Allocator traits and std::heap

[nikomatsakis]

📢 This feature has a dedicated working group, please direct comments and concerns to the working group's repo.

The remainder of this post is no longer an accurate summary of the current state; see that dedicated working group instead.

Old content

Original Post:

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FCP proposal: #32838 (comment)
FCP checkboxes: #32838 (comment)

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Tracking issue for rust-lang/rfcs#1398 and the std::heap module.

• land struct Layout, trait Allocator, and default implementations in alloc crate (Allocator integration #42313)
• decide where parts should live (e.g. default impls has dependency on alloc crate, but Layout/Allocator could be in libcore...) (Allocator integration #42313)
• fixme from source code: audit default implementations (in Layout for overflow errors, (potentially switching to overflowing_add and overflowing_mul as necessary).
• decide if realloc_in_place should be replaced with grow_in_place and shrink_in_place (comment) (Allocator integration #42313)
• review arguments for/against associated error type (see subthread here)
• determine what the requirements are on the alignment provided to fn dealloc. (See discussion on allocator rfc and global allocator rfc and trait Alloc PR.)
  • Is it required to deallocate with the exact align that you allocate with? Concerns have been raised that allocators like jemalloc don't require this, and it's difficult to envision an allocator that does require this. (more discussion). @ruuda and @rkruppe look like they've got the most thoughts so far on this.
• should AllocErr be Error instead? (comment)
• Is it required to deallocate with the exact size that you allocate with? With the usable_size business we may wish to allow, for example, that you if you allocate with (size, align) you must deallocate with a size somewhere in the range of size...usable_size(size, align). It appears that jemalloc is totally ok with this (doesn't require you to deallocate with a precise size you allocate with) and this would also allow Vec to naturally take advantage of the excess capacity jemalloc gives it when it does an allocation. (although actually doing this is also somewhat orthogonal to this decision, we're just empowering Vec). So far @gankro has most of the thoughts on this. (@alexcrichton believes this was settled in Allocator integration #42313 due to the definition of "fits")
• similar to previous question: Is it required to deallocate with the exact alignment that you allocated with? (See comment from 5 June 2017)
• OSX/alloc_system is buggy on huge alignments (e.g. an align of 1 << 32) if huge requested align, alloc_system heap::allocate on OS X returns unaligned values #30170 Add precondition to Layout that the align fit in a u32. #43217
• should Layout provide a fn stride(&self) method? (See also Separate size and stride for types rfcs#1397, Collapse trailing padding #17027 )
• Allocator::owns as a method? Alloc: Add owns method #44302

State of std::heap after #42313:

pub struct Layout { /* ... */ }

impl Layout {
    pub fn new<T>() -> Self;
    pub fn for_value<T: ?Sized>(t: &T) -> Self;
    pub fn array<T>(n: usize) -> Option<Self>;
    pub fn from_size_align(size: usize, align: usize) -> Option<Layout>;
    pub unsafe fn from_size_align_unchecked(size: usize, align: usize) -> Layout;

    pub fn size(&self) -> usize;
    pub fn align(&self) -> usize;
    pub fn align_to(&self, align: usize) -> Self;
    pub fn padding_needed_for(&self, align: usize) -> usize;
    pub fn repeat(&self, n: usize) -> Option<(Self, usize)>;
    pub fn extend(&self, next: Self) -> Option<(Self, usize)>;
    pub fn repeat_packed(&self, n: usize) -> Option<Self>;
    pub fn extend_packed(&self, next: Self) -> Option<(Self, usize)>;
}

pub enum AllocErr {
    Exhausted { request: Layout },
    Unsupported { details: &'static str },
}

impl AllocErr {
    pub fn invalid_input(details: &'static str) -> Self;
    pub fn is_memory_exhausted(&self) -> bool;
    pub fn is_request_unsupported(&self) -> bool;
    pub fn description(&self) -> &str;
}

pub struct CannotReallocInPlace;

pub struct Excess(pub *mut u8, pub usize);

pub unsafe trait Alloc {
    // required
    unsafe fn alloc(&mut self, layout: Layout) -> Result<*mut u8, AllocErr>;
    unsafe fn dealloc(&mut self, ptr: *mut u8, layout: Layout);

    // provided
    fn oom(&mut self, _: AllocErr) -> !;
    fn usable_size(&self, layout: &Layout) -> (usize, usize);
    unsafe fn realloc(&mut self,
                      ptr: *mut u8,
                      layout: Layout,
                      new_layout: Layout) -> Result<*mut u8, AllocErr>;
    unsafe fn alloc_zeroed(&mut self, layout: Layout) -> Result<*mut u8, AllocErr>;
    unsafe fn alloc_excess(&mut self, layout: Layout) -> Result<Excess, AllocErr>;
    unsafe fn realloc_excess(&mut self,
                             ptr: *mut u8,
                             layout: Layout,
                             new_layout: Layout) -> Result<Excess, AllocErr>;
    unsafe fn grow_in_place(&mut self,
                            ptr: *mut u8,
                            layout: Layout,
                            new_layout: Layout) -> Result<(), CannotReallocInPlace>;
    unsafe fn shrink_in_place(&mut self,
                              ptr: *mut u8,
                              layout: Layout,
                              new_layout: Layout) -> Result<(), CannotReallocInPlace>;

    // convenience
    fn alloc_one<T>(&mut self) -> Result<Unique<T>, AllocErr>
        where Self: Sized;
    unsafe fn dealloc_one<T>(&mut self, ptr: Unique<T>)
        where Self: Sized;
    fn alloc_array<T>(&mut self, n: usize) -> Result<Unique<T>, AllocErr>
        where Self: Sized;
    unsafe fn realloc_array<T>(&mut self,
                               ptr: Unique<T>,
                               n_old: usize,
                               n_new: usize) -> Result<Unique<T>, AllocErr>
        where Self: Sized;
    unsafe fn dealloc_array<T>(&mut self, ptr: Unique<T>, n: usize) -> Result<(), AllocErr>
        where Self: Sized;
}

/// The global default allocator
pub struct Heap;

impl Alloc for Heap {
    // ...
}

impl<'a> Alloc for &'a Heap {
    // ...
}

/// The "system" allocator
pub struct System;

impl Alloc for System {
    // ...
}

impl<'a> Alloc for &'a System {
    // ...
}

[gereeter]

I unfortunately wasn't paying close enough attention to mention this in the RFC discussion, but I think that realloc_in_place should be replaced by two functions, grow_in_place and shrink_in_place, for two reasons:

• I can't think of a single use case (short of implementing realloc or realloc_in_place) where it is unknown whether the size of the allocation is increasing or decreasing. Using more specialized methods makes it slightly more clear what is going on.
• The code paths for growing and shrinking allocations tend to be radically different - growing involves testing whether adjacent blocks of memory are free and claiming them, while shrinking involves carving off properly sized subblocks and freeing them. While the cost of a branch inside realloc_in_place is quite small, using grow and shrink better captures the distinct tasks that an allocator needs to perform.

Note that these can be added backwards-compatibly next to realloc_in_place, but this would constrain which functions would be by default implemented in terms of which others.

For consistency, realloc would probably also want to be split into grow and split, but the only advantage to having an overloadable realloc function that I know of is to be able to use mmap's remap option, which does not have such a distinction.

[gereeter]

Additionally, I think that the default implementations of realloc and realloc_in_place should be slightly adjusted - instead of checking against the usable_size, realloc should just first try to realloc_in_place. In turn, realloc_in_place should by default check against the usable size and return success in the case of a small change instead of universally returning failure.

This makes it easier to produce a high-performance implementation of realloc: all that is required is improving realloc_in_place. However, the default performance of realloc does not suffer, as the check against the usable_size is still performed.

[pnkfelix]

Another issue: The doc for fn realloc_in_place says that if it returns Ok, then one is assured that ptr now "fits" new_layout.

To me this implies that it must check that the alignment of the given address matches any constraint implied by new_layout.

However, I don't think the spec for the underlying fn reallocate_inplace function implies that it will perform any such check.

• Furthermore, it seems reasonable that any client diving into using fn realloc_in_place will themselves be ensuring that the alignments work (in practice I suspect it means that the same alignment is required everywhere for the given use case...)

So, should the implementation of fn realloc_in_place really be burdened with checking that the alignment of the given ptr is compatible with that of new_layout? It is probably better in this case (of this one method) to push that requirement back to the caller...

[pnkfelix]

@gereeter you make good points; I will add them to the check list I am accumulating in the issue description.

[pnkfelix]

(at this point I am waiting for #[may_dangle] support to ride the train into the beta channel so that I will then be able to use it for std collections as part of allocator integration)