[LLVMdev] transitions from C code to LLVM JIT code and back

[dpenry]

Keith Bauer wrote:
>> What I have not seen an example of, and want to know if it's
>> possible, is having LLVM generated code calling back out to plain C
>> code during its run.
>>     
>
> When I was first looking at LLVM, I googled a lot, and found somebody
> saying to do it this way:
>
> http://onesadcookie.com/trac/browser/AbandonedExperiments/Argh/CodeGen.g
> (lines 43-51).  I'm pretty sure now though that that's the wrong way
> to do it, that you can just add an existing C function to LLVM's
> function table and it'll get called automatically.  There was somebody
> talking about this on this list recently...
>
>   


My students have had some success with doing calls from LLVM-generated 
code into outside-compiled code when using the JIT in 2.0 (we haven't 
upgraded to 2.1 yet) and there were only two things we had to do:

1) make sure all the functions we wanted to call were actually linked 
into the JIT binary or brought in by loading a dynamic library.  Because 
of the nature of our project we chose the former and just copied lli.cpp 
to our own project/tools and built a new JIT with the stuff we wanted 
linked in.

2) if you link the functions into the JIT, you have to make the JIT's 
symbols available for lookup; this may actually be automatically done in 
2.0, but you can guarantee it by adding a call:
  sys::DynamicLibrary::LoadLibraryPermanently(0);
into main of your copy of the JIT

We didn't have to declare the C functions to LLVM or anything like that; 
llvm-ld assumes if it doesn't have a definition for a symbol, then it 
must be coming from somewhere else, unlike most system linkers.

Note that going the other way (calling LLVM-generated code from outside 
code) is quite a bit harder because the symbol you're calling can't be 
resolved to an address without making LLVM API calls.  We haven't had to 
do this in our project.

Disclaimer: I'm pretty new to LLVM and there might be something horribly 
wrong about the way we did this, but it seems to work fine for us.

--David