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49 lines
2.4 KiB
TeX
49 lines
2.4 KiB
TeX
\pagebreak
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\section{\code{scan} Directive}
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\label{sec:scan}
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\index{directives!scan@\code{scan}}
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\index{scan directive@\code{scan} directive}
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\index{reduction clause@\code{reduction} clause!inscan modifier@\code{inscan} modifier}
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\index{inscan modifier@\code{inscan} modifier}
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The following examples illustrate how to parallelize a loop that saves
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the \emph{prefix sum} of a reduction. This is accomplished by using
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the \code{inscan} modifier in the \code{reduction} clause for the input
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variable of the scan, and specifying with a \code{scan} directive whether
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the storage statement includes or excludes the scan input of the present
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iteration (\texttt{k}).
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\index{scan directive@\code{scan} directive!inclusive clause@\code{inclusive} clause}
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\index{scan directive@\code{scan} directive!exclusive clause@\code{exclusive} clause}
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\index{clauses!inclusive@\code{inclusive}}
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\index{inclusive clause@\code{inclusive} clause}
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\index{clauses!exclusive@\code{exclusive}}
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\index{exclusive clause@\code{exclusive} clause}
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Basically, the \code{inscan} modifier connects a loop and/or SIMD reduction to
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the scan operation, and a \code{scan} construct with an \code{inclusive} or
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\code{exclusive} clause specifies whether the ``scan phase'' (lexical block
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before and after the directive, respectively) is to use an \plc{inclusive} or
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\plc{exclusive} scan value for the list item (\texttt{x}).
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The first example uses the \plc{inclusive} scan operation on a composite
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loop-SIMD construct. The \code{scan} directive separates the reduction
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statement on variable \texttt{x} from the use of \texttt{x} (saving to array \texttt{b}).
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The order of the statements in this example indicates that
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value \texttt{a[k]} (\texttt{a(k)} in Fortran) is included in the computation of
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the prefix sum \texttt{b[k]} (\texttt{b(k)} in Fortran) for iteration \texttt{k}.
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\cexample[5.0]{scan}{1}
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\ffreeexample[5.0]{scan}{1}
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The second example uses the \plc{exclusive} scan operation on a composite
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loop-SIMD construct. The \code{scan} directive separates the use of \texttt{x}
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(saving to array \texttt{b}) from the reduction statement on variable \texttt{x}.
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The order of the statements in this example indicates that
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value \texttt{a[k]} (\texttt{a(k)} in Fortran) is excluded from the computation
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of the prefix sum \texttt{b[k]} (\texttt{b(k)} in Fortran) for iteration \texttt{k}.
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\cexample[5.0]{scan}{2}
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\ffreeexample[5.0]{scan}{2}
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