Re: Some dwarf "nitpicks"

[Mark Wielaard <mjw at redhat dot com>]

On Mon, 2011-03-07 at 09:56 -0800, Roland McGrath wrote:
> > $ echo > empty.c 
> > $ gcc -g -c empty.c 
> > $ dwarflint empty.o 
> > error: .debug_abbrev: couldn't load CU headers; assuming CUs are of
> > latest DWARF flavor.
> > error: .debug_abbrev: no abbreviations.
> > error: .debug_info: data not found.
> 
> This produces zero-length .debug_info and .debug_line sections.  There
> is nothing wrong with that, though it would of course also be fine if
> it didn't emit the ".section" directives at all.  
> 
> It produces a .debug_abbrev section that is nothing but a single 0
> byte.  That is superfluous and it would be better if it omitted it
> entirely, so I would recommend reporting that as a GCC bug.  (Many
> such files will add up to lots of wasted zero bytes in the linked
> output.)  

Bug and proposed patch:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48041

> > $ echo "static int empty;" > empty.c
> > $ gcc -g -c empty.c 
> > $ eu-dwarflint empty.o 
> > error: .debug_line: table 0: sequence of opcodes not terminated with
> > DW_LNE_end_sequence.
> 
> This produces a zero-length .debug_line section but a CU with a
> DW_AT_stmt_list attribute pointing to its start.  That is invalid
> DWARF and hence a GCC bug.  The place where DW_AT_stmt_list points
> must have a valid line information header, even if that says it's an
> empty table.  I think what would be most correct here is to omit the
> DW_AT_stmt_list attribute entirely from that CU DIE.

Actually, the header is there, but there are no tables.
Reported with possible patch:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48045

But since the debug_line table header is actually there, dwarflint is
still not really happy. empty.o as produced by my patched gcc attached.

$ eu-dwarflint empty.o
warning: .debug_line: table 0: the file #1 `empty.c' is not used.
error: .debug_line: table 0: sequence of opcodes not terminated with
DW_LNE_end_sequence.

So maybe a better solution would be to just add a dummy end_sequence.

Cheers,

Mark

Attachment: empty.o
Description: application/object