What does the DWARF information generated by your GHC branch look like?

Peter Wortmann scpmw at leeds.ac.uk
Fri Feb 28 19:49:32 UTC 2014

[copy of the dropped reply, for anybody interested]

Johan Tibell wrote:
> I enjoyed reading your paper [1] and I have some questions.

Thanks! The DWARF patches are currently under review for Trac #3693. Any
feedback would be very appreciated:

>  * What does the generated DWARF information look like?

So far we generate:
- .debug_info: Information about all generated procedures and blocks.
- .debug_line: Source-code links for all generated code
- .debug_frame: Unwind information for the GHC stack
- .debug_ghc: Everything we can't properly represent as DWARF

>  will you fill in the .debug_line section so that standard tools like
> "perf report" and gprof can be used on Haskell code?

Yes, even though from a few quick tests the results of "perf report"
aren't too useful, as source code links are pretty coarse and jump
around a lot - especially for optimised Haskell code. There's the option
to instead annotate with source code links to a generated ".dump-simpl"
file, which might turn out to be more useful.

> Code pointers would be appreciated.

Is this about how .debug_line information is generated? We take the same
approach as LLVM (and GCC, I think) and simply annotate the assembly
with suitable .file & .loc directives. That way we can leave all the
heavy lifting to the assembler.

Current patch is here:

>  * Does your GHC allow DWARF information to be generated without
> actually using any of the RTS (e.g. eventlog) machinery?

The RTS just serves as a DWARF interpreter for its own executable (+
libraries) in this, so yes, it's fully independent. On the other hand,
having special code allows us to avoid a few subtleties about Haskell
code that are hard to communicate to standard debugging tools
(especially concerning stack tracing).

> Another way to ask the same question, do you have a ghc -g flag that
> has no implication for the runtime settings?

Right now -g does not affect the RTS at all. We might want to change
that at some point though so we can get rid of the libdwarf dependency.

>  * Do you generate DW_TAG_subprogram sections in the .debug_info
> section so that other tools can figure out the name of Haskell
> functions?

Yes, we are setting the "name" attribute to a suitable Haskell name.
Sadly, at least GDB seems to ignore it and falls back to the symbol
name. I investigated this some time ago, and I think the reason was that
it doesn't recognize the Haskell language ID (which isn't standardized,
obviously). Simply pretending to be C(++) might fix this, but I would be
a bit scared of other side-effects.

  Peter Wortmann

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