Implement and document -z unique / -z nounique handling in gold

* layout.cc (Layout::finish_dynamic_section): Set DF_GNU_1_UNIQUE.
	* options.h (class General_options): Handle -z unique, -z nounique.

gold/ChangeLog

@@ -1,3 +1,9 @@
+2020-12-15  Vivek Das Mohapatra  <vivek@collabora.com>
+
+	Implement -z unique / -z nounique options.
+	* layout.cc (Layout::finish_dynamic_section): Set DF_GNU_1_UNIQUE.
+	* options.h (class General_options): Handle -z unique, -z nounique.
+
 2020-12-13  Cary Coutant  <ccoutant@gmail.com>
 
 	PR gold/23539

gold/layout.cc

@@ -5371,6 +5371,12 @@ Layout::finish_dynamic_section(const Input_objects* input_objects,
     flags |= elfcpp::DF_1_PIE;
   if (flags != 0)
     odyn->add_constant(elfcpp::DT_FLAGS_1, flags);
+
+  flags = 0;
+  if (parameters->options().unique())
+    flags |= elfcpp::DF_GNU_1_UNIQUE;
+  if (flags != 0)
+    odyn->add_constant(elfcpp::DT_GNU_FLAGS_1, flags);
 }
 
 // Set the size of the _DYNAMIC symbol table to be the size of the

gold/options.h

@@ -1469,6 +1469,9 @@ class General_options
   DEFINE_bool(interpose, options::DASH_Z, '\0', false,
 	      N_("Mark object to interpose all DSOs but executable"),
 	      NULL);
+  DEFINE_bool(unique, options::DASH_Z, '\0', false,
+	      N_("Mark DSO to be loaded at most once, and only in the main namespace"),
+	      N_("Do not mark the DSO as one to be loaded only in the main namespace"));
   DEFINE_bool_alias(lazy, now, options::DASH_Z, '\0',
 		    N_("Mark object for lazy runtime binding"),
 		    NULL, true);