\pagebreak
\section{Atomic Compare}
\label{sec:cas}

\index{constructs!atomic@\code{atomic}}
\index{atomic construct@\code{atomic} construct}
\index{clauses!capture@\code{capture}}
\index{clauses!compare@\code{compare}}
\index{capture clause@\code{capture} clause}
\index{compare clause@\code{compare} clause}

In OpenMP 5.1 the \scode{compare} clause was added to the extended-atomic clauses.
The \scode{compare} clause extends the semantics to perform the \scode{atomic}
update conditionally. 

In the following C/C++ example, two formats of structured blocks
are shown for associated \scode{atomic} constructs with the \scode{compare} clause.
In the first \scode{atomic} construct, the format forms a conditional update statement.
In the second \scode{atomic} construct the format forms a conditional expression statement.
The ``greater than'' and ``less than'' forms are not available with the Fortran \scode{compare}
clause.  One can use the \splc{max} and \splc{min} functions with the \scode{atomic}~\scode{update}
construct to perform the C/C++ example operations.

\cexample[5.1]{cas}{1}
%\ffreeexample[5.1]{cas}{1}

In OpenMP 5.1 the \scode{compare} clause was also added to support Compare And
Swap (CAS) semantics. In the following example the \splc{enqueue} routine
(a naive implementation of a Michael and Scott enqueue function), uses the
\scode{compare} clause, with the \scode{capture} clause, to perform and compare
(\splc{q->head == node->next}) and swap (\splc{if-else} assignments) of the
form: 
\begin{description}[noitemsep,labelindent=5mm,widest=f90]
\item \splc{{ r = x == e; if(r) { x = d; } else { v = x; } }}.
\end{description}
The example program concurrently enqueues nodes from an array of nodes (\splc{nodes[N]}).
Since the equivalence of Fortran pointers can be determined only with a function (such as associated),
no Fortran version is provided here. The use of the associated function in an atomic compare syntax is
being considered in a future release.

\cexample[5.1]{cas}{2}
%\ffreeexample[5.1]{cas}{2}