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

[via llvm-dev]

Hello Mats,

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?
I guess that the third point describes what I want at best. But I also 
want to dump local variables and identify the line of the crash - without 
the need of a external debugger. I would like to write it self, to 
understand it more - but I don't know how the LLVM can help me with this.

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.
But walking through the debug symbols is my problem - I don't know how to 
understand them. When the code gets jitted I get section with the name 
"debug" but I don't know how to handle them. So I hoped that I maybe could 
convert the meta data from the BC files to DWARF as you mentioned. But I 
don't know if there is a tool for this or maybe some functions. I looked 
at llvm::Module and llvm::ExecutionEngine because I use them for jitting - 
but there was no helpful function I thought...

Kind regards
Björn





From:   mats petersson <mats at planetcatfish.com>
To:     bjoern.gaier at horiba.com
Cc:     llvm-dev <llvm-dev at lists.llvm.org>
Date:   19.01.2018 16:03
Subject:        Re: [llvm-dev] [JIT] Evaluating Debug-Metadata in bitcode
Sent by:        mats.o.petersson at googlemail.com



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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Als GmbH eingetragen im Handelsregister Bad Homburg v.d.H. HRB 9816, 
USt.ID-Nr. DE 114 165 789
Geschäftsführer: Dr. Hiroshi Nakamura, Dr. Robert Plank, Markus Bode, 
Heiko Lampert, Hiroshi Kawamura, Takashi Nagano, Takeshi Fukushima.

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