[gcc r17-1437] libgomp.texi: Mention ROCR_VISIBLE_DEVICES in context of USM

[Tobias Burnus]

https://gcc.gnu.org/g:b6f7035408cc08acef6f05a65d5da1d5f9e29a33

commit r17-1437-gb6f7035408cc08acef6f05a65d5da1d5f9e29a33
Author: Tobias Burnus <tburnus@baylibre.com>
Date:   Tue Jun 9 08:21:18 2026 +0200

    libgomp.texi: Mention ROCR_VISIBLE_DEVICES in context of USM
    
    Re-add half-sentence that got lost in commit r16-5799-gfe5c59ea997316.
    
    Currently, USM is only supported if all AMD GPUs support USM; hence,
    on a mixed system (e.g. CPU with GPU of type APU plus separate, discrete
    GPU not supporting USM/SVM), host fallback always happens.
    
    Workaround is to use ROCR_VISIBLE_DEVICES. Additionally, the
    "not-supported" wording should refer here to USM/SVM support not
    to XNACK, which has been clarified here.
    
    libgomp/ChangeLog:
    
            * libgomp.texi (GCN): Mention ROCR_VISIBLE_DEVICES.

Diff:
---
 libgomp/libgomp.texi | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/libgomp/libgomp.texi b/libgomp/libgomp.texi
index 7bc313a6fabf..48ebfccb3b80 100644
--- a/libgomp/libgomp.texi
+++ b/libgomp/libgomp.texi
@@ -7236,8 +7236,10 @@ The implementation remark:
       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'').
+      child processes).  If any AMD GPU device does not support unified-shared
+      memory, all AMD GPUs are removed from the list of available devices
+      (``host fallback''); consider using @code{ROCR_VISIBLE_DEVICES} to only
+      enable devices supporting it.
 @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