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libgomp, amdgcn: document HSA_XNACK

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Mention that the HSA_XNACK variable is automatically set by the toolchain.

libgomp/ChangeLog:

	* libgomp.texi (AMD GCN): Mention HSA_XNACK is set automatically.
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Andrew Stubbs
2025-11-28 16:20:46 +00:00
parent 1927a43b1e
commit fe5c59ea99
1 changed files with 8 additions and 7 deletions
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libgomp/libgomp.texi
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@@ -7157,13 +7157,14 @@ The implementation remark:
such that the next reverse offload region is only executed after the previous
one returned.
@item OpenMP code that has a @code{requires} directive with @code{self_maps} or
@code{unified_shared_memory} is only supported if all AMD GPUs have the
@code{HSA_AMD_SYSTEM_INFO_SVM_ACCESSIBLE_BY_DEFAULT} property; for
discrete GPUs, this may require setting the @code{HSA_XNACK} environment
variable to @samp{1}; for systems with both an APU and a discrete GPU that
does not support XNACK, consider using @code{ROCR_VISIBLE_DEVICES} to
enable only the APU. If not supported, all AMD GPU devices are removed
from the list of available devices (``host fallback'').
@code{unified_shared_memory} is only supported if @emph{all} the AMD GPUs
present have the @code{HSA_AMD_SYSTEM_INFO_SVM_ACCESSIBLE_BY_DEFAULT}
property; some systems require the "xnack" feature enabled for this to be
true, in which case the runtime will attempt to set the @code{HSA_XNACK}
environment variable to @samp{1} automatically (user-set values are not
overridden, and the setting only affects the executable itself and any
child processes). If any AMD GPU device is not supported, all AMD GPUs
are removed from the list of available devices (``host fallback'').
@item The available stack size can be changed using the @code{GCN_STACK_SIZE}
environment variable; the default is 32 kiB per thread.
@item Low-latency memory (@code{omp_low_lat_mem_space}) is supported when the