[LLVMdev] RFC: Improving our DWARF (and ELF) emission testing capabilities

[Daniel Berlin]

On Fri, Jan 18, 2013 at 4:00 PM, Eli Bendersky <eliben at google.com> wrote:
> Hi All,
>
> While working on some recent patches for x32 support, I ran into an
> unpleasant limitation the LLVM eco-system has with testing DWARF
> emission. We currently have several approaches, neither of which is
> great:
>
> 1. llvm-dwarfdump: the best approach when it works. But unfortunately
> lib/DebugInfo supports only a (small) subset of DWARF. Tricky sections
> like debug_frame aren't supported.

Could you point out what you mean?
In particular, what parts you think it does not support (since you say
it supports a small subset).
What do you want out of debug_frame, past simple parsing?
Anything else requires real evaluation.

I ask because I wrote a DWARF reader that google uses internally, and
then was open sourced and contributed to google breakpad.
(see http://code.google.com/p/google-breakpad/source/browse/trunk/src/common/dwarf/,
in particular dwarf2reader.cc).

> 2. Relying of assembly directive emissions (i.e. .cfi_*), which is
> cumbersome and misses a lot of things like actual DWARF encoding.

Err, .cfi_ and used because the encoding is tricky to get right, and
assemblers are better at optimizing it.
However, i'll point out that breakpad also has a CFI assembler
(http://code.google.com/p/google-breakpad/source/browse/trunk/src/common/dwarf/cfi_assembler.cc)

>
> The long-term solution for DWARF would be to enhance lib/DebugInfo to
> the point where it can handle all interesting DWARF sections. But this
> is a lofty goal, since DWARF parsing is notoriously hard and this
> would require a large investment of time and effort.
?????
Having written about 6 DWARF parsers, I strongly disagree it is either
notoriously hard or a large investment of time and effort.  People
have written DWARF parsers on the weekend.  One of the reasons DWARF
is popular is because it is relatively simple to *parse*, even though
semantic extraction is more difficult.

In any case, I mention the above project (google-breakpad) because i'd
be more than happy to get that DWARF related code relicensed to the
LLVM license if someone wanted it.
>