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52 lines
2.4 KiB
TeX
52 lines
2.4 KiB
TeX
\section{Task Dependences for \kcode{taskloop} Construct}
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\label{sec:taskloop_depend}
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\index{dependences!taskloop dependences}
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\index{task_iteration directive@\kcode{task_iteration} directive!depend clause@\kcode{depend} clause}
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\index{task_iteration directive@\kcode{task_iteration} directive}
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\index{directives!task_iteration@\kcode{task_iteration}}
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\index{taskloop construct@\kcode{taskloop} construct}
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\index{constructs!taskloop@\kcode{taskloop}}
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\index{depend clause@\kcode{depend} clause}
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\index{clauses!depend@\kcode{depend}}
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Dependences for tasks generated from a \kcode{taskloop} construct can
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be specified using the \kcode{task_iteration} directive nested in
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the beginning of the associated loop body.
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In the following example, taskloop TL1 contains
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a \kcode{task_iteration} directive with the \kcode{depend} clauses
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that specify task dependences across loop iterations on variable \ucode{A}
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(\ucode{A[i] $\rightarrow$ A[i-1]}).
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The \kcode{nogroup} clause for the \kcode{taskloop} construct removes
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the implicit taskgroup for a taskloop so that dependences across taskloops and
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with other tasks can be specified.
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For taskloop TL2, the dependence (\ucode{A[i] $\rightarrow$ A[i-4]})
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is specified for every 4 loop iterations
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as defined by the \kcode{if} clause that matches with
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the chunk size 4 specified in the \kcode{grainsize} clause for taskloop tasks.
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The dependences are generated only for those iterations where
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the \kcode{if} condition evaluates to \plc{true}.
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For instance, the first task generated from TL2 will update elements
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\ucode{A[1:4]} with depend clauses \kcode{depend(inout: \ucode{A[4]})}
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and \kcode{depend(in: \ucode{A[0]})}. This ensures element \ucode{A[4]}
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(thus elements \ucode{A[1:3]}) will be available from TL1 before executing
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the task.
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The last task T3 will wait for the availability of \ucode{A[n-1]}
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(or \ucode{A(n)} in Fortran) before printing the result.
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\cexample[6.0]{taskloop_dep}{1}
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\ffreeexample[6.0]{taskloop_dep}{1}
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The following example shows the use of the \kcode{task_iteration}
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directive for specifying task dependences in a multi-dimensional loop nest
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from multiple loop iterations in taskloop TL4.
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Similar to the previous example, the \kcode{nogroup} clause removes
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the implicit taskgroup for the \kcode{taskloop} construct so that
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dependences with other tasks (T5 in this case) can be specified.
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\cexample[6.0]{taskloop_dep}{2}
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\ffreeexample[6.0]{taskloop_dep}{2}
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