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69 lines
3.8 KiB
TeX
69 lines
3.8 KiB
TeX
%\pagebreak
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\section{\kcode{defaultmap} Clause}
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\label{sec:defaultmap}
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\index{target construct@\kcode{target} construct!defaultmap clause@\kcode{defaultmap} clause}
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\index{clauses!defaultmap@\kcode{defaultmap}}
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\index{defaultmap clause@\kcode{defaultmap} clause}
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\index{defaultmap clause@\kcode{defaultmap} clause!implicit behavior}
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\index{defaultmap clause@\kcode{defaultmap} clause!variable category}
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The implicitly determined data-mapping and data-sharing attribute
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rules of variables referenced in a \kcode{target} construct can be
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changed by the \kcode{defaultmap} clause.
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As of OpenMP 5.0, the implicit behavior is specified as
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\kcode{alloc}, \kcode{to}, \kcode{from}, \kcode{tofrom},
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\kcode{firstprivate}, \kcode{none}, \kcode{default} or \kcode{present},
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and is optionally applied to a variable category specified as \kcode{scalar}, \kcode{aggregate}, \kcode{allocatable},
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or \kcode{pointer}.
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A referenced variable that is in a specified ``category'' is treated as having
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the specified implicit behavior. In C/C++, \kcode{scalar} refers to
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base-language scalar variables, except pointers. In Fortran it refers to a
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scalar variable, as defined by the base language, of intrinsic type but
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excluding the character type. The \kcode{aggregate} category refers to arrays and
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structures (which includes variables of any derived type and of character type for Fortran). Fortran
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has the additional category of \kcode{allocatable} for variables that have the
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allocatable attribute. The \kcode{pointer} category refers to pointers, which
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for Fortran are variables that have the pointer attribute.
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In the example below, the first \kcode{target} construct uses \kcode{defaultmap}
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clauses to set data-mapping and possibly data-sharing attributes that reproduce
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the default rules for implicitly determined data-mapping and data-sharing
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attributes for variables in the construct. That is, if the \kcode{defaultmap}
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clauses were removed, the results would be identical. As of OpenMP 5.2
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the same effect can now be achieved by \kcode{defaultmap(default)} with
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the \kcode{target} construct.
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In the second \kcode{target} construct all implicit behavior is removed
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by specifying the \kcode{none} implicit behavior in the \kcode{defaultmap} clause.
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Hence, all variables that don't have predetermined attributes must be given an
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explicit data-mapping or data-sharing attribute. A scalar (\ucode{s}), an
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array (\ucode{A}) and a structure (\ucode{S} for the C/C++ example and
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\ucode{D} for the Fortran example) are explicitly mapped with the
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\kcode{tofrom} map type.
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The third \kcode{target} construct shows another usual case for using the
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\kcode{defaultmap} clause. The default mapping for (non-pointer) scalar
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variables is specified. Here, the default implicit mapping for \ucode{s3} is
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\kcode{tofrom} as specified in the \kcode{defaultmap} clause, while \ucode{s1}
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and \ucode{s2} are instead explicitly treated as \kcode{firstprivate}.
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In the fourth \kcode{target} construct all arrays and structures are given
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\kcode{firstprivate} implicit behavior by default with the use of the
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\kcode{aggregate} variable category. For the Fortran example, the
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\kcode{allocatable} category is used in a separate \kcode{defaultmap} clause to
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specify default \kcode{firstprivate} implicit behavior for referenced
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allocatable variables (in this case, \ucode{H}).
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% (Common use cases for C/C++ heap storage can be found in
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% \specref{sec:pointer_mapping}.)
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The fifth \kcode{target} construct shows a case for using the
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\kcode{defaultmap} clause with the \kcode{all} variable category which was introduced in
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OpenMP 5.2. The scalar variables \ucode{s1} and \ucode{s2} are mapped \kcode{to}.
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\ucode{s3} is only mapped \kcode{from} due to the explicit map specified.
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%\pagebreak
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\cexample[5.2]{target_defaultmap}{1}
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\ffreeexample[5.2]{target_defaultmap}{1}
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