ada: Introduce -gnateH switch to force reverse Bit_Order threshold to 64

This can be helpful for legacy code that still makes use of an original
reverse Bit_Order clause, i.e. without a Scalar_Storage_Order clause.

gcc/ada/

	* doc/gnat_ugn/building_executable_programs_with_gnat.rst (Compiler
	Switches): Document -gnateH.
	* opt.ads (Reverse_Bit_Order_Threshold): New variable.
	* sem_ch13.adb (Adjust_Record_For_Reverse_Bit_Order): Use its value
	if it is nonnegative instead of System_Max_Integer_Size.
	* switch-c.adb (Scan_Front_End_Switches): Deal with -gnateH.
	* usage.adb (Usage): Print -gnateH.
	* gnat_ugn.texi: Regenerate.

6 files changed, 40 insertions, 2 deletions

diff --git a/gcc/ada/doc/gnat_ugn/building_executable_programs_with_gnat.rst b/gcc/ada/doc/gnat_ugn/building_executable_programs_with_gnat.rst
index 20e003d..8e47967 100644
--- a/gcc/ada/doc/gnat_ugn/building_executable_programs_with_gnat.rst
+++ b/gcc/ada/doc/gnat_ugn/building_executable_programs_with_gnat.rst
@@ -1612,6 +1612,14 @@ Alphabetical List of All Switches
   Save result of preprocessing in a text file.
 
 
+.. index:: -gnateH  (gcc)
+
+:switch:`-gnateH`
+  Set the threshold from which the RM 13.5.1(13.3/2) clause applies to 64.
+  This is useful only on 64-bit plaforms where this threshold is 128, but
+  used to be 64 in earlier versions of the compiler.
+
+
 .. index:: -gnatei  (gcc)
 
 :switch:`-gnatei{nnn}`
diff --git a/gcc/ada/gnat_ugn.texi b/gcc/ada/gnat_ugn.texi
index 88123df..021c267 100644
--- a/gcc/ada/gnat_ugn.texi
+++ b/gcc/ada/gnat_ugn.texi
@@ -19,7 +19,7 @@
 
 @copying
 @quotation
-GNAT User's Guide for Native Platforms , Jun 01, 2023
+GNAT User's Guide for Native Platforms , Jun 16, 2023
 
 AdaCore
 
@@ -9075,6 +9075,18 @@ information.
 Save result of preprocessing in a text file.
 @end table
 
+@geindex -gnateH (gcc)
+
+
+@table @asis
+
+@item @code{-gnateH}
+
+Set the threshold from which the RM 13.5.1(13.3/2) clause applies to 64.
+This is useful only on 64-bit plaforms where this threshold is 128, but
+used to be 64 in earlier versions of the compiler.
+@end table
+
 @geindex -gnatei (gcc)
 
 
diff --git a/gcc/ada/opt.ads b/gcc/ada/opt.ads
index bcafba9..87399c8 100644
--- a/gcc/ada/opt.ads
+++ b/gcc/ada/opt.ads
@@ -1342,6 +1342,11 @@ package Opt is
    --  GNATPREP
    --  Set to True if -C switch used.
 
+   Reverse_Bit_Order_Threshold : Int := -1;
+   --  GNAT
+   --  Set to the threshold from which the RM 13.5.1(13.3/2) clause applies,
+   --  or -1 if the size of the largest machine scalar is to be used.
+
    RTS_Lib_Path_Name : String_Ptr := null;
    RTS_Src_Path_Name : String_Ptr := null;
    --  GNAT
diff --git a/gcc/ada/sem_ch13.adb b/gcc/ada/sem_ch13.adb
index 6562732..c3ea8d63 100644
--- a/gcc/ada/sem_ch13.adb
+++ b/gcc/ada/sem_ch13.adb
@@ -426,7 +426,9 @@ package body Sem_Ch13 is
 
    procedure Adjust_Record_For_Reverse_Bit_Order (R : Entity_Id) is
       Max_Machine_Scalar_Size : constant Uint :=
-                                  UI_From_Int (System_Max_Integer_Size);
+        UI_From_Int (if Reverse_Bit_Order_Threshold >= 0
+                     then Reverse_Bit_Order_Threshold
+                     else System_Max_Integer_Size);
       --  We use this as the maximum machine scalar size
 
       SSU : constant Uint := UI_From_Int (System_Storage_Unit);
diff --git a/gcc/ada/switch-c.adb b/gcc/ada/switch-c.adb
index f6207e4..bbbb536 100644
--- a/gcc/ada/switch-c.adb
+++ b/gcc/ada/switch-c.adb
@@ -635,6 +635,12 @@ package body Switch.C is
                      Generate_Processed_File := True;
                      Ptr := Ptr + 1;
 
+                  --  -gnateH (set reverse Bit_Order threshold to 64)
+
+                  when 'H' =>
+                     Reverse_Bit_Order_Threshold := 64;
+                     Ptr := Ptr + 1;
+
                   --  -gnatei (max number of instantiations)
 
                   when 'i' =>
diff --git a/gcc/ada/usage.adb b/gcc/ada/usage.adb
index 58cfa78..681ece5 100644
--- a/gcc/ada/usage.adb
+++ b/gcc/ada/usage.adb
@@ -199,6 +199,11 @@ begin
    Write_Switch_Char ("eG");
    Write_Line ("Generate preprocessed source");
 
+   --  Line for -gnateH switch
+
+   Write_Switch_Char ("eH");
+   Write_Line ("Set reverse Bit_Order threshold to 64");
+
    --  Line for -gnatei switch
 
    Write_Switch_Char ("einn");