[−][src]Struct static_alloc::bump::Bump
Allocator drawing from an inner, statically sized memory resource.
The type parameter T
is used only to annotate the required size and alignment of the region
and has no futher use. Note that in particular there is no safe way to retrieve or unwrap an
inner instance even if the Bump
was not constructed as a shared global static. Nevertheless,
the choice of type makes it easier to reason about potentially required extra space due to
alignment padding.
This type is always Sync
to allow creating static
instances. This works only because
there is no actual instance of T
contained inside.
Usage as global allocator
You can use the stable rust attribute to use an instance of this type as the global allocator.
use static_alloc::Bump; #[global_allocator] static A: Bump<[u8; 1 << 16]> = Bump::uninit(); fn main() { }
Take care, some runtime features of Rust will allocate some memory before or after your own code. In particular, it was found to be be tricky to predict the usage of the builtin test framework which seemingly allocates some structures per test.
Usage as a non-dropping local allocator
It is also possible to use a Bump
as a stack local allocator or a specialized allocator. The
interface offers some utilities for allocating values from references to shared or unshared
instances directly. Note: this will never call the Drop
implementation of the allocated
type. In particular, it would almost surely not be safe to Pin
the values, except if there is
a guarantee for the Bump
itself to not be deallocated either.
use static_alloc::Bump; let local: Bump<[u64; 3]> = Bump::uninit(); let one = local.leak(0_u64).unwrap(); let two = local.leak(1_u64).unwrap(); let three = local.leak(2_u64).unwrap(); // Exhausted the space. assert!(local.leak(3_u64).is_err());
Mind that the supplied type parameter influenced both size and alignment and a [u8; 24]
does not guarantee being able to allocation three u64
even though most targets have a minimum
alignment requirement of 16 and it works fine on those.
// Just enough space for `u128` but no alignment requirement. let local: Bump<[u8; 16]> = Bump::uninit(); // May or may not return an err. let _ = local.leak(0_u128);
Instead use the type parameter to Bump
as a hint for the best alignment.
// Enough space and align for `u128`. let local: Bump<[u128; 1]> = Bump::uninit(); assert!(local.leak(0_u128).is_ok());
Usage as a (local) bag of bits
It is of course entirely possible to use a local instance instead of a single global allocator.
For example you could utilize the pointer interface directly to build a #[no_std]
dynamic
data structure in an environment without extern lib alloc
. This feature was the original
motivation behind the crate but no such data structures are provided here so a quick sketch of
the idea must do:
use core::alloc; use static_alloc::Bump; #[repr(align(4096))] struct PageTable { // some non-trivial type. } impl PageTable { /// Avoid stack allocation of the full struct. pub unsafe fn new(into: *mut u8) -> &'static mut Self { // ... } } // Allocator for pages for page tables. Provides 64 pages. When the // program/kernel is provided as an ELF the bootloader reserves // memory for us as part of the loading process that we can use // purely for page tables. Replaces asm `paging: .BYTE <size>;` static Paging: Bump<[u8; 1 << 18]> = Bump::uninit(); fn main() { let layout = alloc::Layout::new::<PageTable>(); let memory = Paging.alloc(layout).unwrap(); let table = unsafe { PageTable::new(memory.as_ptr()) }; }
A similar structure would of course work to allocate some non-'static' objects from a temporary
Bump`.
More insights
The ordering used is currently SeqCst
. This enforces a single global sequence of observed
effects on the slab level. The author is fully aware that this is not strictly necessary. In
fact, even AcqRel
may not be required as the monotonic bump allocator does not synchronize
other memory itself. If you bring forward a PR with a formalized reasoning for relaxing the
requirements to Relaxed
(llvm Monotonic
) it will be greatly appreciated (even more if you
demonstrate performance gains).
WIP: slices.
Methods
impl<T> Bump<T>
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pub const fn uninit() -> Self
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Make a new allocatable slab of certain byte size and alignment.
The storage will contain uninitialized bytes.
pub fn zeroed() -> Self
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Make a new allocatable slab of certain byte size and alignment.
The storage will contain zeroed bytes. This is not yet available
as a const fn
which currently limits its potential usefulness
but there is no good reason not to provide it regardless.
pub const fn new(storage: T) -> Self
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Make a new allocatable slab provided with some bytes it can hand out.
Note that storage
will never be dropped and there is no way to get it back.
pub fn alloc(&self, layout: Layout) -> Option<NonNull<u8>>
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Allocate a region of memory.
This is a safe alternative to GlobalAlloc::alloc.
Panics
This function will panic if the requested layout has a size of 0
. For the use in a
GlobalAlloc
this is explicitely forbidden to request and would allow any behaviour but we
instead strictly check it.
pub fn alloc_at(
&self,
layout: Layout,
level: Level
) -> Result<Allocation, Failure>
[src]
&self,
layout: Layout,
level: Level
) -> Result<Allocation, Failure>
Try to allocate some layout with a precise base location.
The base location is the currently consumed byte count, without correction for the alignment of the allocation. This will succeed if it can be allocate exactly at the expected location.
Panics
This function may panic if the provided level
is from a different slab.
pub fn get_layout(&self, layout: Layout) -> Option<Allocation>
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Get an allocation with detailed layout.
Provides an Uninit
wrapping several aspects of initialization in a safe interface,
bound by the lifetime of the reference to the allocator.
pub fn get_layout_at(
&self,
layout: Layout,
at: Level
) -> Result<Allocation, Failure>
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&self,
layout: Layout,
at: Level
) -> Result<Allocation, Failure>
Get an allocation with detailed layout at a specific level.
Provides an Uninit
wrapping several aspects of initialization in a safe interface,
bound by the lifetime of the reference to the allocator.
Since the underlying allocation is the same, it would be unsafe
but justified to fuse
this allocation with the preceding or succeeding one.
pub fn get<V>(&self) -> Option<Allocation<V>>
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Get an allocation for a specific type.
It is not yet initialized but provides a safe interface for that initialization.
Usage
use core::cell::{Ref, RefCell}; let slab: Bump<[Ref<'static, usize>; 1]> = Bump::uninit(); let data = RefCell::new(0xff); // We can place a `Ref` here but we did not yet. let alloc = slab.get::<Ref<usize>>().unwrap(); let cell_ref = unsafe { alloc.leak(data.borrow()) }; assert_eq!(**cell_ref, 0xff);
pub fn get_at<V>(&self, level: Level) -> Result<Allocation<V>, Failure>
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Get an allocation for a specific type at a specific level.
See get
for usage.
pub fn level(&self) -> Level
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Observe the current level.
Keep in mind that concurrent usage of the same slab may modify the level before you are
able to use it in alloc_at
. Calling this method provides also no other guarantees on
synchronization of memory accesses, only that the values observed by the caller are a
monotonically increasing seequence.
pub fn leak<V>(&self, val: V) -> Result<&mut V, LeakError<V>>
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Allocate a value for the lifetime of the allocator.
The value is leaked in the sense that
- the drop implementation of the allocated value is never called;
- reusing the memory for another allocation in the same
Bump
requires manual unsafe code to handle dropping and reinitialization.
However, it does not mean that the underlying memory used for the allocated value is never
reclaimed. If the Bump
itself is a stack value then it will get reclaimed together with
it.
Safety notice
It is important to understand that it is undefined behaviour to reuse the allocation for
the whole lifetime of the returned reference. That is, dropping the allocation in-place
while the reference is still within its lifetime comes with the exact same unsafety caveats
as ManuallyDrop::drop
.
#[derive(Debug, Default)] struct FooBar { // ... } let local: Bump<[FooBar; 3]> = Bump::uninit(); let one = local.leak(FooBar::default()).unwrap(); // Dangerous but justifiable. let one = unsafe { // Ensures there is no current mutable borrow. core::ptr::drop_in_place(&mut *one); };
Usage
use static_alloc::Bump; let local: Bump<[u64; 3]> = Bump::uninit(); let one = local.leak(0_u64).unwrap(); assert_eq!(*one, 0); *one = 42;
Limitations
Only sized values can be allocated in this manner for now, unsized values are blocked on
stabilization of ptr::slice_from_raw_parts
. We can not otherwise get a fat pointer to
the allocated region.
pub fn leak_at<V>(
&self,
val: V,
level: Level
) -> Result<(&mut V, Level), LeakError<V>>
[src]
&self,
val: V,
level: Level
) -> Result<(&mut V, Level), LeakError<V>>
Allocate a value with a precise location.
See leak
for basics on allocation of values.
The level is an identifer for a base location (more at level
). This will succeed if it
can be allocate exactly at the expected location.
This method will return the new level of the slab allocator. A next allocation at the
returned level will be placed next to this allocation, only separated by necessary padding
from alignment. In particular, this is the same strategy as applied for the placement of
#[repr(C)]
struct members. (Except for the final padding at the last member to the full
struct alignment.)
Usage
use static_alloc::Bump; let local: Bump<[u64; 3]> = Bump::uninit(); let base = local.level(); let (one, level) = local.leak_at(1_u64, base).unwrap(); // Will panic when an allocation happens in between. let (two, _) = local.leak_at(2_u64, level).unwrap(); assert_eq!((one as *const u64).wrapping_offset(1), two);
Trait Implementations
impl<T> GlobalAlloc for Bump<T>
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unsafe fn alloc(&self, layout: Layout) -> *mut u8
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unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout)
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unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8
1.28.0[src]
unsafe fn realloc(
&self,
ptr: *mut u8,
layout: Layout,
new_size: usize
) -> *mut u8
1.28.0[src]
&self,
ptr: *mut u8,
layout: Layout,
new_size: usize
) -> *mut u8
impl<'alloc, T> LocalAlloc<'alloc> for Bump<T>
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fn alloc(&'alloc self, layout: NonZeroLayout) -> Option<Allocation<'alloc>>
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unsafe fn dealloc(&'alloc self, _: Allocation<'alloc>)
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fn alloc_zeroed(
&'alloc self,
layout: NonZeroLayout
) -> Option<Allocation<'alloc>>
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&'alloc self,
layout: NonZeroLayout
) -> Option<Allocation<'alloc>>
unsafe fn realloc(
&'alloc self,
alloc: Allocation<'alloc>,
layout: NonZeroLayout
) -> Option<Allocation<'alloc>>
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&'alloc self,
alloc: Allocation<'alloc>,
layout: NonZeroLayout
) -> Option<Allocation<'alloc>>
impl<T> Sync for Bump<T>
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Auto Trait Implementations
Blanket Implementations
impl<T> Any for T where
T: 'static + ?Sized,
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T: 'static + ?Sized,
impl<T> Borrow<T> for T where
T: ?Sized,
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T: ?Sized,
impl<T> BorrowMut<T> for T where
T: ?Sized,
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T: ?Sized,
fn borrow_mut(&mut self) -> &mut T
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impl<T> From<T> for T
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impl<T, U> Into<U> for T where
U: From<T>,
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U: From<T>,
impl<T, U> TryFrom<U> for T where
U: Into<T>,
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U: Into<T>,
type Error = Infallible
The type returned in the event of a conversion error.
fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>
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impl<T, U> TryInto<U> for T where
U: TryFrom<T>,
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U: TryFrom<T>,