bit POD's Unstuffed::Other(f64::from_bits(data))
} else {
Unstuffed::Ptr((data & !(SIGN_BIT | QNAN)).try_into().unwrap())
}
} }
fn stuff_ptr(addr: usize) -> u64 { fn stuff_ptr(addr: usize) -> u64 {
// add the QNAN and SIGN_BIT // add the QNAN and SIGN_BIT
SIGN_BIT | QNAN | u64::try_from(addr).unwrap() SIGN_BIT | QNAN | u64::try_from(addr).unwrap()
} }
fn extract_ptr(inner: u64) -> usize {
// keep everything except for QNAN and SIGN_BIT
(inner & !(SIGN_BIT | QNAN)).try_into().unwrap()
}
} }
// a very, very crude representation of an object // a very, very crude representation of an object
@ -79,20 +73,19 @@ type Object = HashMap<String, u32>;
// our value type // our value type
type Value = StuffedPtr<Object, NanBoxStrategy, u64>; type Value = StuffedPtr<Object, NanBoxStrategy, u64>;
fn main() {
let float: Value = StuffedPtr::new_other(123.5); let float: Value = StuffedPtr::new_other(123.5);
assert_eq!(float.copy_other(), Some(123.5)); assert_eq!(float.other(), Some(123.5));
let object: Object = HashMap::from([("a".to_owned(), 457)]); let object: Object = HashMap::from([("a".to_owned(), 457)]);
let boxed = Box::new(object); let boxed = Box::new(object);
let ptr: Value = StuffedPtr::new_ptr(Box::into_raw(boxed)); let ptr: Value = StuffedPtr::new_ptr(Box::into_raw(boxed));
let object = unsafe { &*ptr.get_ptr().unwrap() }; let object = unsafe { &*ptr.ptr().unwrap() };
assert_eq!(object.get("a"), Some(&457)); assert_eq!(object.get("a"), Some(&457));
drop(unsafe { Box::from_raw(ptr.get_ptr().unwrap()) }); drop(unsafe { Box::from_raw(ptr.ptr().unwrap()) });
// be careful, `ptr` is a dangling pointer now! // be careful, `ptr` is a dangling pointer now!
}
``` ```
# MSRV-Policy # MSRV-Policy