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Examples_declare_target.tex
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Examples_declare_target.tex
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\pagebreak
\chapter{\code{declare} \code{target} Construct}
\label{chap:declare_target}
\section{\code{declare} \code{target} and \code{end} \code{declare} \code{target} for a Function}
The following example shows how the \code{declare} \code{target} directive
is used to indicate that the corresponding call inside a \code{target} region
is to a \code{fib} function that can execute on the default target device.
A version of the function is also available on the host device. When the \code{if}
clause conditional expression on the \code{target} construct evaluates to \plc{false},
the \code{target} region (thus \code{fib}) will execute on the host device.
For C/C++ codes the declaration of the function \code{fib} appears between the \code{declare}
\code{target} and \code{end} \code{declare} \code{target} directives.
\cexample{declare_target}{1c}
The Fortran \code{fib} subroutine contains a \code{declare} \code{target} declaration
to indicate to the compiler to create an device executable version of the procedure.
The subroutine name has not been included on the \code{declare} \code{target}
directive and is, therefore, implicitly assumed.
The program uses the \code{module\_fib} module, which presents an explicit interface to
the compiler with the \code{declare} \code{target} declarations for processing
the \code{fib} call.
\fexample{declare_target}{1f}
The next Fortran example shows the use of an external subroutine. Without an explicit
interface (through module use or an interface block) the \code{declare} \code{target}
declarations within a external subroutine are unknown to the main program unit;
therefore, a \code{declare} \code{target} must be provided within the program
scope for the compiler to determine that a target binary should be available.
\fexample{declare_target}{2f}
\section{\code{declare} \code{target} Construct for Class Type}
\cppspecificstart
The following example shows how the \code{declare} \code{target} and \code{end}
\code{declare} \code{target} directives are used to enclose the declaration
of a variable \plc{varY} with a class type \code{typeY}. The member function \code{typeY::foo()} cannot
be accessed on a target device because its declaration did not appear between \code{declare}
\code{target} and \code{end} \code{declare} \code{target} directives.
\cnexample{declare_target}{2c}
\cppspecificend
\section{\code{declare} \code{target} and \code{end} \code{declare} \code{target} for Variables}
The following examples show how the \code{declare} \code{target} and \code{end}
\code{declare} \code{target} directives are used to indicate that global variables
are mapped to the implicit device data environment of each target device.
In the following example, the declarations of the variables \plc{p}, \plc{v1}, and \plc{v2} appear
between \code{declare} \code{target} and \code{end} \code{declare} \code{target}
directives indicating that the variables are mapped to the implicit device data
environment of each target device. The \code{target} \code{update} directive
is then used to manage the consistency of the variables \plc{p}, \plc{v1}, and \plc{v2} between the
data environment of the encountering host device task and the implicit device data
environment of the default target device.
\cexample{declare_target}{3c}
The Fortran version of the above C code uses a different syntax. Fortran modules
use a list syntax on the \code{declare} \code{target} directive to declare
mapped variables.
\fexample{declare_target}{3f}
The following example also indicates that the function \code{Pfun()} is available on the
target device, as well as the variable \plc{Q}, which is mapped to the implicit device
data environment of each target device. The \code{target} \code{update} directive
is then used to manage the consistency of the variable \plc{Q} between the data environment
of the encountering host device task and the implicit device data environment of
the default target device.
In the following example, the function and variable declarations appear between
the \code{declare} \code{target} and \code{end} \code{declare} \code{target}
directives.
\cexample{declare_target}{4c}
The Fortran version of the above C code uses a different syntax. In Fortran modules
a list syntax on the \code{declare} \code{target} directive is used to declare
mapped variables and procedures. The \plc{N} and \plc{Q} variables are declared as a comma
separated list. When the \code{declare} \code{target} directive is used to
declare just the procedure, the procedure name need not be listed -- it is implicitly
assumed, as illustrated in the \code{Pfun()} function.
\fexample{declare_target}{4f}
\section{\code{declare} \code{target} and \code{end} \code{declare} \code{target} with \code{declare} \code{simd}}
The following example shows how the \code{declare} \code{target} and \code{end}
\code{declare} \code{target} directives are used to indicate that a function
is available on a target device. The \code{declare} \code{simd} directive indicates
that there is a SIMD version of the function \code{P()} that is available on the target
device as well as one that is available on the host device.
\cexample{declare_target}{5c}
The Fortran version of the above C code uses a different syntax. Fortran modules
use a list syntax of the \code{declare} \code{target} declaration for the mapping.
Here the \plc{N} and \plc{Q} variables are declared in the list form as a comma separated list.
The function declaration does not use a list and implicitly assumes the function
name. In this Fortran example row and column indices are reversed relative to the
C/C++ example, as is usual for codes optimized for memory access.
\fexample{declare_target}{5f}