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|
use crate::data::{Pointer, Value};
use std::collections::{HashMap, VecDeque};
use std::iter::Iterator;
fn rewrite_pointers(
stack: &mut [Value],
rewrites: &HashMap<Pointer, Pointer>,
) -> Result<(), String> {
for val in stack.iter_mut() {
let p = if val.is_pointer() {
val.to_pointer().unwrap()
} else {
continue;
};
let Pointer(u) = p;
*val = Value::from_pointer(
*rewrites
.get(&p)
.ok_or(format!("no rewrite found for {:x}", u))?,
);
}
return Ok(());
}
fn open_ptr(p: Pointer) -> (usize, usize) {
let Pointer(u) = p;
return (u / 8, u % 8);
}
pub struct Heap {
heap: Vec<u64>,
spare_heap: Vec<u64>,
last_reachable_cells: usize,
}
impl Heap {
pub fn new() -> Self {
Heap {
heap: Vec::with_capacity(512),
spare_heap: Vec::with_capacity(512),
last_reachable_cells: 0,
}
}
fn alloc_size(&self, p: Pointer) -> Result<usize, String> {
let (u, _) = open_ptr(p);
if u == 0 {
return Err(String::from("cannot get the size of a nil pointer"));
}
// Alloc size is stored just below the pointer.
let size = self.heap[u - 1];
return Ok(usize::try_from(size).unwrap() >> 1);
}
fn is_bytevector(&mut self, p: Pointer) -> Result<bool, String> {
let (u, _) = open_ptr(p);
let size = self.heap[u - 1];
// Low bit 1 means bytevector.
return Ok(size & 1 != 0);
}
fn walk_gc_roots(
&mut self,
roots: &[Value],
rewrites: &mut HashMap<Pointer, Pointer>,
) -> Result<(), String> {
let mut queue: VecDeque<Value> = VecDeque::with_capacity(roots.len());
queue.extend(roots);
while let Some(val) = queue.pop_front() {
let p = if val.is_pointer() {
val.to_pointer().unwrap()
} else {
continue;
};
if rewrites.contains_key(&p) {
// Already copied this one.
continue;
}
let (u, _) = open_ptr(p);
let object_size = self.alloc_size(p)?;
let object_size_words = object_size / 8;
// Copy object and size.
rewrites.insert(p, Pointer(self.spare_heap.len() * 8 + 8));
self.spare_heap
.extend(&self.heap[u - 1..u + object_size_words]);
if self.is_bytevector(p)? {
// Don't process bytevectors recursively. We're all done.
continue;
}
for i in u..u + object_size_words {
queue.push_back(Value(self.heap[i]));
}
}
return Ok(());
}
fn collect_garbage(&mut self, locals: &mut [Value]) -> Result<(), String> {
self.spare_heap.truncate(0);
let mut rewrites = HashMap::new();
self.walk_gc_roots(locals, &mut rewrites)?;
// Rewrite values.
rewrite_pointers(locals, &rewrites)?;
// Activate the new heap!
std::mem::swap(&mut self.heap, &mut self.spare_heap);
// Walk objects in the heap and rewrite pointers. First object is at address 8.
let mut i = 8;
while i < self.heap.len() * 8 {
let p = Pointer(i);
if self.is_bytevector(p)? {
i += self.alloc_size(p)?;
continue;
}
let sz = self.alloc_size(p)?;
for j in (0..sz).step_by(8) {
let val = self.peek(p, j)?;
let vp = if val.is_pointer() {
val.to_pointer().unwrap()
} else {
continue;
};
let Pointer(u) = vp;
self.poke(
Value::from_pointer(
*rewrites
.get(&vp)
.ok_or(format!("no rewrite found for {:x}", u))?,
),
p,
j,
)?;
}
if sz == 0 {
return Err(String::from("object with size 0 found on the heap"));
}
i += sz + 8;
}
self.last_reachable_cells = self.heap.len();
// Done??
return Ok(());
}
fn alloc_b(
&mut self,
n: usize,
locals: &mut [Value],
bytevector_p: bool,
) -> Result<Pointer, String> {
if self.heap.len() > 2 * self.last_reachable_cells {
self.collect_garbage(locals)?;
}
let n_cells = (n + 7) / 8;
let len_idx = self.heap.len();
self.heap.resize(self.heap.len() + n_cells + 1, 0);
let len_p = &mut self.heap[len_idx];
*len_p = u64::try_from(n).unwrap() << 1;
if bytevector_p {
*len_p |= 1;
}
let obj_start = len_idx + 1;
let p = Pointer(obj_start * 8);
self.heap[obj_start..obj_start + n_cells].fill(0);
return Ok(p);
}
pub fn alloc(&mut self, n: usize, locals: &mut [Value]) -> Result<Pointer, String> {
return self.alloc_b(n * 8, locals, false);
}
pub fn alloc_bytevector(&mut self, n: usize, locals: &mut [Value]) -> Result<Pointer, String> {
return self.alloc_b(n, locals, true);
}
pub fn peek(&self, p: Pointer, offset: usize) -> Result<Value, String> {
let obj_sz = self.alloc_size(p)?;
if offset >= obj_sz {
return Err(format!(
"invalid offset {offset} in an object of size {obj_sz}"
));
}
let (u, _) = open_ptr(p.offset(offset));
if u >= self.heap.len() {
return Err(format!("invalid pointer {:x}", p));
}
return Ok(Value(self.heap[u]));
}
pub fn poke(&mut self, v: Value, p: Pointer, offset: usize) -> Result<(), String> {
let obj_sz = self.alloc_size(p)?;
if offset >= obj_sz {
return Err(format!(
"invalid offset {offset} in an object of size {obj_sz}"
));
}
let (u, _) = open_ptr(p.offset(offset));
let Value(x) = v;
if u >= self.heap.len() {
return Err(format!("invalid pointer {:x}", p));
}
self.heap[u] = x;
return Ok(());
}
pub fn peek_byte(&self, p: Pointer) -> Result<u8, String> {
let (word_cnt, word_offset) = open_ptr(p);
if word_cnt >= self.heap.len() {
return Err(format!("invalid pointer {:x}", p));
}
Ok((self.heap[word_cnt] >> word_offset * 8) as u8)
}
pub fn poke_byte(&mut self, u: u8, p: Pointer) -> Result<(), String> {
let (word_cnt, word_offset) = open_ptr(p);
if word_cnt >= self.heap.len() {
return Err(format!("invalid pointer {:x}", p));
}
let surrounding_word = self.heap[word_cnt];
let mask = !(0xff << word_offset * 8);
self.heap[word_cnt] = surrounding_word & mask | u64::from(u) << word_offset * 8;
Ok(())
}
}
|
