[gdb] Mention CU offset for <artifical> if verbose

Say we're debugging a test-case with CUs with name "<artificial>", meaning
not originating from a single file compilation, and use the verbose setting:
...
$ gdb -iex "set verbose on" -batch cc1
Reading symbols from cc1...
Reading in symbols for <artificial>... \
  and /tmp/trunk/gcc/attribs.c... \
  ...
  and /tmp/trunk/gcc/tree-ssa-reassoc.c... \
  done.
...

From the "/tmp/trunk/gcc/attribs.c" message, it's clear which CU is loaded.  But
that's not the case for the "<artificial>" message.

The message uses the filename field of struct partial_symtab, which is
documented like this:
...
  /* Name of the source file which this partial_symtab defines,
     or if the psymtab is anonymous then a descriptive name for
     debugging purposes, or "".  It must not be NULL.  */
...

So, fix this by setting the filename field to a more descriptive name than
"<artificial>", by appending the CU offset.

This way, we print instead:
...
$ gdb -iex "set verbose on" -batch cc1
Reading symbols from cc1...
Reading in symbols for <artificial>@0x41146d9 \
  and /tmp/trunk/gcc/attribs.c... \
  ... \
  and /tmp/trunk/gcc/tree-ssa-reassoc.c... \
  done.
...

Build and reg-tested on x86_64-linux.

gdb/ChangeLog:

2020-02-09  Tom de Vries  <tdevries@suse.de>

	* dwarf2read.c (process_psymtab_comp_unit_reader): Append CU offset to
	filename if it matches "<artificial>".

1 files changed, 8 insertions, 0 deletions

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index d1622dc..9e66e61 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -7252,9 +7252,17 @@ process_psymtab_comp_unit_reader (const struct die_reader_specs *reader,
   prepare_one_comp_unit (cu, comp_unit_die, pretend_language);
 
   /* Allocate a new partial symbol table structure.  */
+  gdb::unique_xmalloc_ptr<char> debug_filename;
+  static const char artificial[] = "<artificial>";
   filename = dwarf2_string_attr (comp_unit_die, DW_AT_name, cu);
   if (filename == NULL)
     filename = "";
+  else if (strcmp (filename, artificial) == 0)
+    {
+      debug_filename.reset (concat (artificial, "@",
+				    sect_offset_str (per_cu->sect_off), NULL));
+      filename = debug_filename.get ();
+    }
 
   pst = create_partial_symtab (per_cu, filename);