[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: https://github.com/scpmw/ghc/commits/profiling-import
* 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: https://github.com/scpmw/ghc/commit/c5294576
* 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. Greetings, Peter Wortmann