\section{The \code{reduction} Clause}

\label{sec:reduction}

The following example demonstrates the \code{reduction} clause ; note that some

reductions can be expressed in the loop in several ways, as shown for the \code{max}

and \code{min} reductions below:

\cexample{reduction}{1}

\ffreeexample{reduction}{1}

A common implementation of the preceding example is to treat it as if it had been

written as follows:

\cexample{reduction}{2}

\ffreenexample{reduction}{2}

The following program is non-conforming because the reduction is on the

\emph{intrinsic procedure name} \code{MAX} but that name has been redefined to be the variable

named \code{MAX}.

\ffreenexample{reduction}{3}

\begin{figure}[t!]

\linewitharrows{-1}{dashed}{Fortran (cont.)}{8em}

\end{figure}

The following conforming program performs the reduction using the

\emph{intrinsic procedure name} \code{MAX} even though the intrinsic \code{MAX} has been renamed

to \code{REN}.

\ffreenexample{reduction}{4}

The following conforming program performs the reduction using

\plc{intrinsic procedure name} \code{MAX} even though the intrinsic \code{MAX} has been renamed

to \code{MIN}.

\ffreenexample{reduction}{5}

The following example is non-conforming because the initialization (\code{a =

0}) of the original list item \code{a} is not synchronized with the update of

\code{a} as a result of the reduction computation in the \code{for} loop. Therefore,

the example may print an incorrect value for \code{a}.

To avoid this problem, the initialization of the original list item \code{a}

should complete before any update of \code{a} as a result of the \code{reduction}

clause. This can be achieved by adding an explicit barrier after the assignment

\code{a = 0}, or by enclosing the assignment \code{a = 0} in a \code{single}

directive (which has an implied barrier), or by initializing \code{a} before

the start of the \code{parallel} region.

\cexample{reduction}{6}

\fexample{reduction}{6}

The following example demonstrates the reduction of array \plc{a}. In C/C++ this is illustrated by the explicit use of an array section \plc{a[0:N]} in the \code{reduction} clause. The corresponding Fortran example uses array syntax supported in the base language. As of the OpenMP 4.5 specification the explicit use of array section in the \code{reduction} clause in Fortran is not permitted. But this oversight will be fixed in the next release of the specification.

\cexample{reduction}{7}

\ffreeexample{reduction}{7}