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49 lines
2.1 KiB
TeX
49 lines
2.1 KiB
TeX
%\pagebreak
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\section{Target Offload}
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\label{sec:target_offload}
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\index{environment variables!OMP_TARGET_OFFLOAD@\kcode{OMP_TARGET_OFFLOAD}}
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\index{OMP_TARGET_OFFLOAD@\kcode{OMP_TARGET_OFFLOAD}}
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In the OpenMP 5.0 implementation the \kcode{OMP_TARGET_OFFLOAD}
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environment variable was defined to change default offload behavior.
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By default the target code (region) is executed on the host if the target device
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does not exist or the implementation does not support the target device.
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%Last sentence uses words of the 5.0 spec pg. 21 lines 7-8
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In an OpenMP 5.0 compliant implementation, setting the
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\kcode{OMP_TARGET_OFFLOAD} variable to \vcode{MANDATORY} will
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force the program to terminate execution when a \kcode{target}
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construct is encountered and the target device is not supported or is not available.
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With a value \vcode{DEFAULT} the target region will execute on a device if the
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device exists and is supported by the implementation,
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otherwise it will execute on the host.
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Support for the \vcode{DISABLED}
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value is optional; when it is supported the behavior is as if only the
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host device exists (other devices are considered non-existent to the runtime),
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and target regions are executed on the host.
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The following example reports execution behavior for different
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values of the \kcode{OMP_TARGET_OFFLOAD} variable. A handy routine
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for extracting the \kcode{OMP_TARGET_OFFLOAD} environment variable
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value is deployed here, because the OpenMP API does not have a routine
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for obtaining the value. %(\texit{yet}).
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Note:
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The example issues a warning when a pre-5.0 implementation is used,
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indicating that the \kcode{OMP_TARGET_OFFLOAD} is ignored.
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The value of the \kcode{OMP_TARGET_OFFLOAD} variable is reported
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when the \kcode{OMP_DISPLAY_ENV}
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environment variable is set to \vcode{TRUE} or \vcode{VERBOSE}.
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%\pagebreak
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\cexample[5.0]{target_offload_control}{1}[1]
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%\pagebreak
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\ffreeexample[5.0]{target_offload_control}{1}[1]
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% OMP 4.5 target offload 15:9-11
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%If the target device does not exist or the
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%implementation does not support the target device, all target regions associated with that device
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%execute on the host device.
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