[gcc r16-5799] libgomp, amdgcn: document HSA_XNACK

[Andrew Stubbs]

https://gcc.gnu.org/g:fe5c59ea997316da471de29f5720b650114035e9

commit r16-5799-gfe5c59ea997316da471de29f5720b650114035e9
Author: Andrew Stubbs <ams@baylibre.com>
Date:   Fri Nov 28 16:20:46 2025 +0000

    libgomp, amdgcn: document HSA_XNACK
    
    Mention that the HSA_XNACK variable is automatically set by the toolchain.
    
    libgomp/ChangeLog:
    
            * libgomp.texi (AMD GCN): Mention HSA_XNACK is set automatically.

Diff:
---
 libgomp/libgomp.texi | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/libgomp/libgomp.texi b/libgomp/libgomp.texi
index abe8ef75d8e7..ac96d2f29c76 100644
--- a/libgomp/libgomp.texi
+++ b/libgomp/libgomp.texi
@@ -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