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