Index: lib.ads
===================================================================
--- lib.ads	(revision 164906)
+++ lib.ads	(working copy)
@@ -49,6 +49,11 @@ package Lib is
    --  extended source (the main unit, its spec, or one of its subunits). This
    --  flag to implement In_Extended_Main_Source_Unit.
 
+   Analysing_Subunit_Of_Main : Boolean := False;
+   --  Set to True when analyzing a subunit of the main source. When True, if
+   --  the subunit is preprocessed and -gnateG is specified, then the
+   --  preprocessed file (.prep) is written.
+
    --------------------------------------------
    -- General Approach to Library Management --
    --------------------------------------------
Index: frontend.adb
===================================================================
--- frontend.adb	(revision 164906)
+++ frontend.adb	(working copy)
@@ -116,6 +116,11 @@ begin
       Prepcomp.Check_Symbols;
    end if;
 
+   --  We set Parsing_Main_Extended_Source true here to cover processing of all
+   --  the configuration pragma files, as well as the main source unit itself.
+
+   Parsing_Main_Extended_Source := True;
+
    --  Now that the preprocessing situation is established, we are able to
    --  load the main source (this is no longer done by Lib.Load.Initialize).
 
@@ -127,11 +132,6 @@ begin
       return;
    end if;
 
-   --  We set Parsing_Main_Extended_Source true here to cover processing of all
-   --  the configuration pragma files, as well as the main source unit itself.
-
-   Parsing_Main_Extended_Source := True;
-
    --  Read and process configuration pragma files if present
 
    declare
Index: sem_ch10.adb
===================================================================
--- sem_ch10.adb	(revision 164906)
+++ sem_ch10.adb	(working copy)
@@ -1739,12 +1739,17 @@ package body Sem_Ch10 is
          --  Otherwise we must load the subunit and link to it
 
          else
+            --  Make sure that, if the subunit is preprocessed and -gnateG is
+            --  specified, the preprocessed file will be written.
+
+            Lib.Analysing_Subunit_Of_Main := True;
             Unum :=
               Load_Unit
                 (Load_Name  => Subunit_Name,
                  Required   => False,
                  Subunit    => True,
                  Error_Node => N);
+            Lib.Analysing_Subunit_Of_Main := False;
 
             --  Give message if we did not get the unit Emit warning even if
             --  missing subunit is not within main unit, to simplify debugging.
@@ -1822,11 +1827,11 @@ package body Sem_Ch10 is
             end if;
          end if;
 
-         --  The remaining case is when the subunit is not already loaded and
-         --  we are not generating code. In this case we are just performing
-         --  semantic analysis on the parent, and we are not interested in
-         --  the subunit. For subprograms, analyze the stub as a body. For
-         --  other entities the stub has already been marked as completed.
+      --  The remaining case is when the subunit is not already loaded and we
+      --  are not generating code. In this case we are just performing semantic
+      --  analysis on the parent, and we are not interested in the subunit. For
+      --  subprograms, analyze the stub as a body. For other entities the stub
+      --  has already been marked as completed.
 
       else
          Optional_Subunit;
Index: sem_ch12.adb
===================================================================
--- sem_ch12.adb	(revision 164906)
+++ sem_ch12.adb	(working copy)
@@ -5906,12 +5906,19 @@ package body Sem_Ch12 is
                New_Body     : Node_Id;
 
             begin
+               --  Make sure that, if it is a subunit of the main unit that is
+               --  preprocessed and if -gnateG is specified, the preprocessed
+               --  file will be written.
+
+               Lib.Analysing_Subunit_Of_Main :=
+                 Lib.In_Extended_Main_Source_Unit (N);
                Unum :=
                  Load_Unit
                    (Load_Name  => Subunit_Name,
                     Required   => False,
                     Subunit    => True,
                     Error_Node => N);
+               Lib.Analysing_Subunit_Of_Main := False;
 
                --  If the proper body is not found, a warning message will be
                --  emitted when analyzing the stub, or later at the point
Index: sinput-l.adb
===================================================================
--- sinput-l.adb	(revision 164933)
+++ sinput-l.adb	(working copy)
@@ -30,6 +30,7 @@ with Einfo;    use Einfo;
 with Errout;   use Errout;
 with Fname;    use Fname;
 with Hostparm;
+with Lib;      use Lib;
 with Opt;      use Opt;
 with Osint;    use Osint;
 with Output;   use Output;
@@ -552,9 +553,17 @@ package body Sinput.L is
 
                else
                   --  Output the result of the preprocessing, if requested and
-                  --  the source has been modified by the preprocessing.
+                  --  the source has been modified by the preprocessing. Only
+                  --  do that for the main unit (spec, body and subunits).
 
-                  if Generate_Processed_File and then Modified then
+                  if Generate_Processed_File and then
+                     Modified and then
+                     ((Compiler_State = Parsing
+                       and then Parsing_Main_Extended_Source)
+                      or else
+                       (Compiler_State = Analyzing
+                        and then Analysing_Subunit_Of_Main))
+                  then
                      declare
                         FD     : File_Descriptor;
                         NB     : Integer;
