Index: c/c-parser.c
===================================================================
--- c/c-parser.c	(revision 238425)
+++ c/c-parser.c	(working copy)
@@ -301,6 +301,9 @@ c_lex_one_token (c_parser *parser, c_tok
 	    else if (rid_code >= RID_FIRST_ADDR_SPACE
 		     && rid_code <= RID_LAST_ADDR_SPACE)
 	      {
+		addr_space_t as;
+		as = (addr_space_t) (rid_code - RID_FIRST_ADDR_SPACE);
+		targetm.addr_space.diagnose_usage (as, token->location);
 		token->id_kind = C_ID_ADDRSPACE;
 		token->keyword = rid_code;
 		break;
Index: doc/tm.texi
===================================================================
--- doc/tm.texi	(revision 238425)
+++ doc/tm.texi	(working copy)
@@ -10431,6 +10431,17 @@ Define this to define how the address sp
 The result is the value to be used with @code{DW_AT_address_class}.
 @end deftypefn
 
+@deftypefn {Target Hook} void TARGET_ADDR_SPACE_DIAGNOSE_USAGE (addr_space_t @var{as}, location_t @var{loc})
+Define this hook if the availability of an address space depends on
+command line options and some diagnostics should be printed when the
+address space is used.  This hook is called during parsing and allows
+to emit a better diagnostic compared to the case where the address space
+was not registered with @code{c_register_addr_space}.  @var{as} is
+the address space as registered with @code{c_register_addr_space}.
+@var{loc} is the location of the address space qualifier token.
+The default implementation does nothing.
+@end deftypefn
+
 @node Misc
 @section Miscellaneous Parameters
 @cindex parameters, miscellaneous
Index: doc/tm.texi.in
===================================================================
--- doc/tm.texi.in	(revision 238425)
+++ doc/tm.texi.in	(working copy)
@@ -7486,6 +7486,8 @@ c_register_addr_space ("__ea", ADDR_SPAC
 
 @hook TARGET_ADDR_SPACE_DEBUG
 
+@hook TARGET_ADDR_SPACE_DIAGNOSE_USAGE
+
 @node Misc
 @section Miscellaneous Parameters
 @cindex parameters, miscellaneous
Index: target.def
===================================================================
--- target.def	(revision 238425)
+++ target.def	(working copy)
@@ -3241,6 +3241,20 @@ The result is the value to be used with
  int, (addr_space_t as),
  default_addr_space_debug)
 
+/* Function to emit custom diagnostic if an address space is used.  */
+DEFHOOK
+(diagnose_usage,
+ "Define this hook if the availability of an address space depends on\n\
+command line options and some diagnostics should be printed when the\n\
+address space is used.  This hook is called during parsing and allows\n\
+to emit a better diagnostic compared to the case where the address space\n\
+was not registered with @code{c_register_addr_space}.  @var{as} is\n\
+the address space as registered with @code{c_register_addr_space}.\n\
+@var{loc} is the location of the address space qualifier token.\n\
+The default implementation does nothing.",
+ void, (addr_space_t as, location_t loc),
+ default_addr_space_diagnose_usage)
+
 HOOK_VECTOR_END (addr_space)
 
 #undef HOOK_PREFIX
Index: targhooks.c
===================================================================
--- targhooks.c	(revision 238425)
+++ targhooks.c	(working copy)
@@ -1291,6 +1291,15 @@ default_addr_space_debug (addr_space_t a
   return as;
 }
 
+/* The default hook implementation for TARGET_ADDR_SPACE_DIAGNOSE_USAGE.
+   Don't complain about any address space.  */
+
+void
+default_addr_space_diagnose_usage (addr_space_t, location_t)
+{
+}
+	 
+
 /* The default hook for TARGET_ADDR_SPACE_CONVERT. This hook should never be
    called for targets with only a generic address space.  */
 
Index: targhooks.h
===================================================================
--- targhooks.h	(revision 238425)
+++ targhooks.h	(working copy)
@@ -181,6 +181,7 @@ extern rtx default_addr_space_legitimize
 extern bool default_addr_space_subset_p (addr_space_t, addr_space_t);
 extern bool default_addr_space_zero_address_valid (addr_space_t);
 extern int default_addr_space_debug (addr_space_t);
+extern void default_addr_space_diagnose_usage (addr_space_t, location_t);
 extern rtx default_addr_space_convert (rtx, tree, tree);
 extern unsigned int default_case_values_threshold (void);
 extern bool default_have_conditional_execution (void);
