ada: Introduce Cannot_Be_Superflat flag on N_Range nodes

The support of superflat arrays in the language generates an overhead that
the code generator attempts to minimize, but it cannot handle too complex
cases and it would be helpful if the front-end could lend a hand.

This change introduces the Cannot_Be_Superflat flag on N_Range nodes for
this purpose, and sets it on the result of string concatenations when it
is guaranteed to be nonnull.

gcc/ada/

	* gen_il-fields.ads (Opt_Field_Enum): Add Cannot_Be_Superflat.
	* gen_il-gen-gen_nodes.adb (N_Range): Add Cannot_Be_Superflat as
	semantical flag and change Includes_Infinities to semantical.
	* sinfo.ads (Cannot_Be_Superflat): Document it for N_Range.
	* exp_ch4.adb (Expand_Concatenate): Set Cannot_Be_Superflat on the
	range of the result if the result cannot be null.

gcc/ada/exp_ch4.adb

@@ -2536,7 +2536,7 @@ package body Exp_Ch4 is
       --  Reset to False if at least one operand is encountered which is known
       --  at compile time to be non-null. Used for handling the special case
       --  of setting the high bound to the last operand high bound for a null
-      --  result, thus ensuring a proper high bound in the super-flat case.
+      --  result, thus ensuring a proper high bound in the superflat case.
 
       N : constant Nat := List_Length (Opnds);
       --  Number of concatenation operands including possibly null operands
@@ -2726,8 +2726,9 @@ package body Exp_Ch4 is
       --  Local Declarations
 
       Opnd_Typ   : Entity_Id;
-      Slice_Rng  : Entity_Id;
-      Subtyp_Ind : Entity_Id;
+      Slice_Rng  : Node_Id;
+      Subtyp_Ind : Node_Id;
+      Subtyp_Rng : Node_Id;
       Ent        : Entity_Id;
       Len        : Unat;
       J          : Nat;
@@ -3184,7 +3185,7 @@ package body Exp_Ch4 is
 
       --  Handle the exceptional case where the result is null, in which case
       --  case the bounds come from the last operand (so that we get the proper
-      --  bounds if the last operand is super-flat).
+      --  bounds if the last operand is superflat).
 
       if Result_May_Be_Null then
          Low_Bound :=
@@ -3239,6 +3240,12 @@ package body Exp_Ch4 is
          Slice_Rng := Empty;
       end if;
 
+      Subtyp_Rng := Make_Range (Loc, Low_Bound, High_Bound);
+
+      --  If the result cannot be null then the range cannot be superflat
+
+      Set_Cannot_Be_Superflat (Subtyp_Rng, not Result_May_Be_Null);
+
       --  Now we construct an array object with appropriate bounds. We mark
       --  the target as internal to prevent useless initialization when
       --  Initialize_Scalars is enabled. Also since this is the actual result
@@ -3249,10 +3256,7 @@ package body Exp_Ch4 is
           Subtype_Mark => New_Occurrence_Of (Atyp, Loc),
           Constraint   =>
             Make_Index_Or_Discriminant_Constraint (Loc,
-              Constraints => New_List (
-                Make_Range (Loc,
-                  Low_Bound  => Low_Bound,
-                  High_Bound => High_Bound))));
+              Constraints => New_List (Subtyp_Rng)));
 
       Ent := Make_Temporary (Loc, 'S');
       Set_Is_Internal       (Ent);

gcc/ada/gen_il-fields.ads

@@ -87,6 +87,7 @@ package Gen_IL.Fields is
       Body_Required,
       Body_To_Inline,
       Box_Present,
+      Cannot_Be_Superflat,
       Char_Literal_Value,
       Chars,
       Check_Address_Alignment,

gcc/ada/gen_il-gen-gen_nodes.adb

@@ -531,7 +531,8 @@ begin -- Gen_IL.Gen.Gen_Nodes
    Cc (N_Range, N_Subexpr,
        (Sy (Low_Bound, Node_Id),
         Sy (High_Bound, Node_Id),
-        Sy (Includes_Infinities, Flag)));
+        Sm (Cannot_Be_Superflat, Flag),
+        Sm (Includes_Infinities, Flag)));
 
    Cc (N_Reference, N_Subexpr,
        (Sy (Prefix, Node_Id)));

gcc/ada/sinfo.ads

@@ -932,6 +932,12 @@ package Sinfo is
    --    a pragma Import or Interface applies, in which case no body is
    --    permitted (in Ada 83 or Ada 95).
 
+   --  Cannot_Be_Superflat
+   --    This flag is present in N_Range nodes. It is set if the range is of a
+   --    discrete type and cannot be superflat, i.e. it is guaranteed that the
+   --    inequality High_Bound >= Low_Bound - 1 is true. At the time of this
+   --    writing, it is only used by the code generator to streamline things.
+
    --  Cleanup_Actions
    --    Present in block statements created for transient blocks, contains
    --    additional cleanup actions carried over from the transient scope.
@@ -3081,6 +3087,7 @@ package Sinfo is
       --  Sloc points to ..
       --  Low_Bound
       --  High_Bound
+      --  Cannot_Be_Superflat
       --  Includes_Infinities
       --  plus fields for expression