[PATCH] inconsistent DWARF2 sections generated by --gdwarf2

[Bob Wilson <bwilson@tensilica.com>]

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The code for dwarf2_finish() in gas/dwarf2dbg.c appears to be inconsistent.  If 
you assemble a file that contains no instructions, e.g., just a symbol 
declaration: ".global some_symbol", with --gdwarf2, GAS will create a 
.debug_line section but no corresponding .debug_info section.  This is probably 
harmless for GDB, but it breaks readelf, which expects a one-to-one 
correspondence between .debug_info and .debug_line sections.  Running "readelf 
-wl" on such a file results in the following error message:

   readelf: Error: Not enough comp units for .debug_lines section

It looks like the problem was introduced by the following change:

2002-08-01  H.J. Lu  <hjl@gnu.org>
	    Daniel Jacobowitz  <drow@mvista.com>

	* dwarf2dbg.c (dwarf2_finish): Don't emit unreferenced
	.debug_line section unless it has line information.

I read through the discussions surrounding that patch to try to understand the 
issues involved, but I'm not sure if I succeeded.  I _think_ this patch is 
correct -- assuming the .debug_info section is needed, it must either be 
provided in the input or generated later in dwarf2_finish.  In the latter case, 
the test to "do nothing" should be consistent with the test to generate 
.debug_info, etc.  An alternative would be changing GAS to write out a 
.debug_info section even when no line numbers are specified, but the following 
comment in out_debug_info() suggests that might not work: "We're not supposed to 
get called unless at least one line number entry was emitted...."

I'll be happy to commit this if someone will confirm that it is the right thing 
to do and approve the patch.  It doesn't cause any gas testsuite regressions for 
an xtensa-elf target.

--Bob


gas ChangeLog
2004-11-15  Bob Wilson  <bob.wilson@acm.org>

	* dwarf2dbg.c (dwarf2_finish): Don't write a .debug_line section
	without a corresponding .debug_info section.


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Index: dwarf2dbg.c
===================================================================
RCS file: /cvs/src/src/gas/dwarf2dbg.c,v
retrieving revision 1.71
diff -u -p -r1.71 dwarf2dbg.c
--- dwarf2dbg.c	2 Nov 2004 09:49:25 -0000	1.71
+++ dwarf2dbg.c	15 Nov 2004 18:53:45 -0000
@@ -1356,13 +1356,12 @@ dwarf2_finish (void)
   struct line_seg *s;
 
   /* We don't need to do anything unless:
-     - Some debug information was recorded via .file/.loc
-     - or, we are generating DWARF2 information ourself (--gdwarf2)
+     - Some debug information was recorded via .file/.loc or
+       generated by GAS (--gdwarf2)
      - or, there is a user-provided .debug_info section which could
        reference the file table in the .debug_line section we generate
        below.  */
   if (all_segs == NULL
-      && debug_type != DEBUG_DWARF2
       && (bfd_get_section_by_name (stdoutput, ".debug_info") == NULL
 	  || files_in_use == 0))
     return;