[llvm-dev] [ORC JIT] Getting symbols from object files before any lookup is done

[Gaier, Bjoern via llvm-dev]

Hey Lang,

Yes my intention was to use the “lookup” function of the executionsession to get the address of every symbol in the object file or module. In general I would prefer to even get the address of the hidden symbols. Just for learning though.
However I have a big issue, if I search in the LLVM code base on my machine for “JITDylibLookupFlags” or “MatchAllSymbols” I will not find a thing.

Seems like I’m not running the latest LLVM version – I will download the LLVM10 for those changes now - or are they in some branch I’m not aware of?

Thank you for the help :3

Kind greetings
Björn

From: Lang Hames <lhames at gmail.com>
Sent: 29 April 2020 23:10
To: Gaier, Bjoern <Bjoern.Gaier at horiba.com>
Cc: LLVM Developers Mailing List <llvm-dev at lists.llvm.org>
Subject: Re: [ORC JIT] Getting symbols from object files before any lookup is done

Hi Bjoern,

The general answer is to scan the interface of whatever program representation you are adding to the JIT. In the object file case you would create an llvm::object::ObjectFile and then iterate over its symbols. As you noticed you have to be careful with linkage (e.g. private and available-externally definitions aren't really part of the interface) and you have to be aware that the compilation of higher level program representations can cause new definitions to be created (e.g. for constant pool entries).

What are you planning to do with the symbols once you have them? If you are going to issue a lookup you will have to be careful with the lookup and visibility flags. A lookup using JITDylibLookupFlags::MatchExportedSymbolsOnly will fail if you look up a hidden (i.e. non-exported) symbol. You can either not look up hidden symbols, or you can use JITDylibLookupFlags::MatchAllSymbols.

Regards,
Lang.

On Wed, Apr 22, 2020 at 4:51 AM Gaier, Bjoern <Bjoern.Gaier at horiba.com<mailto:Bjoern.Gaier at horiba.com>> wrote:
Hello LLVM-Mailing-List and hello Lang (I figured somebody would add you….so I did it directly. Sorry),

I'm still playing around with the ORC JIT version which is available in LLVM 10. My goal is to get the address of every defined symbol in a module and an object file before the code is emitted. With the help of Lang I do this now with the lookup function of the Execution Session. Thank you for this!

However, my next issue is: How can I find every defined symbol in my object files? Currently I attached a callback function with the "setNotifyLoaded" function of "RTDyldObjectLinkingLayer". Which is really handy, because I can simply iterate over the symbols of the object files.
However this only works when I already know what is inside my object files, which kinda kills the purpose.

For a workaround I iterate through all the global symbols and functions of the IR module I'm loading, skipping symbols that are not defined in the module. However, I noticed when I do a lookup on those symbols, I will get some "additional" symbols from the notify loaded callback function having addresses I will miss then. Also this does not help me when I'm loading an object file from disk cause I don't have such a function there.

Okay... explaining stuff is not my best practise so.... Here a tl;dr:
How can I get all symbols from an object file when it is loaded/added to ORC JIT?

Sorry that I still have questions >o<

Kind greetings
Björn
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, Takashi Nagano, Junichi Tajika, Ergin Cansiz.
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, Takashi Nagano, Junichi Tajika, Ergin Cansiz.
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