Document Number: P2835R2
Date: 2024-01-10
Reply to: Gonzalo Brito Gadeschi <gonzalob _at_ nvidia.com>
Authors: Gonzalo Brito Gadeschi
Audience: LEWG

Expose `std::atomic_ref` 's object address

Changelog

R2: Preparation for maling list review
- Update links to compiler explorer.
- Update API design rationale.
- Update __cpp_lib_atomic_ref macro.
R1:
- Add alternative API designs.
R0: initial revision (Varna)

Introduction

std::atomic_ref prevents applications from obtaining the address of the object referenced by *this and, therefore, from reasoning about contention on accesses to the object, which is crucial for performance (see “Usecases” section).

Applications that need to reason about contention for performance cannot use std::atomic_ref but may be able to use std::atomic& or std::atomic* instead.

That is not always possible, e.g., if object’s type is outside the application’s control. Then, a pair<atomic_ref<T>, T*> may be passed around instead. However, this is not ergonomic, and always having a pointer available slightly increases the hazard of accidentally accessing the object via a raw pointer while an atomic_ref object is still live.

This paper proposes to add a .data() member function to std::atomic_ref instead, which can be used when the application needs to access the underliyng object’s address, e.g., to be able to reason about contention.

Tony tables

Before	After
`std::atomic<int>& ref;` `auto* addr = &ref;`	`std::atomic_ref ref;` `auto* addr = ref.data();`

Alternatives

Currently, it is not possible to obtain a pointer to the underlying object of an std::atomic_ref, and therefore not possible to accidentally access the object concurrently through a raw pointer while the std::atomic_ref is still live.

The proposed API introduces a data member function that returns a T const*. A program that accidentally dereferences this pointer while there are live std::atomic_ref referencing the object exhibits undefined behavior.

To make this accidental usage of this API harder, we could:

Change the return type to void const*: void const* data() const noexcept;.
Change the return type to uintptr_t and the name to address: uintptr_t address() const noexcept;.

Wording

Add the following to [atomics.ref.generic.general].

namespace std {
  template<class T> struct atomic_ref {
    // ...
    T const* data() const noexcept;
    // ...
  };
}

Add the following to [atomic.ref.ops]:

T const* data() const noexcept;

* Returns: pointer to the object referenced by *this.

Update __cpp_lib_atomic_ref version macro in <version> synopsis [version.syn] to the C++ version this feature is introduced in:


#define __cpp_lib_atomic_ref 201806______L // freestanding, also in <atomic>

Use cases

The main use case is detecting contention, and using that information to optimize concurrent algorithms.

Discovery Patterns

Some hardware architectures have instructions to “discover” different threads of the same programm that are running on the same core and are execution the same “program step”.

In those hardware architectures, these instructions can be used to aggregate atomic operations performed by different threads into a single operation performed by one thread. The pattern looks like this:












void unsynchronized_aggregated_faa(atomic<int>& acc, int upd) {
  // Find all spatially-close threads executing this program step 
  // with same values of "acc" and "upd".
  auto thread_mask = __discover_threads_with_same(acc, upd);
  auto thread_count = popcount(thread_mask);
  
  // These threads elect a leader, which aggregates their updates
  // and performs a single atomic RMW operation instead of one
  // per thread:
  if(__pick_one(thread_mask))
     acc.fetch_add(thread_count * upd, memory_order_relaxed);
}

On NVIDIA GPUs, this optimization can significantly increase the performance of certain algoriths, like “arrive” operations on barriers. In this example (godbolt), even with a small number of threads, ~1.25x speed ups are measured.

Expose std::atomic_ref 's object address