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96 lines
4.9 KiB
TeX
96 lines
4.9 KiB
TeX
\cchapter{Data Environment}{data_environment}
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\label{chap:data_environment}
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The OpenMP \plc{data environment} contains data attributes of variables and
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objects. Many constructs (such as \kcode{parallel}, \kcode{simd}, \kcode{task})
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accept clauses to control \plc{data-sharing} attributes
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of referenced variables in the construct, where \plc{data-sharing} applies to
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whether the attribute of the variable is \plc{shared},
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is \plc{private} storage, or has special operational characteristics
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(as found in the \kcode{firstprivate}, \kcode{lastprivate}, \kcode{linear}, or \kcode{reduction} clause).
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The data environment for a device (distinguished as a \plc{device data environment})
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is controlled on the host by \plc{data-mapping} attributes, which determine the
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relationship of the data on the host, the \plc{original} data, and the data on the
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device, the \plc{corresponding} data.
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\bigskip
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DATA-SHARING ATTRIBUTES
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Data-sharing attributes of variables can be classified as being \plc{predetermined},
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\plc{explicitly determined} or \plc{implicitly determined}.
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Certain variables and objects have predetermined attributes.
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A commonly found case is the loop iteration variable in associated loops
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of a \kcode{for} or \kcode{do} construct. It has a private data-sharing attribute.
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Variables with predetermined data-sharing attributes cannot be listed in a data-sharing clause; but there are some
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exceptions (mainly concerning loop iteration variables).
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Variables with explicitly determined data-sharing attributes are those that are
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referenced in a given construct and are listed in a data-sharing attribute
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clause on the construct. Some of the common data-sharing clauses are:
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\kcode{shared}, \kcode{private}, \kcode{firstprivate}, \kcode{lastprivate},
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\kcode{linear}, and \kcode{reduction}. % Are these all of them?
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Variables with implicitly determined data-sharing attributes are those
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that are referenced in a given construct, do not have predetermined
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data-sharing attributes, and are not listed in a data-sharing
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attribute clause of an enclosing construct.
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For a complete list of variables and objects with predetermined and
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implicitly determined attributes, please refer to the
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\docref{Data-sharing Attribute Rules for Variables Referenced in a Construct}
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subsection of the OpenMP Specifications document.
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\bigskip
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DATA-MAPPING ATTRIBUTES
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The \kcode{map} clause on a device construct explicitly specifies how the list items in
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the clause are mapped from the encountering task's data environment (on the host)
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to the corresponding item in the device data environment (on the device).
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The common \plc{list items} are arrays, array sections, scalars, pointers, and
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structure elements (members).
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Procedures and global variables have predetermined data mapping if they appear
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within the list or block of a \kcode{declare target} directive. Also, a C/C++ pointer
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is mapped as a zero-length array section, as is a C++ variable that is a reference to a pointer.
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% Waiting for response from Eric on this.
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Without explicit mapping, non-scalar and non-pointer variables within the scope of the \kcode{target}
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construct are implicitly mapped with a \plc{map-type} of \kcode{tofrom}.
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Without explicit mapping, scalar variables within the scope of the \kcode{target}
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construct are not mapped, but have an implicit firstprivate data-sharing
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attribute. (That is, the value of the original variable is given to a private
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variable of the same name on the device.) This behavior can be changed with
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the \kcode{defaultmap} clause.
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The \kcode{map} clause can appear on \kcode{target}, \kcode{target data} and
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\kcode{target enter/exit data} constructs. The operations of creation and
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removal of device storage as well as assignment of the original list item
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values to the corresponding list items may be complicated when the list
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item appears on multiple constructs or when the host and device storage
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is shared. In these cases the item's reference count, the number of times
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it has been referenced (increment by 1 on entry and decrement by 1 on exit) in nested (structured)
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map regions and/or accumulative (unstructured) mappings, determines the operation.
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Details of the \kcode{map} clause and reference count operation are specified
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in the \docref{\kcode{map} Clause} subsection of the OpenMP Specifications document.
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%===== Examples Sections =====
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\input{data_environment/threadprivate}
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\input{data_environment/default_none}
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\input{data_environment/private}
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\input{data_environment/fort_loopvar}
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\input{data_environment/fort_sp_common}
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\input{data_environment/fort_sa_private}
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\input{data_environment/fort_shared_var}
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\input{data_environment/carrays_fpriv}
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\input{data_environment/lastprivate}
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\input{data_environment/reduction}
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\input{data_environment/udr}
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\input{data_environment/induction}
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\input{data_environment/scan}
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\input{data_environment/copyin}
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\input{data_environment/copyprivate}
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\input{data_environment/cpp_reference}
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\input{data_environment/associate}
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