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80 lines
3.2 KiB
TeX
80 lines
3.2 KiB
TeX
\cchapter{Devices}{devices}
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\label{chap:devices}
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The \kcode{target} construct consists of a \kcode{target} directive
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and an execution region. The \kcode{target} region is executed on
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the default device or the device specified in the \kcode{device}
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clause.
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In OpenMP version 4.0, by default, all variables within the lexical
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scope of the construct are copied \plc{to} and \plc{from} the
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device, unless the device is the host, or the data exists on the
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device from a previously executed data-type construct that
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has created space on the device and possibly copied host
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data to the device storage.
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The constructs that explicitly
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create storage, transfer data, and free storage on the device
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are categorized as structured and unstructured. The
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\kcode{target data} construct is structured. It creates
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a data region around \kcode{target} constructs, and is
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convenient for providing persistent data throughout multiple
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\kcode{target} regions. The \kcode{target enter data} and
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\kcode{target exit data} constructs are unstructured, because
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they can occur anywhere and do not support a ``structure''
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(a region) for enclosing \kcode{target} constructs, as does the
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\kcode{target data} construct.
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The \kcode{map} clause is used on \kcode{target}
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constructs and the data-type constructs to map host data. It
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specifies the device storage and data movement \plc{to} and \plc{from}
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the device, and controls on the storage duration.
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There is an important change in the OpenMP 4.5 specification
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that alters the data model for scalar variables and C/C++ pointer variables.
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The default behavior for scalar variables and C/C++ pointer variables
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in a 4.5 compliant code is \kcode{firstprivate}. Example
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codes that have been updated to reflect this new behavior are
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annotated with a description that describes changes required
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for correct execution. Often it is a simple matter of mapping
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the variable as \kcode{tofrom} to obtain the intended 4.0 behavior.
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In OpenMP version 4.5 the mechanism for target
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execution is specified as occurring through a \plc{target task}.
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When the \kcode{target} construct is encountered a new
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target task is generated. The target task
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completes after the \kcode{target} region has executed and all data
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transfers have finished.
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This new specification does not affect the execution of
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pre-4.5 code; it is a necessary element for asynchronous
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execution of the \kcode{target} region when using the new \kcode{nowait}
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clause introduced in OpenMP 4.5.
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%===== Examples Sections =====
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\input{devices/target}
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\input{devices/target_defaultmap}
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\input{devices/target_pointer_mapping}
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\input{devices/target_structure_mapping}
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\input{devices/target_fort_allocatable_array_mapping}
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\input{devices/array_sections}
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\input{devices/usm}
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\input{devices/C++_virtual_functions}
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\input{devices/array_shaping}
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\input{devices/target_mapper}
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\input{devices/target_data}
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\input{devices/target_unstructured_data}
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\input{devices/target_update}
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\input{devices/declare_target}
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\input{devices/lambda_expressions}
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\input{devices/teams}
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\input{devices/async_target_depend}
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\input{devices/async_target_with_tasks}
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\input{devices/async_target_nowait}
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\input{devices/async_target_nowait_depend}
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\input{devices/async_target_nowait_arg}
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\input{devices/device}
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\input{devices/device_env_traits}
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