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Rework Arena code
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@pczarn
pczarn committed Jan 5, 2016
1 parent 0d3160c commit 5f1b1ec
Showing 1 changed file with 82 additions and 85 deletions.
167 changes: 82 additions & 85 deletions src/libarena/lib.rs
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Expand Up @@ -50,12 +50,11 @@ use std::ptr;
use alloc::heap;
use alloc::raw_vec::RawVec;

// The way arena uses arrays is really deeply awful. The arrays are
// allocated, and have capacities reserved, but the fill for the array
// will always stay at 0.
struct Chunk {
data: RawVec<u8>,
/// Index of the first unused byte.
fill: Cell<usize>,
/// Indicates whether objects with destructors are stored in this chunk.
is_copy: Cell<bool>,
}

Expand All @@ -75,12 +74,37 @@ impl Chunk {
unsafe fn as_ptr(&self) -> *const u8 {
self.data.ptr()
}

// Walk down a chunk, running the destructors for any objects stored
// in it.
unsafe fn destroy(&self) {
let mut idx = 0;
let buf = self.as_ptr();
let fill = self.fill.get();

while idx < fill {
let tydesc_data = buf.offset(idx as isize) as *const usize;
let (tydesc, is_done) = un_bitpack_tydesc_ptr(*tydesc_data);
let (size, align) = ((*tydesc).size, (*tydesc).align);

let after_tydesc = idx + mem::size_of::<*const TyDesc>();

let start = round_up(after_tydesc, align);

if is_done {
((*tydesc).drop_glue)(buf.offset(start as isize) as *const i8);
}

// Find where the next tydesc lives
idx = round_up(start + size, mem::align_of::<*const TyDesc>());
}
}
}

/// A slower reflection-based arena that can allocate objects of any type.
///
/// This arena uses `Vec<u8>` as a backing store to allocate objects from. For
/// each allocated object, the arena stores a pointer to the type descriptor
/// This arena uses `RawVec<u8>` as a backing store to allocate objects from.
/// For each allocated object, the arena stores a pointer to the type descriptor
/// followed by the object (potentially with alignment padding after each
/// element). When the arena is destroyed, it iterates through all of its
/// chunks, and uses the tydesc information to trace through the objects,
Expand Down Expand Up @@ -127,10 +151,10 @@ impl<'a> Arena<'a> {
impl<'longer_than_self> Drop for Arena<'longer_than_self> {
fn drop(&mut self) {
unsafe {
destroy_chunk(&*self.head.borrow());
self.head.borrow().destroy();
for chunk in self.chunks.borrow().iter() {
if !chunk.is_copy.get() {
destroy_chunk(chunk);
chunk.destroy();
}
}
}
Expand All @@ -142,31 +166,6 @@ fn round_up(base: usize, align: usize) -> usize {
(base.checked_add(align - 1)).unwrap() & !(align - 1)
}

// Walk down a chunk, running the destructors for any objects stored
// in it.
unsafe fn destroy_chunk(chunk: &Chunk) {
let mut idx = 0;
let buf = chunk.as_ptr();
let fill = chunk.fill.get();

while idx < fill {
let tydesc_data = buf.offset(idx as isize) as *const usize;
let (tydesc, is_done) = un_bitpack_tydesc_ptr(*tydesc_data);
let (size, align) = ((*tydesc).size, (*tydesc).align);

let after_tydesc = idx + mem::size_of::<*const TyDesc>();

let start = round_up(after_tydesc, align);

if is_done {
((*tydesc).drop_glue)(buf.offset(start as isize) as *const i8);
}

// Find where the next tydesc lives
idx = round_up(start + size, mem::align_of::<*const TyDesc>());
}
}

// We encode whether the object a tydesc describes has been
// initialized in the arena in the low bit of the tydesc pointer. This
// is necessary in order to properly do cleanup if a panic occurs
Expand All @@ -183,6 +182,9 @@ fn un_bitpack_tydesc_ptr(p: usize) -> (*const TyDesc, bool) {
// HACK(eddyb) TyDesc replacement using a trait object vtable.
// This could be replaced in the future with a custom DST layout,
// or `&'static (drop_glue, size, align)` created by a `const fn`.
// Requirements:
// * rvalue promotion (issue #1056)
// * mem::{size_of, align_of} must be const fns
struct TyDesc {
drop_glue: fn(*const i8),
size: usize,
Expand All @@ -198,45 +200,52 @@ impl<T: ?Sized> AllTypes for T {}
unsafe fn get_tydesc<T>() -> *const TyDesc {
use std::raw::TraitObject;

let ptr = &*(1 as *const T);
let ptr = &*(heap::EMPTY as *const T);

// Can use any trait that is implemented for all types.
let obj = mem::transmute::<&AllTypes, TraitObject>(ptr);
obj.vtable as *const TyDesc
}

impl<'longer_than_self> Arena<'longer_than_self> {
#[inline]
fn chunk_size(&self) -> usize {
self.copy_head.borrow().capacity()
}

// Functions for the POD part of the arena
// Grows a given chunk and returns `false`, or replaces it with a bigger
// chunk and returns `true`.
// This method is shared by both parts of the arena.
#[cold]
fn alloc_copy_grow(&self, n_bytes: usize, align: usize) -> *const u8 {
// Allocate a new chunk.
let new_min_chunk_size = cmp::max(n_bytes, self.chunk_size());
let new_chunk = Chunk::new((new_min_chunk_size + 1).next_power_of_two(), true);
let mut copy_head = self.copy_head.borrow_mut();
let old_chunk = mem::replace(&mut *copy_head, new_chunk);
self.chunks.borrow_mut().push(old_chunk);

self.alloc_copy_inner(n_bytes, align)
fn alloc_grow(&self, head: &mut Chunk, used_cap: usize, n_bytes: usize) -> bool {
if head.data.reserve_in_place(used_cap, n_bytes) {
// In-place reallocation succeeded.
false
} else {
// Allocate a new chunk.
let new_min_chunk_size = cmp::max(n_bytes, head.capacity());
let new_chunk = Chunk::new((new_min_chunk_size + 1).next_power_of_two(), false);
let old_chunk = mem::replace(head, new_chunk);
if old_chunk.fill.get() != 0 {
self.chunks.borrow_mut().push(old_chunk);
}
true
}
}

// Functions for the copyable part of the arena.

#[inline]
fn alloc_copy_inner(&self, n_bytes: usize, align: usize) -> *const u8 {
let start = round_up(self.copy_head.borrow().fill.get(), align);
let chunk_size = self.chunk_size();

let end = start + n_bytes;
if end > chunk_size {
if !self.copy_head.borrow_mut().data.reserve_in_place(start, n_bytes) {
return self.alloc_copy_grow(n_bytes, align);
let mut copy_head = self.copy_head.borrow_mut();
let fill = copy_head.fill.get();
let mut start = round_up(fill, align);
let mut end = start + n_bytes;

if end > copy_head.capacity() {
if self.alloc_grow(&mut *copy_head, fill, end - fill) {
// Continuing with a newly allocated chunk
start = 0;
end = n_bytes;
copy_head.is_copy.set(true);
}
}

let copy_head = self.copy_head.borrow();
copy_head.fill.set(end);

unsafe { copy_head.as_ptr().offset(start as isize) }
Expand All @@ -254,40 +263,28 @@ impl<'longer_than_self> Arena<'longer_than_self> {
}
}

// Functions for the non-POD part of the arena
fn alloc_noncopy_grow(&self, n_bytes: usize, align: usize) -> (*const u8, *const u8) {
// Allocate a new chunk.
let new_min_chunk_size = cmp::max(n_bytes, self.chunk_size());
let new_chunk = Chunk::new((new_min_chunk_size + 1).next_power_of_two(), false);
let mut head = self.head.borrow_mut();
let old_chunk = mem::replace(&mut *head, new_chunk);
self.chunks.borrow_mut().push(old_chunk);

self.alloc_noncopy_inner(n_bytes, align)
}
// Functions for the non-copyable part of the arena.

#[inline]
fn alloc_noncopy_inner(&self, n_bytes: usize, align: usize) -> (*const u8, *const u8) {
// Be careful to not maintain any `head` borrows active, because
// `alloc_noncopy_grow` borrows it mutably.
let (start, end, tydesc_start, head_capacity) = {
let head = self.head.borrow();
let fill = head.fill.get();

let tydesc_start = fill;
let after_tydesc = fill + mem::size_of::<*const TyDesc>();
let start = round_up(after_tydesc, align);
let end = start + n_bytes;

(start, end, tydesc_start, head.capacity())
};

if end > head_capacity {
return self.alloc_noncopy_grow(n_bytes, align);
let mut head = self.head.borrow_mut();
let fill = head.fill.get();

let mut tydesc_start = fill;
let after_tydesc = fill + mem::size_of::<*const TyDesc>();
let mut start = round_up(after_tydesc, align);
let mut end = round_up(start + n_bytes, mem::align_of::<*const TyDesc>());

if end > head.capacity() {
if self.alloc_grow(&mut *head, tydesc_start, end - tydesc_start) {
// Continuing with a newly allocated chunk
tydesc_start = 0;
start = round_up(mem::size_of::<*const TyDesc>(), align);
end = round_up(start + n_bytes, mem::align_of::<*const TyDesc>());
}
}

let head = self.head.borrow();
head.fill.set(round_up(end, mem::align_of::<*const TyDesc>()));
head.fill.set(end);

unsafe {
let buf = head.as_ptr();
Expand Down

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