Unformatted version of **ai12s/ai12-0321-1.txt version 1.2**

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!standard A.19(0) 19-03-07 AI12-0321-1/01

!class Amendment 19-03-07

!status work item 19-03-07

!status received 19-03-07

!priority Low

!difficulty Easy

!subject Support for Arithmetic Atomic Operations, Test and Set, and Atomic Exchange

!class Amendment 19-03-07

!status work item 19-03-07

!status received 19-03-07

!priority Low

!difficulty Easy

!subject Support for Arithmetic Atomic Operations, Test and Set, and Atomic Exchange

!summary

!problem

On multiprocessor platforms, relying on locks for synchronization can be
problematic. One issue is that while a task is blocked, it can be difficult or
impossible to offer guarantees for progress. Other problems associated with
locks includes deadlock, livelock, and priority inversion. Locking can also be a
significant detriment to performance, and reduce opportunities for parallelism.

Lock-free data structures guarantee system-wide progress, while wait-free data
structures, in addition to being lock-free, also guarantee per-thread progresss.

Lock-free data structures can also be used to improve performance by increasing
the amount of time spent executing in parallel since access to the data
structure does not need to be serialised.

AI12-0234-1 provides access to compare-and-swap operations, but there are
other useful operations that could be provided as atomic primitives.

Atomic arithmetic, such as being able to atomically increment an atomic
counter is a common use for atomic operations. Another common need in this
area is to create spin-locks in user space.

Ada should provide some simple primitives that can be mapped to hardware
instructions that allow such updates to perform as expected.

!proposal

This proposal depends on AI12-0234-1, which defines the parent package
Ada.Atomic_Operations.

The solution is to provide a set of standard library calls that map to commonly
available atomic hardware instructions such as compare and swap,
test and set, and atomic increment. These subprograms are to be intrinsic calls,
and the generic libraries will assert an error if a particular target platform
does not support such atomic operations.

The libraries are generic libraries in order to support operations on discrete
types of different sizes, and we require that the actual types for the generics
be atomic types, so that Ada's semantics of atomic types can be associated
with these primitive operations.

!wording

A.19.3 The Generic Function Atomic_Operations.Atomic_Exchange

The language-defined generic function Atomic_Operations.Atomic_Exchange
provides an operation to atomically exchange the values of two atomic
objects.

Static Semantics

The library function Ada.Atomic_Operations.Atomic_Exchange has the
following declaration:

Writes the value denoted by Value into Item, and returns the previous
value denoted by Item.

A.19.3 The Package Atomic_Operations.Test_And_Set

The language-defined package Atomic_Operations.Test_And_Set
provides an operation to atomically set and clear an atomic flag object.

Static Semantics

The library package Ada.Atomic_Operations.Test_And_Set has the
following declaration:

Test_And_Set_Flag represents the state of an atomic flag object.
An atomic flag object can either be considered to be set or cleared.

function Atomic_Test_And_Set

(Item : aliased in out Test_And_Set_Flag) return Boolean
with Convention => Intrinsic;

Performs an atomic test-and-set operation on Item. Item is set to some
implementation defined nonzero "set" value and the return value is true
if and only if the previous contents were "set".

procedure Atomic_Clear

(Item : aliased in out Test_And_Set_Flag)
with Convention => Intrinsic;

Performs an atomic clear operation on Item. After the operation, Item
contains 0. This call should be used in conjunction with
Atomic_Test_And_Set.

A.19.4 The Package Atomic_Operations.Arithmetic

The language-defined package Atomic_Operations.Arithmetic
provides operations to perform arithmetic atomically on objects of
modular types.

Static Semantics

The library package Ada.Atomic_Operations.Arithmetic has the
following declaration:

The following functions perform the operation suggested by the name,
and return the result of the operation.

i.e. Item := Item op Value; return Item;

The following functions perform the operation suggested by the name,
and return the value that had previously been in Item.

i.e. Tmp := Item; Item := Item op Value; return Tmp;

!discussion

The approach taken to improving support for lock free data structures is to
provide a set of libraries of low level atomic primitives similar to the library
that is provided by gcc for C and C++.

The library of intrinsic primitives might be of interest to those wishing to
implement specific lock free algoriths, particularly if porting those applications
from other languages.

It was considered whether signed arithmetic functions should be provided.
While it is possible, the routines would be trickier to write because
we would want Ada semantics on arithmetic overflow, which doesn't happen
with the low-level gcc instrinsic functions. This would mean using the
pre-fetch library calls to obtain the value before the operation, and then
applying the operation again on a result variable in addition to the
atomic variable, so that any overflow could be detected, and an exception
raised. THe modular arithmetic functions are simpler, and can be simply
mapped to the underlying calls, since arithmetic overflow handling is not
needed. We could add signed arithmetic in the future, if the need is
great enough.

!ASIS

No ASIS effect.

!ACATS test

An ACATS C-Test is needed to check that the new capabilities are supported.

!appendix

From: Brad Moore Sent: Thursday, March 07, 2019 6:03 PM Here is my first draft for a new AI on additional atomic operations. This content was split off from AI12-0234-1. [This is version /01 of this AI - Editor.] I eliminated functions that did not seem to be needed, such as atomic load, atomic store, and atomic fences. I also eliminated the signed arithmetic package. The modular arithmetic better maps to the underlying calls, since the signed onces would need to support detection of arithmetic overflow, which the underlying gcc instrinsic functions do not do. We could add signed arithmetic functions in the future if the need was great enough. ****************************************************************

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