Index: sem_prag.adb
===================================================================
--- sem_prag.adb	(revision 164088)
+++ sem_prag.adb	(working copy)
@@ -4907,8 +4907,8 @@ package body Sem_Prag is
       --  form created by the parser.
 
       procedure Set_Mechanism_Value (Ent : Entity_Id; Mech_Name : Node_Id) is
-         Class : Node_Id;
-         Param : Node_Id;
+         Class        : Node_Id;
+         Param        : Node_Id;
          Mech_Name_Id : Name_Id;
 
          procedure Bad_Class;
@@ -4957,7 +4957,15 @@ package body Sem_Prag is
 
             elsif Chars (Mech_Name) = Name_Descriptor then
                Check_VMS (Mech_Name);
-               Set_Mechanism (Ent, By_Descriptor);
+
+               --  Descriptor => Short_Descriptor if pragma was given
+
+               if Short_Descriptors then
+                  Set_Mechanism (Ent, By_Short_Descriptor);
+               else
+                  Set_Mechanism (Ent, By_Descriptor);
+               end if;
+
                return;
 
             elsif Chars (Mech_Name) = Name_Short_Descriptor then
@@ -4980,7 +4988,6 @@ package body Sem_Prag is
          --  Note: this form is parsed as an indexed component
 
          elsif Nkind (Mech_Name) = N_Indexed_Component then
-
             Class := First (Expressions (Mech_Name));
 
             if Nkind (Prefix (Mech_Name)) /= N_Identifier
@@ -4991,6 +4998,14 @@ package body Sem_Prag is
                Bad_Mechanism;
             else
                Mech_Name_Id := Chars (Prefix (Mech_Name));
+
+               --  Change Descriptor => Short_Descriptor if pragma was given
+
+               if Mech_Name_Id = Name_Descriptor
+                 and then Short_Descriptors
+               then
+                  Mech_Name_Id := Name_Short_Descriptor;
+               end if;
             end if;
 
          --  MECHANISM_NAME ::= descriptor (Class => CLASS_NAME) |
@@ -5000,7 +5015,6 @@ package body Sem_Prag is
          --  Note: this form is parsed as a function call
 
          elsif Nkind (Mech_Name) = N_Function_Call then
-
             Param := First (Parameter_Associations (Mech_Name));
 
             if Nkind (Name (Mech_Name)) /= N_Identifier
@@ -5028,72 +5042,72 @@ package body Sem_Prag is
             Bad_Class;
 
          elsif Mech_Name_Id = Name_Descriptor
-               and then Chars (Class) = Name_UBS
+           and then Chars (Class) = Name_UBS
          then
             Set_Mechanism (Ent, By_Descriptor_UBS);
 
          elsif Mech_Name_Id = Name_Descriptor
-               and then Chars (Class) = Name_UBSB
+           and then Chars (Class) = Name_UBSB
          then
             Set_Mechanism (Ent, By_Descriptor_UBSB);
 
          elsif Mech_Name_Id = Name_Descriptor
-               and then Chars (Class) = Name_UBA
+           and then Chars (Class) = Name_UBA
          then
             Set_Mechanism (Ent, By_Descriptor_UBA);
 
          elsif Mech_Name_Id = Name_Descriptor
-               and then Chars (Class) = Name_S
+           and then Chars (Class) = Name_S
          then
             Set_Mechanism (Ent, By_Descriptor_S);
 
          elsif Mech_Name_Id = Name_Descriptor
-               and then Chars (Class) = Name_SB
+           and then Chars (Class) = Name_SB
          then
             Set_Mechanism (Ent, By_Descriptor_SB);
 
          elsif Mech_Name_Id = Name_Descriptor
-               and then Chars (Class) = Name_A
+           and then Chars (Class) = Name_A
          then
             Set_Mechanism (Ent, By_Descriptor_A);
 
          elsif Mech_Name_Id = Name_Descriptor
-               and then Chars (Class) = Name_NCA
+           and then Chars (Class) = Name_NCA
          then
             Set_Mechanism (Ent, By_Descriptor_NCA);
 
          elsif Mech_Name_Id = Name_Short_Descriptor
-               and then Chars (Class) = Name_UBS
+           and then Chars (Class) = Name_UBS
          then
             Set_Mechanism (Ent, By_Short_Descriptor_UBS);
 
          elsif Mech_Name_Id = Name_Short_Descriptor
-               and then Chars (Class) = Name_UBSB
+           and then Chars (Class) = Name_UBSB
          then
             Set_Mechanism (Ent, By_Short_Descriptor_UBSB);
 
          elsif Mech_Name_Id = Name_Short_Descriptor
-               and then Chars (Class) = Name_UBA
+           and then Chars (Class) = Name_UBA
          then
             Set_Mechanism (Ent, By_Short_Descriptor_UBA);
 
          elsif Mech_Name_Id = Name_Short_Descriptor
-               and then Chars (Class) = Name_S
+           and then Chars (Class) = Name_S
          then
             Set_Mechanism (Ent, By_Short_Descriptor_S);
 
          elsif Mech_Name_Id = Name_Short_Descriptor
-               and then Chars (Class) = Name_SB
+           and then Chars (Class) = Name_SB
          then
             Set_Mechanism (Ent, By_Short_Descriptor_SB);
 
          elsif Mech_Name_Id = Name_Short_Descriptor
-               and then Chars (Class) = Name_A
+           and then Chars (Class) = Name_A
          then
             Set_Mechanism (Ent, By_Short_Descriptor_A);
 
          elsif Mech_Name_Id = Name_Short_Descriptor
-               and then Chars (Class) = Name_NCA
+           and then Chars (Class) = Name_NCA
          then
             Set_Mechanism (Ent, By_Short_Descriptor_NCA);
 
@@ -11052,6 +11066,18 @@ package body Sem_Prag is
             Set_Is_Shared_Passive (Cunit_Ent);
          end Shared_Passive;
 
+         -----------------------
+         -- Short_Descriptors --
+         -----------------------
+
+         --  pragma Short_Descriptors;
+
+         when Pragma_Short_Descriptors =>
+            GNAT_Pragma;
+            Check_Arg_Count (0);
+            Check_Valid_Configuration_Pragma;
+            Short_Descriptors := True;
+
          ----------------------
          -- Source_File_Name --
          ----------------------
@@ -12887,6 +12913,7 @@ package body Sem_Prag is
       Pragma_Share_Generic                 => -1,
       Pragma_Shared                        => -1,
       Pragma_Shared_Passive                => -1,
+      Pragma_Short_Descriptors             =>  0,
       Pragma_Source_File_Name              => -1,
       Pragma_Source_File_Name_Project      => -1,
       Pragma_Source_Reference              => -1,
Index: par-prag.adb
===================================================================
--- par-prag.adb	(revision 164070)
+++ par-prag.adb	(working copy)
@@ -1192,6 +1192,7 @@ begin
            Pragma_Shared                        |
            Pragma_Shared_Passive                |
            Pragma_Short_Circuit_And_Or          |
+           Pragma_Short_Descriptors             |
            Pragma_Storage_Size                  |
            Pragma_Storage_Unit                  |
            Pragma_Static_Elaboration_Desired    |
Index: opt.adb
===================================================================
--- opt.adb	(revision 164000)
+++ opt.adb	(working copy)
@@ -61,6 +61,7 @@ package body Opt is
       Optimize_Alignment_Config             := Optimize_Alignment;
       Persistent_BSS_Mode_Config            := Persistent_BSS_Mode;
       Polling_Required_Config               := Polling_Required;
+      Short_Descriptors_Config              := Short_Descriptors;
       Use_VADS_Size_Config                  := Use_VADS_Size;
 
       --  Reset the indication that Optimize_Alignment was set locally, since
@@ -94,6 +95,7 @@ package body Opt is
       Optimize_Alignment_Local       := Save.Optimize_Alignment_Local;
       Persistent_BSS_Mode            := Save.Persistent_BSS_Mode;
       Polling_Required               := Save.Polling_Required;
+      Short_Descriptors              := Save.Short_Descriptors;
       Use_VADS_Size                  := Save.Use_VADS_Size;
    end Restore_Opt_Config_Switches;
 
@@ -121,6 +123,7 @@ package body Opt is
       Save.Optimize_Alignment_Local       := Optimize_Alignment_Local;
       Save.Persistent_BSS_Mode            := Persistent_BSS_Mode;
       Save.Polling_Required               := Polling_Required;
+      Save.Short_Descriptors              := Short_Descriptors;
       Save.Use_VADS_Size                  := Use_VADS_Size;
    end Save_Opt_Config_Switches;
 
@@ -193,6 +196,7 @@ package body Opt is
       Fast_Math                      := Fast_Math_Config;
       Optimize_Alignment             := Optimize_Alignment_Config;
       Polling_Required               := Polling_Required_Config;
+      Short_Descriptors              := Short_Descriptors_Config;
    end Set_Opt_Config_Switches;
 
    ---------------
Index: opt.ads
===================================================================
--- opt.ads	(revision 164072)
+++ opt.ads	(working copy)
@@ -1089,7 +1089,12 @@ package Opt is
    --  GNAT
    --  Set True if a pragma Short_Circuit_And_Or applies to the current unit.
 
+   Short_Descriptors : Boolean := False;
+   --  GNAT
+   --  Set True if a pragma Short_Descriptors applies to the current unit.
+
    Sprint_Line_Limit : Nat := 72;
+   --  GNAT
    --  Limit values for chopping long lines in Sprint output, can be reset
    --  by use of NNN parameter with -gnatG or -gnatD switches.
 
@@ -1651,6 +1656,14 @@ package Opt is
    --  flag is used to set the initial value for Polling_Required at the start
    --  of analyzing each unit.
 
+   Short_Descriptors_Config : Boolean;
+   --  GNAT
+   --  This is the value of the configuration switch that controls the use of
+   --  Short_Descriptors for setting descriptor default sizes. It can be set
+   --  True by the use of the pragma Short_Descriptors in the gnat.adc file.
+   --  This flag is used to set the initial value for Short_Descriptors at the
+   --  start of analyzing each unit.
+
    Use_VADS_Size_Config : Boolean;
    --  GNAT
    --  This is the value of the configuration switch that controls the use of
@@ -1780,6 +1793,7 @@ private
       Optimize_Alignment_Local       : Boolean;
       Persistent_BSS_Mode            : Boolean;
       Polling_Required               : Boolean;
+      Short_Descriptors              : Boolean;
       Use_VADS_Size                  : Boolean;
    end record;
 
Index: snames.ads-tmpl
===================================================================
--- snames.ads-tmpl	(revision 164070)
+++ snames.ads-tmpl	(working copy)
@@ -386,6 +386,7 @@ package Snames is
    Name_Restriction_Warnings           : constant Name_Id := N + $; -- GNAT
    Name_Reviewable                     : constant Name_Id := N + $;
    Name_Short_Circuit_And_Or           : constant Name_Id := N + $; -- GNAT
+   Name_Short_Descriptors              : constant Name_Id := N + $; -- GNAT
    Name_Source_File_Name               : constant Name_Id := N + $; -- GNAT
    Name_Source_File_Name_Project       : constant Name_Id := N + $; -- GNAT
    Name_Style_Checks                   : constant Name_Id := N + $; -- GNAT
@@ -1466,6 +1467,7 @@ package Snames is
       Pragma_Restriction_Warnings,
       Pragma_Reviewable,
       Pragma_Short_Circuit_And_Or,
+      Pragma_Short_Descriptors,
       Pragma_Source_File_Name,
       Pragma_Source_File_Name_Project,
       Pragma_Style_Checks,
