diff --git a/gcc/config/gcn/gcn.cc b/gcc/config/gcn/gcn.cc
index d6531f55190..6a83ff2a1b4 100644
--- a/gcc/config/gcn/gcn.cc
+++ b/gcc/config/gcn/gcn.cc
@@ -70,6 +70,11 @@ static bool ext_gcn_constants_init = 0;
 
 enum gcn_isa gcn_isa = ISA_GCN3;	/* Default to GCN3.  */
 
+/* Record whether the host compiler added "omp unifed memory" attributes to
+   any functions.  We can then pass this on to mkoffload to ensure xnack is
+   compatible there too.  */
+static bool unified_shared_memory_enabled = false;
+
 /* Reserve this much space for LDS (for propagating variables from
    worker-single mode to worker-partitioned mode), per workgroup.  Global
    analysis could calculate an exact bound, but we don't do that yet.
@@ -2942,6 +2947,29 @@ gcn_init_cumulative_args (CUMULATIVE_ARGS *cum /* Argument info to init */ ,
   if (!caller && cfun->machine->normal_function)
     gcn_detect_incoming_pointer_arg (fndecl);
 
+  if (fndecl && lookup_attribute ("omp unified memory",
+				  DECL_ATTRIBUTES (fndecl)))
+    {
+      unified_shared_memory_enabled = true;
+
+      switch (gcn_arch)
+	{
+	case PROCESSOR_FIJI:
+	case PROCESSOR_VEGA10:
+	case PROCESSOR_VEGA20:
+	case PROCESSOR_GFX908:
+	case PROCESSOR_GFX1030:
+	case PROCESSOR_GFX1036:
+	case PROCESSOR_GFX1100:
+	case PROCESSOR_GFX1103:
+	  error ("GPU architecture does not support Unified Shared Memory");
+	  break;
+	default:
+	  if (flag_xnack == HSACO_ATTR_OFF)
+	    error ("Unified Shared Memory is enabled, but XNACK is disabled");
+	}
+    }
+
   reinit_regs ();
 }
 
@@ -6820,12 +6848,14 @@ gcn_hsa_declare_function_name (FILE *file, const char *name,
   fputs (",@function\n", file);
   ASM_OUTPUT_FUNCTION_LABEL (file, name, decl);
 
-  /* This comment is read by mkoffload.  */
+  /* These comments are read by mkoffload.  */
   if (flag_openacc)
     fprintf (file, "\t;; OPENACC-DIMS: %d, %d, %d : %s\n",
 	     oacc_get_fn_dim_size (cfun->decl, GOMP_DIM_GANG),
 	     oacc_get_fn_dim_size (cfun->decl, GOMP_DIM_WORKER),
 	     oacc_get_fn_dim_size (cfun->decl, GOMP_DIM_VECTOR), name);
+  if (unified_shared_memory_enabled)
+    fprintf (asm_out_file, "\t;; MKOFFLOAD OPTIONS: USM+\n");
 }
 
 /* Implement TARGET_ASM_SELECT_SECTION.
diff --git a/gcc/config/gcn/mkoffload.cc b/gcc/config/gcn/mkoffload.cc
index 810298a799b..3dcb6943c45 100644
--- a/gcc/config/gcn/mkoffload.cc
+++ b/gcc/config/gcn/mkoffload.cc
@@ -487,6 +487,7 @@ process_asm (FILE *in, FILE *out, FILE *cfile)
 {
   int fn_count = 0, var_count = 0, ind_fn_count = 0;
   int dims_count = 0, regcount_count = 0;
+  bool unified_shared_memory_enabled = false;
   struct obstack fns_os, dims_os, regcounts_os;
   obstack_init (&fns_os);
   obstack_init (&dims_os);
@@ -511,6 +512,7 @@ process_asm (FILE *in, FILE *out, FILE *cfile)
   fn_count += 2;
 
   char buf[1000];
+  char dummy;
   enum
     { IN_CODE,
       IN_METADATA,
@@ -531,6 +533,9 @@ process_asm (FILE *in, FILE *out, FILE *cfile)
 		dims_count++;
 	      }
 
+	    if (sscanf (buf, " ;; MKOFFLOAD OPTIONS: USM+%c", &dummy) > 0)
+	      unified_shared_memory_enabled = true;
+
 	    break;
 	  }
 	case IN_METADATA:
@@ -591,7 +596,6 @@ process_asm (FILE *in, FILE *out, FILE *cfile)
 	  }
 	}
 
-      char dummy;
       if (sscanf (buf, " .section .gnu.offload_vars%c", &dummy) > 0)
 	{
 	  state = IN_VARS;
@@ -660,6 +664,7 @@ process_asm (FILE *in, FILE *out, FILE *cfile)
   fprintf (cfile, "#include <stdlib.h>\n");
   fprintf (cfile, "#include <stdint.h>\n");
   fprintf (cfile, "#include <stdbool.h>\n\n");
+  fprintf (cfile, "#include <stdio.h>\n\n");
 
   fprintf (cfile, "static const int gcn_num_vars = %d;\n\n", var_count);
   fprintf (cfile, "static const int gcn_num_ind_funcs = %d;\n\n", ind_fn_count);
@@ -713,6 +718,34 @@ process_asm (FILE *in, FILE *out, FILE *cfile)
 	     "}\n\n",
 	     gcn_stack_size);
 
+  /* Emit a constructor function to set the HSA_XNACK environment variable.
+     This must be done before the ROCr runtime library is loaded.
+     We never override a user value (exit empty string), but we do emit a
+     useful diagnostic in the wrong mode (the ROCr message is not good.  */
+  if (TEST_XNACK_OFF (elf_flags) && unified_shared_memory_enabled)
+    fatal_error (input_location,
+		 "conflicting settings; XNACK is forced off but Unified "
+		 "Shared Memory is on");
+  if (!TEST_XNACK_ANY (elf_flags) || unified_shared_memory_enabled)
+    fprintf (cfile,
+	     "static __attribute__((constructor))\n"
+	     "void configure_xnack (void)\n"
+	     "{\n"
+	     "  const char *val = getenv (\"HSA_XNACK\");\n"
+	     "  if (!val || val[0] == '\\0')\n"
+	     "    setenv (\"HSA_XNACK\", \"%d\", true);\n"
+	     "  else if (%s)\n"
+	     "    {\n"
+	     "      fprintf (stderr, \"error: HSA_XNACK=%%s is incompatible; "
+			    "please unset\\n\", val);\n"
+	     "      exit (1);\n"
+	     "    }\n"
+	     "}\n\n",
+	     unified_shared_memory_enabled || TEST_XNACK_ON (elf_flags),
+	     (unified_shared_memory_enabled || TEST_XNACK_ON (elf_flags)
+	      ? "val[0] != '1' || val[1] != '\\0'"
+	      : "val[0] == '1' && val[1] == '\\0'"));
+
   obstack_free (&fns_os, NULL);
   for (i = 0; i < dims_count; i++)
     free (dims[i].name);
diff --git a/gcc/omp-low.cc b/gcc/omp-low.cc
index d3f9ccc4567..b21b48280f2 100644
--- a/gcc/omp-low.cc
+++ b/gcc/omp-low.cc
@@ -2124,6 +2124,10 @@ create_omp_child_function (omp_context *ctx, bool task_copy)
 	DECL_ATTRIBUTES (decl)
 	  = tree_cons (get_identifier (target_attr),
 		       NULL_TREE, DECL_ATTRIBUTES (decl));
+      if (omp_requires_mask & OMP_REQUIRES_UNIFIED_SHARED_MEMORY)
+	DECL_ATTRIBUTES (decl)
+	  = tree_cons (get_identifier ("omp unified memory"),
+		       NULL_TREE, DECL_ATTRIBUTES (decl));
     }
 
   t = build_decl (DECL_SOURCE_LOCATION (decl),
