atomic/doc/platform.qbk

[/
 / Copyright (c) 2009 Helge Bahmann
 /
 / Distributed under the Boost Software License, Version 1.0. (See accompanying
 / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 /]

[section:template_organization Organization of class template layers]

The implementation uses multiple layers of template classes that
inherit from the next lower level each and refine or adapt the respective
underlying class:

* [^boost::atomic<T>] is the topmost-level, providing
  the external interface. Implementation-wise, it does not add anything
  (except for hiding copy constructor and assignment operator).

* [^boost::detail::atomic::internal_atomic&<T,S=sizeof(T),I=is_integral_type<T> >]:
  This layer is mainly responsible for providing the overloaded operators
  mapping to API member functions (e.g. [^+=] to [^fetch_add]).
  The defaulted template parameter [^I] allows
  to expose the correct API functions (via partial template
  specialization): For non-integral types, it only
  publishes the various [^exchange] functions
  as well as load and store, for integral types it
  additionally exports arithmetic and logic operations.
  [br]
  Depending on whether the given type is integral, it
  inherits from either [^boost::detail::atomic::platform_atomic<T,S=sizeof(T)>]
  or [^boost::detail::atomic::platform_atomic_integral<T,S=sizeof(T)>].
  There is however some special-casing: for non-integral types
  of size 1, 2, 4 or 8, it will coerce the datatype into an integer representation
  and delegate to [^boost::detail::atomic::platform_atomic_integral<T,S=sizeof(T)>]
  -- the rationale is that platform implementors only need to provide
  integer-type operations.

* [^boost::detail::atomic::platform_atomic_integral<T,S=sizeof(T)>]
  must provide the full set of operations for an integral type T
  (i.e. [^load], [^store], [^exchange],
  [^compare_exchange_weak], [^compare_exchange_strong],
  [^fetch_add], [^fetch_sub], [^fetch_and],
  [^fetch_or], [^fetch_xor], [^is_lock_free]).
  The default implementation uses locking to emulate atomic operations, so
  this is the level at which implementors should provide template specializations
  to add support for platform-specific atomic operations.
  [br]
  The two separate template parameters allow separate specialization
  on size and type (which, with fixed size, cannot
  specify more than signedness/unsignedness). The rationale is that
  most platform-specific atomic operations usually depend only on the
  operand size, so that common implementations for signed/unsigned
  types are possible. Signedness allows to properly to choose sign-extending
  instructions for the [^load] operation, avoiding later
  conversion. The expectation is that in most implementations this will
  be a normal assignment in C, possibly accompanied by memory
  fences, so that the compiler can automatically choose the correct
  instruction.

* At the lowest level, [^boost::detail::atomic::platform_atomic<T,S=sizeof(T)>]
  provides the most basic atomic operations ([^load], [^store],
  [^exchange], [^compare_exchange_weak],
  [^compare_exchange_strong]) for arbitrarily generic data types.
  The default implementation uses locking as a fallback mechanism.
  Implementors generally do not have to specialize at this level
  (since these will not be used for the common integral type sizes
  of 1, 2, 4 and 8 bytes), but if s/he can if s/he so wishes to
  provide truly atomic operations for "odd" data type sizes.
  Some amount of care must be taken as the "raw" data type
  passed in from the user through [^boost::atomic<T>]
  is visible here -- it thus needs to be type-punned or otherwise
  manipulated byte-by-byte to avoid using overloaded assignment,
  comparison operators and copy constructors.

[endsect]


[section:platform_atomic_implementation Implementing platform-specific atomic operations]

In principle implementors are responsible for providing the
full range of named member functions of an atomic object
(i.e. [^load], [^store], [^exchange],
[^compare_exchange_weak], [^compare_exchange_strong],
[^fetch_add], [^fetch_sub], [^fetch_and],
[^fetch_or], [^fetch_xor], [^is_lock_free]).
These must be implemented as partial template specializations for
[^boost::detail::atomic::platform_atomic_integral<T,S=sizeof(T)>]:

[c++]

  template<typename T>
  class platform_atomic_integral<T, 4>
  {
  public:
    explicit platform_atomic_integral(T v) : i(v) {}
    platform_atomic_integral(void) {}

    T load(memory_order order=memory_order_seq_cst) const volatile
    {
      // platform-specific code
    }
    void store(T v, memory_order order=memory_order_seq_cst) volatile
    {
      // platform-specific code
    }

  private:
    volatile T i;
  };

As noted above, it will usually suffice to specialize on the second
template argument, indicating the size of the data type in bytes.

[section:automatic_buildup Templates for automatic build-up]

Often only a portion of the required operations can be
usefully mapped to machine instructions. Several helper template
classes are provided that can automatically synthesize missing methods to
complete an implementation.

At the minimum, an implementor must provide the
[^load], [^store],
[^compare_exchange_weak] and
[^is_lock_free] methods:

[c++]

  template<typename T>
  class my_atomic_32 {
  public:
    my_atomic_32() {}
    my_atomic_32(T initial_value) : value(initial_value) {}

    T load(memory_order order=memory_order_seq_cst) volatile const
    {
      // platform-specific code
    }
    void store(T new_value, memory_order order=memory_order_seq_cst) volatile
    {
      // platform-specific code
    }
    bool compare_exchange_weak(T &expected, T desired,
      memory_order success_order,
      memory_order_failure_order) volatile
    {
      // platform-specific code
    }
    bool is_lock_free() const volatile {return true;}
  protected:
  // typedef is required for classes inheriting from this
    typedef T integral_type;
  private:
    T value;
  };

The template [^boost::detail::atomic::build_atomic_from_minimal]
can then take care of the rest:

[c++]

  template<typename T>
  class platform_atomic_integral<T, 4>
    : public boost::detail::atomic::build_atomic_from_minimal<my_atomic_32<T> >
  {
  public:
    typedef build_atomic_from_minimal<my_atomic_32<T> > super;

    explicit platform_atomic_integral(T v) : super(v) {}
    platform_atomic_integral(void) {}
  };

There are several helper classes to assist in building "complete"
atomic implementations from different starting points:

* [^build_atomic_from_minimal] requires
  * [^load]
  * [^store]
  * [^compare_exchange_weak] (4-operand version)

* [^build_atomic_from_exchange] requires
  * [^load]
  * [^store]
  * [^compare_exchange_weak] (4-operand version)
  * [^compare_exchange_strong] (4-operand version)
  * [^exchange]

* [^build_atomic_from_add] requires
  * [^load]
  * [^store]
  * [^compare_exchange_weak] (4-operand version)
  * [^compare_exchange_strong] (4-operand version)
  * [^exchange]
  * [^fetch_add]

* [^build_atomic_from_typical] (<I>supported on gcc only</I>) requires
  * [^load]
  * [^store]
  * [^compare_exchange_weak] (4-operand version)
  * [^compare_exchange_strong] (4-operand version)
  * [^exchange]
  * [^fetch_add_var] (protected method)
  * [^fetch_inc] (protected method)
  * [^fetch_dec] (protected method)

  This will generate a [^fetch_add] method
  that calls [^fetch_inc]/[^fetch_dec]
  when the given parameter is a compile-time constant
  equal to +1 or -1 respectively, and [^fetch_add_var]
  in all other cases. This provides a mechanism for
  optimizing the extremely common case of an atomic
  variable being used as a counter.

  The prototypes for these methods to be implemented is:
  [c++]

    template<typename T>
    class my_atomic {
    public:
      T fetch_inc(memory_order order) volatile;
      T fetch_dec(memory_order order) volatile;
      T fetch_add_var(T counter, memory_order order) volatile;
    };

These helper templates are defined in [^boost/atomic/detail/builder.hpp].

[endsect]

[section:automatic_buildup_small Build sub-word-sized atomic data types]

There is one other helper template that can build sub-word-sized
atomic data types even though the underlying architecture allows
only word-sized atomic operations:

[c++]

  template<typename T>
  class platform_atomic_integral<T, 1> :
    public build_atomic_from_larger_type<my_atomic_32<uint32_t>, T>
  {
  public:
    typedef build_atomic_from_larger_type<my_atomic_32<uint32_t>, T> super;

    explicit platform_atomic_integral(T v) : super(v) {}
    platform_atomic_integral(void) {}
  };

The above would create an atomic data type of 1 byte size, and
use masking and shifts to map it to 32-bit atomic operations.
The base type must implement [^load], [^store]
and [^compare_exchange_weak] for this to work.

[endsect]

[section:other_sizes Atomic data types for unusual object sizes]

In unusual circumstances, an implementor may also opt to specialize
[^public boost::detail::atomic::platform_atomic<T,S=sizeof(T)>]
to provide support for atomic objects not fitting an integral size.
If you do that, keep the following things in mind:

* There is no reason to ever do this for object sizes
  of 1, 2, 4 and 8
* Only the following methods need to be implemented:
  * [^load]
  * [^store]
  * [^compare_exchange_weak] (4-operand version)
  * [^compare_exchange_strong] (4-operand version)
  * [^exchange]

The type of the data to be stored in the atomic
variable (template parameter [^T])
is exposed to this class, and the type may have
overloaded assignment and comparison operators --
using these overloaded operators however will result
in an error. The implementor is responsible for
accessing the objects in a way that does not
invoke either of these operators (using e.g.
[^memcpy] or type-casts).

[endsect]

[endsect]

[section:platform_atomic_fences Fences]

Platform implementors need to provide a function performing
the action required for [funcref boost::atomic_thread_fence atomic_thread_fence]
(the fallback implementation will just perform an atomic operation
on an integer object). This is achieved by specializing the
[^boost::detail::atomic::platform_atomic_thread_fence] template
function in the following way:

[c++]

  template<>
  void platform_atomic_thread_fence(memory_order order)
  {
    // platform-specific code here
  }

[endsect]

[section:platform_atomic_puttogether Putting it altogether]

The template specializations should be put into a header file
in the [^boost/atomic/detail] directory, preferably
specifying supported compiler and architecture in its name.

The file [^boost/atomic/detail/platform.hpp] must
subsequently be modified to conditionally include the new
header.

[endsect]