diff --git a/gcc/ada/ChangeLog b/gcc/ada/ChangeLog
index bbd64ff827c..d08765cf165 100644
--- a/gcc/ada/ChangeLog
+++ b/gcc/ada/ChangeLog
@@ -1,3 +1,8 @@
+2011-09-06  Robert Dewar  <dewar@adacore.com>
+
+	* sem_ch10.adb, impunit.ads, impunit.adb (Not_Impl_Defined_Unit): New
+	name for Is_RM_Defined_Unit. Also several fixes to this unit.
+
 2011-09-06  Robert Dewar  <dewar@adacore.com>
 
 	* impunit.ads, impunit.adb (Is_RM_Defined_Unit): New function.
diff --git a/gcc/ada/impunit.adb b/gcc/ada/impunit.adb
index efcca65fef0..be1d05744f9 100644
--- a/gcc/ada/impunit.adb
+++ b/gcc/ada/impunit.adb
@@ -826,11 +826,11 @@ package body Impunit is
          return False;
    end Is_Known_Unit;
 
-   ------------------------
-   -- Is_RM_Defined_Unit --
-   ------------------------
+   ---------------------------
+   -- Not_Impl_Defined_Unit --
+   ---------------------------
 
-   function Is_RM_Defined_Unit (U : Unit_Number_Type) return Boolean is
+   function Not_Impl_Defined_Unit (U : Unit_Number_Type) return Boolean is
       Fname : constant File_Name_Type := Unit_File_Name (U);
 
    begin
@@ -848,21 +848,22 @@ package body Impunit is
          return True;
       end if;
 
-      --  If length of file name is greater than 12, not RM-defined. The value
-      --  12 here is an 8 char name with extension .ads.
+      --  If length of file name is greater than 12, then it's a user unit
+      --  and not a GNAT implementation defined unit.
 
       if Name_Len > 12 then
+         return True;
+      end if;
+
+      --  Implementation defined if unit in the gnat hierarchy
+
+      if (Name_Len = 8 and then Name_Buffer (1 .. 8) = "gnat.ads")
+        or else (Name_Len > 2 and then Name_Buffer (1 .. 2) = "g-")
+      then
          return False;
       end if;
 
-      --  Not RM-defined if length of name greater than 12 (12 is 8 characters
-      --  plus 4 for ".ads" appended at the end).
-
-      if Length_Of_Name (Fname) > 12 then
-         return False;
-      end if;
-
-      --  Not RM defined if file name does not start with a- s- i-
+      --  Not implementation defined if file name does not start with a- s- i-
 
       if Name_Len < 3
         or else Name_Buffer (2) /= '-'
@@ -872,14 +873,14 @@ package body Impunit is
                    and then
                  Name_Buffer (1) /= 's')
       then
-         return False;
+         return True;
       end if;
 
-      --  Not RM defined if file name does not end in .ads. This can happen
+      --  Not impl-defined if file name does not end in .ads. This can happen
       --  when non-standard file names are being used.
 
       if Name_Buffer (Name_Len - 3 .. Name_Len) /= ".ads" then
-         return False;
+         return True;
       end if;
 
       --  Otherwise normalize file name to 8 characters
@@ -891,7 +892,8 @@ package body Impunit is
       end loop;
 
       --  Check our lists of names, if we find a match, return corresponding
-      --  indication of whether the file is RM defined.
+      --  indication of whether the file is RM defined, respecting the RM
+      --  version in which it is defined.
 
       for J in Non_Imp_File_Names_95'Range loop
          if Name_Buffer (1 .. 8) = Non_Imp_File_Names_95 (J).Fname then
@@ -913,9 +915,11 @@ package body Impunit is
          end if;
       end loop;
 
-      --  If no match in any of the lists, not RM defined
+      --  If unit is in System, Ada or Interfaces hierarchies and did not match
+      --  any entry in the list, means it is an internal implementation defined
+      --  unit which the restriction should definition forbid.
 
-      return False;
-   end Is_RM_Defined_Unit;
+      return True;
+   end Not_Impl_Defined_Unit;
 
 end Impunit;
diff --git a/gcc/ada/impunit.ads b/gcc/ada/impunit.ads
index 31d34a9fe51..e5244938ef9 100644
--- a/gcc/ada/impunit.ads
+++ b/gcc/ada/impunit.ads
@@ -72,10 +72,14 @@ package Impunit is
    --  the known library units, and if so, returns True. If the name does not
    --  match any known library unit, False is returned.
 
-   function Is_RM_Defined_Unit (U : Unit_Number_Type) return Boolean;
-   --  This function returns True if U represents a unit that is defined in
-   --  the RM, as defined by the No_Implementation_Units restriction rules.
-   --  It is used to implement this restriction, so if False is returned, it
-   --  means that with'ing the unit violates the restriction.
+   function Not_Impl_Defined_Unit (U : Unit_Number_Type) return Boolean;
+   --  This function returns True if U represents a unit that is permitted by
+   --  the restriction No_Implementation_Units (i.e. a unit in the Ada, System,
+   --  and Interfaces hierarchies that is defined in the RM, or a user defined
+   --  unit. It returns False if U represents a unit that is not permitted by
+   --  this restriction, which includes units in these three hierarchies that
+   --  are GNAT implementation defined. It also returns False for any units in
+   --  the GNAT hierarchy, which is not strictly conforming, but so obviously
+   --  useful that it is a reasonable deviation from the standard.
 
 end Impunit;
diff --git a/gcc/ada/sem_ch10.adb b/gcc/ada/sem_ch10.adb
index 38885ee0a50..c6f18da4e54 100644
--- a/gcc/ada/sem_ch10.adb
+++ b/gcc/ada/sem_ch10.adb
@@ -2380,7 +2380,9 @@ package body Sem_Ch10 is
       --  Check No_Implementation_Units violation
 
       if Restriction_Check_Required (No_Implementation_Units) then
-         if not Is_RM_Defined_Unit (Get_Source_Unit (U)) then
+         if Not_Impl_Defined_Unit (Get_Source_Unit (U)) then
+            null;
+         else
             Check_Restriction (No_Implementation_Units, Nam);
             Restriction_Violation := True;
          end if;