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79 lines
3.2 KiB
TeX
79 lines
3.2 KiB
TeX
\pagebreak
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\section{The \code{collapse} Clause}
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\label{sec:collapse}
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In the following example, the \code{k} and \code{j} loops are associated with
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the loop construct. So the iterations of the \code{k} and \code{j} loops are
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collapsed into one loop with a larger iteration space, and that loop is then divided
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among the threads in the current team. Since the \code{i} loop is not associated
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with the loop construct, it is not collapsed, and the \code{i} loop is executed
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sequentially in its entirety in every iteration of the collapsed \code{k} and
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\code{j} loop.
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The variable \code{j} can be omitted from the \code{private} clause when the
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\code{collapse} clause is used since it is implicitly private. However, if the
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\code{collapse} clause is omitted then \code{j} will be shared if it is omitted
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from the \code{private} clause. In either case, \code{k} is implicitly private
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and could be omitted from the \code{private} clause.
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\cexample[3.0]{collapse}{1}
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\fexample[3.0]{collapse}{1}
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In the next example, the \code{k} and \code{j} loops are associated with the
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loop construct. So the iterations of the \code{k} and \code{j} loops are collapsed
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into one loop with a larger iteration space, and that loop is then divided among
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the threads in the current team.
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The sequential execution of the iterations in the \code{k} and \code{j} loops
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determines the order of the iterations in the collapsed iteration space. This implies
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that in the sequentially last iteration of the collapsed iteration space, \code{k}
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will have the value \code{2} and \code{j} will have the value \code{3}. Since
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\code{klast} and \code{jlast} are \code{lastprivate}, their values are assigned
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by the sequentially last iteration of the collapsed \code{k} and \code{j} loop.
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This example prints: \code{2 3}.
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\cexample[3.0]{collapse}{2}
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\fexample[3.0]{collapse}{2}
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The next example illustrates the interaction of the \code{collapse} and \code{ordered}
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clauses.
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In the example, the loop construct has both a \code{collapse} clause and an \code{ordered}
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clause. The \code{collapse} clause causes the iterations of the \code{k} and
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\code{j} loops to be collapsed into one loop with a larger iteration space, and
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that loop is divided among the threads in the current team. An \code{ordered}
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clause is added to the loop construct, because an ordered region binds to the loop
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region arising from the loop construct.
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According to Section 2.12.8 of the OpenMP 4.0 specification,
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a thread must not execute more than one ordered region that binds
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to the same loop region. So the \code{collapse} clause is required for the example
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to be conforming. With the \code{collapse} clause, the iterations of the \code{k}
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and \code{j} loops are collapsed into one loop, and therefore only one ordered
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region will bind to the collapsed \code{k} and \code{j} loop. Without the \code{collapse}
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clause, there would be two ordered regions that bind to each iteration of the \code{k}
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loop (one arising from the first iteration of the \code{j} loop, and the other
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arising from the second iteration of the \code{j} loop).
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The code prints
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\code{0 1 1}
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\\
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\code{0 1 2}
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\\
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\code{0 2 1}
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\\
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\code{1 2 2}
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\\
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\code{1 3 1}
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\\
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\code{1 3 2}
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\cexample[3.0]{collapse}{3}
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\fexample[3.0]{collapse}{3}
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