[llvm-dev] [JIT] Evaluating Debug-Metadata in bitcode

[mats petersson via llvm-dev]

Hi Björn,

I'm not sure I understand what you are actually trying to achieve.

Do you want to be able to debug from your main code, and step into the
JITed code?
Do you want to be able to set breakpoints, list callstacks, etc in the
JITed code?
Do you want to, from a crash, identify where in your JITed code it went
wrong?

The first two are definitely one or more order(s) of magnitude harder to
solve than the third one, since you basically have to tell your debugger
that "I've now added some more code here, come here to see the symbols",
and would require a fair amount of extra work to make any existing debugger
understand this concept, never mind the exact implementation details.

A post-mortem analysis is much less complicated, since all you'd have to do
is walk through the debug symbols and relate the generated locations to the
actual locations in the code.

Another aspect is local variables (and arguments into functions), although
I'm not sure it's that much of complication above and beyond the solution
of the above parts.

I suspect there are parts of code that can be used, from llc and lld that
can be used to generate dwarf info, but you will probably still have to
deal with tying that back into your generated code.

In my Pascal compiler, which generates real executable code, adding debug
symbols is trivial from the perspective of making it work in a debugger,
but hard in the sense of generating the debug data for the line, file,
function and variable information - all that I need to do to make it go
into the executable file is append -g to the linker options when building.
But since JIT-generated code isn't an executable that you can just apply
debug symbols into, this makes it a fair bit harder (I think, I haven't
actually tried).

--
Mats

On 19 January 2018 at 14:17, via llvm-dev <llvm-dev at lists.llvm.org> wrote:

> Hello LLVM-People,
>
> I'm still a beginner with the LLVM, but I really like the concept and the
> possibilities with the JIT.
>
> Currently I compile simple functions with clang-cl into bitcode files.
> After this I use another program to JIT this bitcode files and execute
> functions of it - like lli.
> Thanks to a lot of mails and so on, I understood that a bitcode file is in
> fact still IR-Code, but with another representation - so when I JIT this
> file with my application, I'm acting like a compiler (like llc).
>
> But what does happen to the debug information? When I generate the files
> in human readable code I can easily discover the metadata and understand
> them. So I want to extend my application, that it can debug the code, which
> was jitted. This seems difficult. When I get the address of a debug break,
> I can only identify the function where the exception occurred. I don't see
> a way to connect the metadata to the offset I could calculate from the
> exception and the function. Also I don't know how to progress the metadata
> when it's in its bitcode form. So - how can I start with this task?
>
> Kind regards
> Björn Gaier
>
>
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