Re: Question: using objdump for disassembly and inserting source code with object compiled with -gsplit-dwarf

["ISHIKAWA,chiaki" <ishikawa at yk dot rim dot or dot jp>]

Dear Nick,

Sorry for my tardy response.
The mail got buried in other mails and I was tied up with my day job :-(

Nick Clifton wrote:
> Hi chiaki,
>
> > However, for a medium size c++ file, I found that the source inclusion is not very good. The function signature is inserted, but no source code lines somehow.
> > Is this to be expected?
>
> No.

I see. Then I think I am seeing a bug.

> > Or is there a known bug with objdump?
>
> It is a bug, but not a known one.  Please could you open a bugzilla report
> for the problem, along with, if possible, a small(ish) test case that
> reproduces the problem.

I will try, but not sure if I will succeed in a reasonable amount of time.

> > It is possible that gcc 5.3 or 6.1 may not be producing correct DWARF
> > in .o and .dwp which objdump can grok
>
> This is entirely possible.  But if so, then it still may be possible to
> have objdump warn the user that the debug info is broken/incomplete/missing.

Right. binutils programs ought to warn the user if they encounter  
unexpected data format.


> As an aside - is GDB able to produce correct source+disassembly listings
> from the same input file(s) ?  If so then this would point more towards
> objdump being at fault.  If not then it may well be a G++ problem.

GDB definitely can print the source line, etc.

However, there is a catch. And I am afraid that it could be the cause of  
the problem.

GDB has a feature that lets us tell where gdb should try to find the  
.dwo file (the external auxiliary debug information file aside from the  
.o file itself).
Specifically, "set debug-file-directory" command is used to list the  
directories where gdb ought to look for the related .dwo files.

The said medium-sized program is linked from object files from various  
directories and thus we need to tell GDB the directories where to look  
for the .dwo files.

E.g.: from my .gdbinit file:
# Path for looking for .dwo files.
source /NREF-COMM-CENTRAL/work-dir/d.dirs
and
/NREF-COMM-CENTRAL/work-dir/d.dirs contains the following line:
set debug-file-directory  
/usr/lib/debug:/other-directories-separated-by-colon-and-not-ending-in-colon

Now, I am not sure. Come to think of it, the source file and line number  
is very likely to be in .o file and .dwo file contains additional data  
type information, etc.
(Aha, now I see. the source file information in .o file is
only relative to the compilation directory, i.e. where the compilation  
takes place, and thus GDB needs to know the full pathname from the side  
channel?: that is what set debug-file-directory does (?).)

If my guess is correct, then
this suggests that objdump (and friends) may need to take an optional  
argument something like
-G list-of-directories-separated-by-colon
to tell it where to look for .dwo files.

Anyway, I will try to see if I can create a smallish file to re-create  
the problem.

TIA

> Cheers
>   Nick

Chiaki