[LLVMdev] Dwarf debugging strangeness, continued...

[Talin]

Another chapter in the long saga of trying to get source-level debugging
working :)

I've switched over to using the direct object-file generation instead of
generating assembly in hopes of getting around the "Fatal error: duplicate
.debug_line sections" binutils bug.

I now have 4 different tools for dumping the DWARF info for an object
module: dwarfdump, objdump, readelf, and pydevtools. Curiously, they don't
all agree on what debugging information is present in an LLVM-generated
object file.

First let me explain what's in the executable. There are several parts:

   - First, there are a bunch of .bc files that are generated by my
   frontend, and which contain debugging metadata (I checked using llvm-dis and
   the metadata appears to be OK). These are then combined by my linker,
   tartln, which combines the functions of 'opt' and 'llc', as well as having
   some custom passes for reflection and garbage collection. (In fact, about
   half of the source code for tartln was lifted directly from opt and llc,
   although this cut & paste operation occurred somewhere around the 2.5
   timeframe, and I've tried to keep the code up to date since then.)
      - The output of this compilation phase is a .o file.
   - Second, there are some .cpp files that were compiled with gcc. These
   contain a few runtime support routines for the language, such as stderr
   output and the code to walk the stack frame (which requires inline
   assembly.) No, I'm not using clang, as I am trying to minimize build
   dependencies.
      - The output of this stage is a static library.
   - Finally, the .o and the static libraries are linked together by passing
   them to gcc. (I wanted to use ld directly, but when I do I get undefined
   symbol errors for all of the libunwind functions....more detail on that in
   another thread if anyone is interested. I know that the unwind functions are
   in some default-linked library, unfortunately which library is not obvious -
   there's no file named libunwind anywhere in my library path)
      - The result of this stage is an executable.

Now, when I attempt to dump the debugging info, the following happens:

   - With dwarfdump -a, I only seem to get a small number of subprogram
   definitions in the output: 25 total. However, with dwarfdump -i, I get 6141
   subprogram defenitions. This is strange given that the docs for dwarfdump
   claim that -a is a superset of -i.
   - With "readelf --debug-dump" and "objdump --dwarf" I get 6170 subprogram
   definitions.
   - pydevtools simply crashes when fed my executable. This may be a bug in
   pydevtools. It's hard to tell what's going on here, I've debugged it a bit,
   and it seems like pydevtools is blowing up because the list of ELF sections
   that it reads is an empty list.
      - It would be great if I could get pydevtools working, because it has
      a GUI browser view for the debugging information - that would be
a big help
      tracking down these problems. Oh well.

When I attempt to debug this in gdb, it acts as if it can only 'see' the
debug definitions coming from gcc, and not any of the ones from my frontend.
Doing a stack dump during execution shows symbolic information for the
gcc-created functions, and bare machine addresses for the other stuff.

Note that all of the above is on Linux - on OS X I get a completely
different set of errors.

At this point I'd pay money to get this solved....it stopped being fun, oh,
about a year ago :)

-- 
-- Talin
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