P0401R2
Providing size feedback in the Allocator interface

1. Introduction

This is a library paper to describe how size feedback can be used with allocators:

• In the case of std::allocator, the language feature proposed in [P0901R5] could be used.

• In the case of custom allocators, the availability of the traits interface allows standard library containers to leverage this functionality.

2. Motivation

Consider code adding elements to vector:

std::vector<int> v = {1, 2, 3};
// Expected: v.capacity() == 3

// Add an additional element, triggering a reallocation.
v.push_back(4);

Many allocators only allocate in fixed-sized chunks of memory, rounding up requests. Our underlying heap allocator received a request for 12 bytes (3 * sizeof(int)) while constructing v. For several implementations, this request is turned into a 16 byte region.

2.1. Why not realloc

realloc poses several problems problems:

• We have unclear strategies for growing a container. Consider the two options (a and b) outlined in the comments of the example below. Assume, as with the §2 Motivation example, that there is a 16 byte size class for our underlying allocator.

  std::vector<int> v = {1, 2, 3};
  // Expected: v.capacity() == 3
  
  // Add an additional element.  Does the vector:
  // a) realloc(sizeof(int) * 4), adding a single element
  // b) realloc(sizeof(int) * 6), attempt to double, needing reallocation
  v.push_back(4);
  
  // Add another element.
  // a) realloc(sizeof(int) * 5), fails in-place, requires reallocation
  // b) size + 1 <= capacity, no reallocation required
  v.push_back(5);
  

  Option a requires every append (after the initial allocation) try to realloc, requiring an interaction with the allocator each time.

  Option b requires guesswork to probe the allocator’s true size. By doubling (the typical growth pattern for std::vector in libc++ and libstdc++), we skip past the existing allocation’s size boundary. While we can use a reduced growth rate, doing so requires more interactions with the allocator (calls to realloc), as well as copies (when we cannot resize in-place).

  Both of these options add extra branching on the control flow for growing the buffer.

• When the allocator cannot resize the buffer in-place, it allocates new memory and memcpy's the contents. For non-trivially copyable types, this is disastrous. The language lacks support for reallocating in-place without moving. While this function could be implemented as magic library function, it could not interact with many underlying malloc implementations (libc, in the case of libc++ and libstdc++) used to build operator new, nor could it interact with user-provided replacement functions.

  In-place reallocation has been considered in [N3495] (throwing std::bad_alloc when unable) and [P0894R1] (returning false when unable). This has sometimes been referred to as an "extend" or an "expand" interface. Extending an existing allocation requires a round-trip to the allocator (and its requisite branch/call overheads) to determine whether the allocation could be expanded. As discussed previously, we need to probe for the true size of the backing object (until we find extend fails) until we give up and apply a multiplicative expansion factor.

For fixed-size allocators it makes more sense, and is much simpler for containers to adapt to, if the allocator is able to over-allocate on the initial request and inform the caller how much memory was made available.

2.2. Why not ask the allocator for the size

We could also explore APIs that answer the question: "If I ask for N bytes, how many do I actually get?" [jemalloc] calls this nallocx.

• nallocx requires an extra call / size calculation. This requires we duplicate work, as the size calculation is performed as part of allocation itself.

• nallocx impairs telemetry for monitoring allocation request sizes. If a nallocx return value is cached, the user appears to be asking for exactly as many bytes as nallocx said they would get.

See also [P0901R5]'s discussion "nallocx: not as awesome as it looks."

3. Proposal

Wording relative to [N4800].

We propose a free function to return the allocation size simultaneously with an allocation.

Amend [allocator.requirements]:

template<typename T = void>
struct sized_ptr_t {
  T *ptr;
  size_t n;
};

template<typename T, typename Allocator = std::allocator<T>>
sized_ptr_t<T> allocate_at_least(Allocator& a, size_t n);

• Effects: Let s be the result of allocate_at_least(a, n). Memory is allocated for at least n objects of type T but objects are not constructed.
• Returns: sized_ptr_t{ptr, m}, where ptr is that memory and m is the number of objects for which memory has been allocated, such that m>=n.
• [ Note: If s.n == 0, the value of s.ptr is unspecified. —end note]

Table 34 - Cpp17Allocator Requirements

a.deallocate(p,n)

(not used)

Requires: p shall be a value returned by an earlier call to allocate or allocate_at_least that has not been invalidated by an intervening call to deallocate. If this memory was obtained by a call to allocate, n shall match the value passed to allocate to obtain this memory. Otherwise, n shall satisfy capacity >= n >= requested where [p, capacity] = allocate_at_least(requested) was used to obtain this memory. Throws: Nothing.

4. Design Considerations

4.1. allocate selection

There are multiple approaches here:

• Return a pointer-size pair, as presented.

• Overload allocate and return via a reference parameter. This potentially hampers optimization depending on ABI, as the value is returned via memory rather than a register.

4.2. deallocate changes

We now require flexibility on the size we pass to deallocate. For container types using this allocator interface, they are faced with the choice of storing both the original size request as well as the provided size (requiring additional auxillary storage), or being unable to reproduce one during the call to deallocate.

As the true size is expected to be useful for the capacity of a string or vector instance, the returned size is available without additional storage requirements. The original (requested) size is unavailable, so we relax the deallocate size parameter requirements for these allocations.

4.3. Interaction with polymorphic_allocator

std::pmr::memory_resource is implemented using virtual functions. Adding new methods, such as the proposed allocate API would require taking an ABI break.

5. Revision History

5.1. R1 → R2

Applied LEWG feedback from [Cologne].

Poll: We want a feature like this.

SF	F	N	A	SA
2	9	2	0	0

• As users may specialize std::allocator_traits, this revision moves to a free function std::allocate_at_least as suggested via a LEWG poll.

Poll: Name choice

5	allocate_with_size
4	overallocate
2	allocate_with_oversubscribe
6	allocate_oversize
14	allocate_at_least

Poll: Prefer a struct rather than a requirement of structured bindings.

SF	F	N	A	SA
2	8	0	2	1