mirror of
https://github.com/OpenMP/Examples.git
synced 2025-04-04 05:41:33 +01:00
97 lines
4.1 KiB
TeX
97 lines
4.1 KiB
TeX
\pagebreak
|
|
\chapter{\code{target} Construct}
|
|
\label{chap:target}
|
|
|
|
\section{\code{target} Construct on \code{parallel} Construct}
|
|
|
|
This following example shows how the \code{target} construct offloads a code
|
|
region to a target device. The variables \plc{p}, \plc{v1}, \plc{v2}, and \plc{N} are implicitly mapped
|
|
to the the target device.
|
|
|
|
\cexample{target}{1c}
|
|
|
|
\fexample{target}{1f}
|
|
|
|
\section{\code{target} Construct with \code{map} Clause}
|
|
|
|
This following example shows how the \code{target} construct offloads a code
|
|
region to a target device. The variables \plc{p}, \plc{v1} and \plc{v2} are explicitly mapped to the
|
|
the target device using the map clause. The variable \plc{N} is implicitly mapped to
|
|
the target device.
|
|
|
|
\cexample{target}{2c}
|
|
|
|
\fexample{target}{2f}
|
|
|
|
\section{\code{map} Clause with \code{to}/\code{from} map-types}
|
|
|
|
The following example shows how the \code{target} construct offloads a code region
|
|
to a target device. In the \code{map} clause, the \code{to} and \code{from}
|
|
map-types define the mapping between the original (host) data and the target (device)
|
|
data. The \code{to} map-type specifies that the data will only be read on the
|
|
device, and the \code{from} map-type specifies that the data will only be written
|
|
to on the device. By specifying a guaranteed access on the device, data transfers
|
|
can be reduced for the \code{target} region.
|
|
|
|
The \code{to} map-type indicates that at the start of the \code{target} region
|
|
the variables \plc{v1} and \plc{v2} are initialized with the values of the corresponding variables
|
|
on the host device, and at the end of the \code{target} region the variables
|
|
\plc{v1} and \plc{v2} are not assigned to their corresponding variables on the host device.
|
|
|
|
The \code{from} map-type indicates that at the start of the \code{target} region
|
|
the variable \plc{p} is not initialized with the value of the corresponding variable
|
|
on the host device, and at the end of the \code{target} region the variable \plc{p}
|
|
is assigned to the corresponding variable on the host device.
|
|
|
|
\cexample{target}{3c}
|
|
|
|
The \code{to} and \code{from} map-types allow programmers to optimize data
|
|
motion. Since data for the \plc{v} arrays are not returned, and data for the \plc{p} array
|
|
are not transferred to the device, only one-half of the data is moved, compared
|
|
to the default behavior of an implicit mapping.
|
|
|
|
\fexample{target}{3f}
|
|
|
|
\section{\code{map} Clause with Array Sections}
|
|
|
|
The following example shows how the \code{target} construct offloads a code region
|
|
to a target device. In the \code{map} clause, map-types are used to optimize
|
|
the mapping of variables to the target device. Because variables \plc{p}, \plc{v1} and \plc{v2} are
|
|
pointers, array section notation must be used to map the arrays. The notation \code{:N}
|
|
is equivalent to \code{0:N}.
|
|
|
|
\cexample{target}{4c}
|
|
|
|
In C, the length of the pointed-to array must be specified. In Fortran the extent
|
|
of the array is known and the length need not be specified. A section of the array
|
|
can be specified with the usual Fortran syntax, as shown in the following example.
|
|
The value 1 is assumed for the lower bound for array section \plc{v2(:N)}.
|
|
|
|
\fexample{target}{4f}
|
|
|
|
A more realistic situation in which an assumed-size array is passed to \code{vec\_mult}
|
|
requires that the length of the arrays be specified, because the compiler does
|
|
not know the size of the storage. A section of the array must be specified with
|
|
the usual Fortran syntax, as shown in the following example. The value 1 is assumed
|
|
for the lower bound for array section \plc{v2(:N)}.
|
|
|
|
\fexample{target}{4bf}
|
|
|
|
\section{\code{target} Construct with \code{if} Clause}
|
|
|
|
The following example shows how the \code{target} construct offloads a code region
|
|
to a target device.
|
|
|
|
The \code{if} clause on the \code{target} construct indicates that if the variable
|
|
\plc{N} is smaller than a given threshold, then the \code{target} region will be executed
|
|
by the host device.
|
|
|
|
The \code{if} clause on the \code{parallel} construct indicates that if the
|
|
variable \plc{N} is smaller than a second threshold then the \code{parallel} region
|
|
is inactive.
|
|
|
|
\cexample{target}{5c}
|
|
|
|
\fexample{target}{5f}
|
|
|