[gcc r16-5558] Ada: Fix incorrect handling of BOM by -r switch of gnatchop

[Eric Botcazou]

https://gcc.gnu.org/g:7ca42b7a4026fcc9635d2c2c8f6ab2b9ce92ccbd

commit r16-5558-g7ca42b7a4026fcc9635d2c2c8f6ab2b9ce92ccbd
Author: Eric Botcazou <ebotcazou@adacore.com>
Date:   Mon Nov 24 19:00:43 2025 +0100

    Ada: Fix incorrect handling of BOM by -r switch of gnatchop
    
    As reported and analyzed in the PR, gnatchop does not correctly propagate
    a BOM present in the source file to the first compilation unit it outputs,
    in the case where the -r switch is specified, because it copies the BOM
    for the first compilation unit as part of the chopping process instead of
    copying it specifically at the start of the unit.
    
    gcc/ada/
            PR ada/81106
            * gnatchop.adb (Gnatchop): If present in the source file, output
            the BOM at the start of every compilation unit.

Diff:
---
 gcc/ada/gnatchop.adb | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/gcc/ada/gnatchop.adb b/gcc/ada/gnatchop.adb
index bc045e1fb7ae..286904a2378b 100644
--- a/gcc/ada/gnatchop.adb
+++ b/gcc/ada/gnatchop.adb
@@ -138,7 +138,7 @@ procedure Gnatchop is
       --  Line number from GNAT output line
 
       Offset : File_Offset;
-      --  Offset name from GNAT output line
+      --  Offset from GNAT output line
 
       SR_Present : Boolean;
       --  Set True if SR parameter present
@@ -1319,8 +1319,7 @@ procedure Gnatchop is
       Success : Boolean;
       TS_Time : OS_Time;
 
-      BOM_Present : Boolean;
-      BOM         : BOM_Kind;
+      BOM : BOM_Kind;
       --  Record presence of UTF8 BOM in input
 
    begin
@@ -1347,7 +1346,6 @@ procedure Gnatchop is
       --  Test for presence of BOM
 
       Read_BOM (Buffer.all, BOM_Length, BOM, XML_Support => False);
-      BOM_Present := BOM /= Unknown;
 
       --  Only chop those units that come from this file
 
@@ -1357,7 +1355,7 @@ procedure Gnatchop is
               (Source    => Buffer,
                Num       => Unit_Number,
                TS_Time   => TS_Time,
-               Write_BOM => BOM_Present and then Unit_Number /= 1,
+               Write_BOM => BOM /= Unknown,
                Success   => Success);
             exit when not Success;
          end if;
@@ -1608,6 +1606,13 @@ procedure Gnatchop is
             String'Write
               (Stream_IO.Stream (File),
                Source.all (Source'First .. Source'First + BOM_Length - 1));
+
+            --  The BOM is part of the first unit so do not write it twice
+
+            if Num = 1 then
+               Info.Offset := Info.Offset + BOM_Length;
+               Length := Length - BOM_Length;
+            end if;
          end if;
 
          --  Prepend configuration pragmas if necessary