Deal with full path in .file 0 directive

Gas uses the directory part, if present, of the .file 0 directive to set
entry 0 of the directory table in DWARF 5, which represents the "current
directory".

Now Gas also uses the file part of the same directive to set entry 0 of the
file table, which represents the "current compilation file".  But the latter
need not be located in the former so GCC will use a full path in the file
part when it is passed a full path:

gcc -c /full/path/test.c -save-temps

yields:

 .file 0 "/current/directory" "/full/path/test.c"

in the assembly file and:

 The Directory Table (offset 0x22, lines 2, columns 1):
  Entry Name
  0     (indirect line string, offset: 0x25): /current/directory
  1     (indirect line string, offset: 0x38): /full/path

 The File Name Table (offset 0x30, lines 2, columns 2):
  Entry Dir     Name
  0     0       (indirect line string, offset: 0x43): /full/path/test.c

in the object file.  Note the full path and the questionable Dir value in
the 0 entry of the file table.

1 files changed, 10 insertions, 7 deletions

diff --git a/gas/doc/as.texi b/gas/doc/as.texi
index 0cc7455..9c1924d 100644
--- a/gas/doc/as.texi
+++ b/gas/doc/as.texi
@@ -5416,19 +5416,22 @@ table is shared with the @code{.debug_info} section of the DWARF2 debugging
 information, and thus the user must know the exact indices that table
 entries will have.
 
-If DWARF-5 support has been enabled via the @option{-gdwarf-5} option then
-an extended version of the @code{file} is also allowed:
+If DWARF5 support has been enabled via the @option{-gdwarf-5} option then
+an extended version of @code{.file} is also allowed:
 
 @smallexample
 .file @var{fileno} [@var{dirname}] @var{filename} [md5 @var{value}]
 @end smallexample
 
 With this version a separate directory name is allowed, although if this is
-used then @var{filename} should not contain any directory components.  In
-addtion an md5 hash value of the contents of @var{filename} can be provided.
-This will be stored in the the file table as well, and can be used by tools
-reading the debug information to verify that the contents of the source file
-match the contents of the compiled file.
+used then @var{filename} should not contain any directory component, except
+for @var{fileno} equal to 0: in this case, @var{dirname} is expected to be
+the current directory and @var{filename} the currently processed file, and
+the latter need not be located in the former. In addtion an MD5 hash value
+of the contents of @var{filename} can be provided. This will be stored in
+the the file table as well, and can be used by tools reading the debug
+information to verify that the contents of the source file match the
+contents of the compiled file.
 
 @node Fill
 @section @code{.fill @var{repeat} , @var{size} , @var{value}}