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52 lines
2.9 KiB
TeX
52 lines
2.9 KiB
TeX
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\section{Fortran Comments (Free Source Form)}
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\label{sec:fortran_free_format_comments}
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\index{directive syntax!free form, Fortran}
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\index{free form syntax, Fortran}
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OpenMP directives in Fortran codes with free source form are specified as comments
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that use the \code{!\$omp} sentinel, followed by the
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directive name, and required and optional clauses. Lines are continued with an ending ampersand (\code{\&}),
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and the continued line begins with \code{!\$omp} or \code{!\$omp\&}. Comments may appear on the
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same line as the directive. Directives are case insensitive.
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In the example below the first directive (DIR 1) specifies the %parallel work-sharing
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\code{parallel}~\code{do} combined directive, with a \code{num\_threads} clause, and a comment.
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The second directive (DIR 2) shows the same directive split across two lines.
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The next nested directives (DIR 3 and 4) show the previous combined directive as
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two separate directives.
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Here, an \code{end} directive (\code{end}~\code{parallel}) must be specified to demarcate the range (region)
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of the \code{parallel} directive.
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\ffreeexample{directive_syntax_F_free_comment}{1}
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\clearpage
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As of OpenMP 5.1, \code{block} and \code{end}~\code{block} statements can be used to designate
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a structured block for an OpenMP region, and any paired OpenMP \code{end} directive becomes optional,
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as shown in the next example. Note, the variables \plc{i} and \plc{thrd\_no} are declared within the
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block structure and are hence private.
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It was necessary to explicitly declare the \plc{i} variable, due to the \code{implicit none} statement;
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it could have also been declared outside the structured block.
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\ffreeexample[5.1]{directive_syntax_F_block}{1}
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A Fortran BLOCK construct may eliminate the need for a paired \scode{end} directive for an OpenMP construct,
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as illustrated in the following example.
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The first \code{parallel} construct is specified with an OpenMP loosely structured block
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(where the first executable construct is not a Fortran 2008 BLOCK construct).
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A paired \scode{end} directive must end the OpenMP construct.
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The second \code{parallel} construct is specified with an OpenMP strictly structured block
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(consists only of a single Fortran BLOCK construct).
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The paired \scode{end} directive is optional in this case, and is not used here.
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The next two \code{parallel} directives form an enclosing outer \code{parallel} construct
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and a nested inner \code{parallel} construct. The first \code{end}~\code{parallel} directive
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that subsequently appears terminates the inner \code{parallel} construct,
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because a paired \scode{end} directive immediately following a BLOCK construct that is
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a strictly structured block of an OpenMP construct is treated as the terminating end directive
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of that construct.
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The next \code{end}~\code{parallel} directive is required to terminate the outer \code{parallel} construct.
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\ffreeexample[5.1]{directive_syntax_F_block}{2}
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