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66 lines
2.6 KiB
TeX
66 lines
2.6 KiB
TeX
\pagebreak
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\section{The \code{reduction} Clause}
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\label{sec:reduction}
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The following example demonstrates the \code{reduction} clause ; note that some
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reductions can be expressed in the loop in several ways, as shown for the \code{max}
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and \code{min} reductions below:
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\cexample{reduction}{1}
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\ffreeexample{reduction}{1}
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A common implementation of the preceding example is to treat it as if it had been
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written as follows:
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\cexample{reduction}{2}
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\fortranspecificstart
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\ffreenexample{reduction}{2}
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The following program is non-conforming because the reduction is on the
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\emph{intrinsic procedure name} \code{MAX} but that name has been redefined to be the variable
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named \code{MAX}.
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\ffreenexample{reduction}{3}
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% blue line floater at top of this page for "Fortran, cont."
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\begin{figure}[t!]
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\linewitharrows{-1}{dashed}{Fortran (cont.)}{8em}
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\end{figure}
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The following conforming program performs the reduction using the
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\emph{intrinsic procedure name} \code{MAX} even though the intrinsic \code{MAX} has been renamed
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to \code{REN}.
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\ffreenexample{reduction}{4}
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The following conforming program performs the reduction using
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\plc{intrinsic procedure name} \code{MAX} even though the intrinsic \code{MAX} has been renamed
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to \code{MIN}.
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\ffreenexample{reduction}{5}
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\fortranspecificend
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The following example is non-conforming because the initialization (\code{a =
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0}) of the original list item \code{a} is not synchronized with the update of
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\code{a} as a result of the reduction computation in the \code{for} loop. Therefore,
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the example may print an incorrect value for \code{a}.
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To avoid this problem, the initialization of the original list item \code{a}
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should complete before any update of \code{a} as a result of the \code{reduction}
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clause. This can be achieved by adding an explicit barrier after the assignment
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\code{a = 0}, or by enclosing the assignment \code{a = 0} in a \code{single}
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directive (which has an implied barrier), or by initializing \code{a} before
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the start of the \code{parallel} region.
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\cexample{reduction}{6}
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\fexample{reduction}{6}
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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.
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\cexample{reduction}{7}
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\ffreeexample{reduction}{7}
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