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auto merge of #14768 : riccieri/rust/detransmute-arena, r=alexcrichton 

**Update**

I've reimplemented this using `Cell` and `RefCell`, as suggested by @alexcrichton. By taking care with the duration of the borrows, I was able to maintain the recursive allocation feature (now covered by a test) without the use of `Unsafe`, and without breaking the non-aliasing `&mut` invariant.

**Original**

Changes both `Arena` and `TypedArena` to contain an inner struct wrapped in a `Unsafe`, and change field access to go through those instead of transmuting `&self` to `&mut self`.

Part of #13933
This commit is contained in:
bors 2014-06-10 18:07:07 -07:00
parent b1302f9c4f 47b72e388d
commit f92a8facf9
1 changed files with 90 additions and 70 deletions
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160
src/libarena/lib.rs
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@ -81,8 +81,8 @@ pub struct Arena {
// The head is separated out from the list as a unbenchmarked // The head is separated out from the list as a unbenchmarked
// microoptimization, to avoid needing to case on the list to access the // microoptimization, to avoid needing to case on the list to access the
// head. // head.
head: Chunk, head: RefCell<Chunk>,
copy_head: Chunk, copy_head: RefCell<Chunk>,
chunks: RefCell<Vec<Chunk>>, chunks: RefCell<Vec<Chunk>>,
} }
@ -95,8 +95,8 @@ impl Arena {
/// Allocate a new Arena with `initial_size` bytes preallocated. /// Allocate a new Arena with `initial_size` bytes preallocated.
pub fn new_with_size(initial_size: uint) -> Arena { pub fn new_with_size(initial_size: uint) -> Arena {
Arena { Arena {
head: chunk(initial_size, false), head: RefCell::new(chunk(initial_size, false)),
copy_head: chunk(initial_size, true), copy_head: RefCell::new(chunk(initial_size, true)),
chunks: RefCell::new(Vec::new()), chunks: RefCell::new(Vec::new()),
} }
} }
@ -114,7 +114,7 @@ fn chunk(size: uint, is_copy: bool) -> Chunk {
impl Drop for Arena { impl Drop for Arena {
fn drop(&mut self) { fn drop(&mut self) {
unsafe { unsafe {
destroy_chunk(&self.head); destroy_chunk(&*self.head.borrow());
for chunk in self.chunks.borrow().iter() { for chunk in self.chunks.borrow().iter() {
if !chunk.is_copy.get() { if !chunk.is_copy.get() {
destroy_chunk(chunk); destroy_chunk(chunk);
@ -171,38 +171,40 @@ fn un_bitpack_tydesc_ptr(p: uint) -> (*TyDesc, bool) {
impl Arena { impl Arena {
fn chunk_size(&self) -> uint { fn chunk_size(&self) -> uint {
self.copy_head.capacity() self.copy_head.borrow().capacity()
} }
// Functions for the POD part of the arena // Functions for the POD part of the arena
fn alloc_copy_grow(&mut self, n_bytes: uint, align: uint) -> *u8 { fn alloc_copy_grow(&self, n_bytes: uint, align: uint) -> *u8 {
// Allocate a new chunk. // Allocate a new chunk.
let new_min_chunk_size = cmp::max(n_bytes, self.chunk_size()); let new_min_chunk_size = cmp::max(n_bytes, self.chunk_size());
self.chunks.borrow_mut().push(self.copy_head.clone()); self.chunks.borrow_mut().push(self.copy_head.borrow().clone());
self.copy_head =
*self.copy_head.borrow_mut() =
chunk(num::next_power_of_two(new_min_chunk_size + 1u), true); chunk(num::next_power_of_two(new_min_chunk_size + 1u), true);
return self.alloc_copy_inner(n_bytes, align); return self.alloc_copy_inner(n_bytes, align);
} }
#[inline] #[inline]
fn alloc_copy_inner(&mut self, n_bytes: uint, align: uint) -> *u8 { fn alloc_copy_inner(&self, n_bytes: uint, align: uint) -> *u8 {
let start = round_up(self.copy_head.borrow().fill.get(), align);
let end = start + n_bytes;
if end > self.chunk_size() {
return self.alloc_copy_grow(n_bytes, align);
}
let copy_head = self.copy_head.borrow();
copy_head.fill.set(end);
unsafe { unsafe {
let start = round_up(self.copy_head.fill.get(), align); copy_head.as_ptr().offset(start as int)
let end = start + n_bytes;
if end > self.chunk_size() {
return self.alloc_copy_grow(n_bytes, align);
}
self.copy_head.fill.set(end);
//debug!("idx = {}, size = {}, align = {}, fill = {}",
// start, n_bytes, align, head.fill.get());
self.copy_head.as_ptr().offset(start as int)
} }
} }
#[inline] #[inline]
fn alloc_copy<'a, T>(&'a mut self, op: || -> T) -> &'a T { fn alloc_copy<'a, T>(&'a self, op: || -> T) -> &'a T {
unsafe { unsafe {
let ptr = self.alloc_copy_inner(mem::size_of::<T>(), let ptr = self.alloc_copy_inner(mem::size_of::<T>(),
mem::min_align_of::<T>()); mem::min_align_of::<T>());
@ -213,42 +215,48 @@ impl Arena {
} }
// Functions for the non-POD part of the arena // Functions for the non-POD part of the arena
fn alloc_noncopy_grow(&mut self, n_bytes: uint, align: uint) fn alloc_noncopy_grow(&self, n_bytes: uint, align: uint) -> (*u8, *u8) {
-> (*u8, *u8) {
// Allocate a new chunk. // Allocate a new chunk.
let new_min_chunk_size = cmp::max(n_bytes, self.chunk_size()); let new_min_chunk_size = cmp::max(n_bytes, self.chunk_size());
self.chunks.borrow_mut().push(self.head.clone()); self.chunks.borrow_mut().push(self.head.borrow().clone());
self.head =
*self.head.borrow_mut() =
chunk(num::next_power_of_two(new_min_chunk_size + 1u), false); chunk(num::next_power_of_two(new_min_chunk_size + 1u), false);
return self.alloc_noncopy_inner(n_bytes, align); return self.alloc_noncopy_inner(n_bytes, align);
} }
#[inline] #[inline]
fn alloc_noncopy_inner(&mut self, n_bytes: uint, align: uint) fn alloc_noncopy_inner(&self, n_bytes: uint, align: uint) -> (*u8, *u8) {
-> (*u8, *u8) { // Be careful to not maintain any `head` borrows active, because
unsafe { // `alloc_noncopy_grow` borrows it mutably.
let tydesc_start = self.head.fill.get(); let (start, end, tydesc_start, head_capacity) = {
let after_tydesc = self.head.fill.get() + mem::size_of::<*TyDesc>(); let head = self.head.borrow();
let fill = head.fill.get();
let tydesc_start = fill;
let after_tydesc = fill + mem::size_of::<*TyDesc>();
let start = round_up(after_tydesc, align); let start = round_up(after_tydesc, align);
let end = start + n_bytes; let end = start + n_bytes;
if end > self.head.capacity() { (start, end, tydesc_start, head.capacity())
return self.alloc_noncopy_grow(n_bytes, align); };
}
self.head.fill.set(round_up(end, mem::align_of::<*TyDesc>())); if end > head_capacity {
return self.alloc_noncopy_grow(n_bytes, align);
}
//debug!("idx = {}, size = {}, align = {}, fill = {}", let head = self.head.borrow();
// start, n_bytes, align, head.fill); head.fill.set(round_up(end, mem::align_of::<*TyDesc>()));
let buf = self.head.as_ptr(); unsafe {
let buf = head.as_ptr();
return (buf.offset(tydesc_start as int), buf.offset(start as int)); return (buf.offset(tydesc_start as int), buf.offset(start as int));
} }
} }
#[inline] #[inline]
fn alloc_noncopy<'a, T>(&'a mut self, op: || -> T) -> &'a T { fn alloc_noncopy<'a, T>(&'a self, op: || -> T) -> &'a T {
unsafe { unsafe {
let tydesc = get_tydesc::<T>(); let tydesc = get_tydesc::<T>();
let (ty_ptr, ptr) = let (ty_ptr, ptr) =
@ -274,12 +282,10 @@ impl Arena {
#[inline] #[inline]
pub fn alloc<'a, T>(&'a self, op: || -> T) -> &'a T { pub fn alloc<'a, T>(&'a self, op: || -> T) -> &'a T {
unsafe { unsafe {
// FIXME #13933: Remove/justify all `&T` to `&mut T` transmutes
let this: &mut Arena = mem::transmute::<&_, &mut _>(self);
if intrinsics::needs_drop::<T>() { if intrinsics::needs_drop::<T>() {
this.alloc_noncopy(op) self.alloc_noncopy(op)
} else { } else {
this.alloc_copy(op) self.alloc_copy(op)
} }
} }
} }
@ -298,6 +304,20 @@ fn test_arena_destructors() {
} }
} }
#[test]
fn test_arena_alloc_nested() {
struct Inner { value: uint }
struct Outer<'a> { inner: &'a Inner }
let arena = Arena::new();
let result = arena.alloc(|| Outer {
inner: arena.alloc(|| Inner { value: 10 })
});
assert_eq!(result.inner.value, 10);
}
#[test] #[test]
#[should_fail] #[should_fail]
fn test_arena_destructors_fail() { fn test_arena_destructors_fail() {
@ -325,19 +345,20 @@ fn test_arena_destructors_fail() {
/// run again for these objects. /// run again for these objects.
pub struct TypedArena<T> { pub struct TypedArena<T> {
/// A pointer to the next object to be allocated. /// A pointer to the next object to be allocated.
ptr: *T, ptr: Cell<*T>,
/// A pointer to the end of the allocated area. When this pointer is /// A pointer to the end of the allocated area. When this pointer is
/// reached, a new chunk is allocated. /// reached, a new chunk is allocated.
end: *T, end: Cell<*T>,
/// A pointer to the first arena segment. /// A pointer to the first arena segment.
first: Option<Box<TypedArenaChunk<T>>>, first: RefCell<TypedArenaChunkRef<T>>,
} }
type TypedArenaChunkRef<T> = Option<Box<TypedArenaChunk<T>>>;
struct TypedArenaChunk<T> { struct TypedArenaChunk<T> {
/// Pointer to the next arena segment. /// Pointer to the next arena segment.
next: Option<Box<TypedArenaChunk<T>>>, next: TypedArenaChunkRef<T>,
/// The number of elements that this chunk can hold. /// The number of elements that this chunk can hold.
capacity: uint, capacity: uint,
@ -423,39 +444,38 @@ impl<T> TypedArena<T> {
pub fn with_capacity(capacity: uint) -> TypedArena<T> { pub fn with_capacity(capacity: uint) -> TypedArena<T> {
let chunk = TypedArenaChunk::<T>::new(None, capacity); let chunk = TypedArenaChunk::<T>::new(None, capacity);
TypedArena { TypedArena {
ptr: chunk.start() as *T, ptr: Cell::new(chunk.start() as *T),
end: chunk.end() as *T, end: Cell::new(chunk.end() as *T),
first: Some(chunk), first: RefCell::new(Some(chunk)),
} }
} }
/// Allocates an object in the TypedArena, returning a reference to it. /// Allocates an object in the TypedArena, returning a reference to it.
#[inline] #[inline]
pub fn alloc<'a>(&'a self, object: T) -> &'a T { pub fn alloc<'a>(&'a self, object: T) -> &'a T {
unsafe { if self.ptr == self.end {
// FIXME #13933: Remove/justify all `&T` to `&mut T` transmutes self.grow()
let this: &mut TypedArena<T> = mem::transmute::<&_, &mut _>(self);
if this.ptr == this.end {
this.grow()
}
let ptr: &'a mut T = mem::transmute(this.ptr);
ptr::write(ptr, object);
this.ptr = this.ptr.offset(1);
let ptr: &'a T = ptr;
ptr
} }
let ptr: &'a T = unsafe {
let ptr: &'a mut T = mem::transmute(self.ptr);
ptr::write(ptr, object);
self.ptr.set(self.ptr.get().offset(1));
ptr
};
ptr
} }
/// Grows the arena. /// Grows the arena.
#[inline(never)] #[inline(never)]
fn grow(&mut self) { fn grow(&self) {
let chunk = self.first.take_unwrap(); let chunk = self.first.borrow_mut().take_unwrap();
let new_capacity = chunk.capacity.checked_mul(&2).unwrap(); let new_capacity = chunk.capacity.checked_mul(&2).unwrap();
let chunk = TypedArenaChunk::<T>::new(Some(chunk), new_capacity); let chunk = TypedArenaChunk::<T>::new(Some(chunk), new_capacity);
self.ptr = chunk.start() as *T; self.ptr.set(chunk.start() as *T);
self.end = chunk.end() as *T; self.end.set(chunk.end() as *T);
self.first = Some(chunk) *self.first.borrow_mut() = Some(chunk)
} }
} }
@ -463,13 +483,13 @@ impl<T> TypedArena<T> {
impl<T> Drop for TypedArena<T> { impl<T> Drop for TypedArena<T> {
fn drop(&mut self) { fn drop(&mut self) {
// Determine how much was filled. // Determine how much was filled.
let start = self.first.get_ref().start() as uint; let start = self.first.borrow().get_ref().start() as uint;
let end = self.ptr as uint; let end = self.ptr.get() as uint;
let diff = (end - start) / mem::size_of::<T>(); let diff = (end - start) / mem::size_of::<T>();
// Pass that to the `destroy` method. // Pass that to the `destroy` method.
unsafe { unsafe {
self.first.get_mut_ref().destroy(diff) self.first.borrow_mut().get_mut_ref().destroy(diff)
} }
} }
} }