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cleanup the slab types

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- get rid of HeaderOrIdxPtr
- add IndexPtrSlab
- add UntypedArenaSlab
- add to_untyped for converting a typed slab into an unsyped slab
- remove TypedArenaPtr::new, as they shouldn't be created outside of this module
This commit is contained in:
Bennet Bleßmann
2024-07-07 15:47:50 +02:00
parent 1b19ae81d5
commit 689632b51c
2 changed files with 95 additions and 70 deletions
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164
src/arena.rs
View File

@@ -296,14 +296,6 @@ where
}
impl<T: ?Sized + ArenaAllocated> TypedArenaPtr<T> {
/// # Safety
/// - the pointers referee type is correct, safe code depends on the correctness of the type argument
/// - the pointer is allocated in the arena
#[inline]
pub const unsafe fn new(data: *mut T::Payload) -> Self {
unsafe { TypedArenaPtr(ptr::NonNull::new_unchecked(data)) }
}
#[inline]
pub fn as_ptr(&self) -> *mut T::Payload {
self.0.as_ptr()
@@ -398,14 +390,14 @@ pub trait ArenaAllocated {
/// # Safety
/// - the caller must guarantee that the pointee type of UntypedArenaPtr is Self
/// - the pointer must be non-null
unsafe fn typed_ptr(ptr: UntypedArenaPtr) -> TypedArenaPtr<Self>
where
Self::Payload: Sized,
{
// safety:
// - allocated in an arena as from an UntypedArenaPtr
// - caller guarantees the type is correct
unsafe { TypedArenaPtr::new(ptr.payload_offset().cast_mut().cast::<Self::Payload>()) }
TypedArenaPtr(NonNull::new_unchecked(
ptr.payload_offset().cast_mut().cast::<Self::Payload>(),
))
}
#[allow(clippy::missing_safety_doc)]
@@ -417,20 +409,14 @@ pub trait ArenaAllocated {
let slab = Box::new(TypedAllocSlab {
slab: AllocSlab {
next: arena.base.take(),
#[cfg(target_pointer_width = "32")]
_padding: 0,
header: HeaderOrIdxPtr {
header: ArenaHeader::build_with(size as u64, Self::tag()),
},
header: ArenaHeader::build_with(size as u64, Self::tag()),
},
payload: value,
});
let raw_box = Box::into_raw(slab);
// safety: Box::into_raw retuns a pointer to a valid allocation
let allocated_ptr = unsafe { TypedAllocSlab::to_typed_arena_ptr(raw_box) };
let (allocated_ptr, untyped_slab) = slab.to_untyped();
arena.base = Some(NonNull::new(raw_box.cast::<AllocSlab>()).unwrap());
arena.base = Some(untyped_slab);
allocated_ptr
}
@@ -655,94 +641,132 @@ impl ArenaAllocated for IndexPtr {
/// # Safety
/// - the caller must guarantee that the pointee type of UntypedArenaPtr is T
/// - the pointer must be non-null
unsafe fn typed_ptr(ptr: UntypedArenaPtr) -> TypedArenaPtr<Self> {
unsafe { TypedArenaPtr::new(ptr.get_ptr().cast_mut().cast::<IndexPtr>()) }
TypedArenaPtr(NonNull::new_unchecked(
ptr.get_ptr().cast_mut().cast::<IndexPtr>(),
))
}
#[inline]
fn alloc(arena: &mut Arena, value: Self) -> TypedArenaPtr<Self> {
let slab = Box::new(AllocSlab {
let slab = Box::new(IndexPtrSlab {
next: arena.base.take(),
#[cfg(target_pointer_width = "32")]
_padding: 0,
header: HeaderOrIdxPtr { idx_ptr: value },
index_ptr: value,
});
let raw_box = Box::into_raw(slab);
let allocated_ptr =
unsafe { TypedArenaPtr::new(ptr::addr_of_mut!((*raw_box).header.idx_ptr)) };
arena.base = Some(NonNull::new(raw_box).unwrap());
let (allocated_ptr, untyped_slab) = slab.to_untyped();
arena.base = Some(untyped_slab);
allocated_ptr
}
/// # Safety
/// - ptr points to an allocated slab of the correct kind
unsafe fn dealloc(ptr: NonNull<TypedAllocSlab<Self>>) {
drop(unsafe { Box::from_raw(ptr.as_ptr().cast::<AllocSlab>()) });
drop(unsafe { Box::from_raw(ptr.as_ptr().cast::<IndexPtrSlab>()) });
}
}
#[repr(C)]
union HeaderOrIdxPtr {
#[derive(Debug)]
pub struct AllocSlab {
next: Option<UntypedArenaSlab>,
header: ArenaHeader,
idx_ptr: IndexPtr,
}
#[repr(C)]
#[derive(Debug)]
pub struct IndexPtrSlab {
next: Option<UntypedArenaSlab>,
index_ptr: IndexPtr,
}
const _: () = {
if std::mem::size_of::<ArenaHeader>() != std::mem::size_of::<IndexPtr>() {
panic!("Size of ArenaHeader != IndexPtr")
if std::mem::align_of::<AllocSlab>() < std::mem::align_of::<*const ()>() {
panic!("alignment of AllocSlab is too low");
}
if std::mem::offset_of!(AllocSlab, header) % std::mem::align_of::<*const ()>() != 0 {
panic!("alignment of header not a multiple of pointers alignment");
}
if std::mem::offset_of!(AllocSlab, header) != std::mem::offset_of!(IndexPtrSlab, index_ptr) {
panic!("IndexPtrSlab.index_ptr and AllocSlab.header are at different offsets");
}
};
impl Debug for HeaderOrIdxPtr {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
unsafe { &self.header }.fmt(f)
}
}
impl IndexPtrSlab {
#[inline]
pub fn to_untyped(self: Box<Self>) -> (TypedArenaPtr<IndexPtr>, UntypedArenaSlab) {
let raw_box = Box::into_raw(self);
impl Clone for HeaderOrIdxPtr {
fn clone(&self) -> Self {
// safety:
// - we created the pointer from a valid reference
// - both ArenaHeader and IndexPtr are plain old datatypes, i.e. no managed resources that need to be cloned
unsafe { std::ptr::read(self) }
// safety: the pointer from Box::into_raw fullfills addr_of_mut's saftey requirements
let index_ptr_ptr = unsafe { ptr::addr_of_mut!((*raw_box).index_ptr) };
let allocated_ptr = TypedArenaPtr(
// safety: the pointer points into a valid allocation so it is non null
unsafe { NonNull::new_unchecked(index_ptr_ptr) },
);
let untyped_arena = UntypedArenaSlab {
// safety: pointer from Box::into_raw is never null
slab: unsafe { NonNull::new_unchecked(raw_box.cast::<AllocSlab>()) },
};
(allocated_ptr, untyped_arena)
}
}
#[repr(C)]
#[derive(Clone, Debug)]
pub struct AllocSlab {
next: Option<NonNull<AllocSlab>>,
#[cfg(target_pointer_width = "32")]
_padding: u32,
header: HeaderOrIdxPtr,
}
#[repr(C)]
#[derive(Clone, Debug)]
#[derive(Debug)]
pub struct TypedAllocSlab<T: ?Sized + ArenaAllocated> {
slab: AllocSlab,
payload: T::Payload,
}
impl<T: ?Sized + ArenaAllocated> TypedAllocSlab<T> {
pub fn tag(&self) -> ArenaHeaderTag {
self.slab.header.tag()
}
pub fn payload(&mut self) -> &mut T::Payload {
&mut self.payload
}
/// # Safety
/// - ptr points to a valid allocation of Self
#[inline]
pub unsafe fn to_typed_arena_ptr(ptr: *mut Self) -> TypedArenaPtr<T> {
// safety:
// - this is the arena allocation of corresponding type
unsafe { TypedArenaPtr::new(addr_of_mut!((*ptr).payload)) }
pub fn to_untyped(self: Box<Self>) -> (TypedArenaPtr<T>, UntypedArenaSlab) {
let raw_box = Box::into_raw(self);
// safety: the pointer from Box::into_raw fullfills addr_of_mut's saftey requirements
let payload_ptr = unsafe { addr_of_mut!((*raw_box).payload) };
(
TypedArenaPtr(unsafe {
// safety: the pointer points into a valid allocation so it is non null
ptr::NonNull::new_unchecked(payload_ptr)
}),
UntypedArenaSlab {
// safety: pointer from Box::into_raw is never null
slab: unsafe { NonNull::new_unchecked(raw_box.cast::<AllocSlab>()) },
},
)
}
}
#[derive(Debug)]
#[repr(transparent)]
pub struct UntypedArenaSlab {
slab: NonNull<AllocSlab>,
}
impl Drop for UntypedArenaSlab {
fn drop(&mut self) {
unsafe { drop_slab_in_place(self.slab) };
}
}
#[derive(Debug)]
pub struct Arena {
base: Option<NonNull<AllocSlab>>,
base: Option<UntypedArenaSlab>,
pub f64_tbl: Arc<F64Table>,
}
@@ -767,7 +791,7 @@ unsafe fn drop_slab_in_place(value: NonNull<AllocSlab>) {
};
}
match (unsafe { value.as_ref() }).header.header.tag() {
match (unsafe { value.as_ref() }).header.tag() {
ArenaHeaderTag::Integer => {
drop_typed_slab_in_place!(Integer, value);
}
@@ -839,18 +863,18 @@ unsafe fn drop_slab_in_place(value: NonNull<AllocSlab>) {
impl Drop for Arena {
fn drop(&mut self) {
// we un-nest UntypedArenaSlab to prevent stackoverflow due to the recursive drop
let mut ptr = self.base.take();
while let Some(slab) = ptr {
unsafe {
ptr = slab.as_ref().next;
drop_slab_in_place(slab);
}
while let Some(mut slab) = ptr {
ptr = unsafe { slab.slab.as_mut() }.next.take();
drop(slab);
}
}
}
const_assert!(mem::size_of::<AllocSlab>() == 16);
const_assert!(mem::size_of::<AllocSlab>() <= 24);
const_assert!(mem::size_of::<OrderedFloat<f64>>() == 8);
#[cfg(test)]

1
src/types.rs
View File

@@ -710,6 +710,7 @@ impl UntypedArenaPtr {
/// # Safety
/// - this UntypedArenaPtr actuall pointee type is T
/// - the pointer must be non-null
#[inline]
pub unsafe fn as_typed_ptr<T: ?Sized + ArenaAllocated>(self) -> TypedArenaPtr<T>
where