[LLVMdev] Understanding resolving external/built-in function references

[James Molloy]

Hi,

> Is it possible to keep adding bitcode incrementally to the
> system, and JIT'ing it, and have the set of functions
> defined in the system keep growing, so that the next hunk
> of bitcode added can have access to all the functions
> defined so far?

Yes. The JIT supports lazy compilation mode where it will generate thunks that only get compiled/linked when called - see JIT::getPointerToFunctionOrStub *.

The above would solve a bijective requirement that earlier code may rely on functions defined in later code - if that is not necessary then the JIT should just work out of the box.

* Note that the lazy compilation mode currently does not work well in a multithreaded environment.

Cheers,

James

> -----Original Message-----
> From: Bill Kelly [mailto:billk at cts.com]
> Sent: 06 June 2011 13:36
> To: llvmdev at cs.uiuc.edu
> Cc: James Molloy
> Subject: Re: [LLVMdev] Understanding resolving external/built-in
> function references
>
> Hi James,
>
> > It depends on how your bitcode is loaded. It can be loaded either by
> llc,
> > which outputs pure ELF/Mach-O, in which case those function
> references get
> > turned into relocations that the linker (ld, Gold) applies.
> >
> > If you're loading bitcode via the JIT, it uses dlsym() to try and
> introspect
> > your runtime environment to find the symbol.
>
> Ah, that makes sense - thanks.
>
>
> > The simple answer is that LLVM bitcode cannot be "loaded and run" as-
> is - it
> > needs to be converted to machine code and relocations are processed
> at this
> > time.
> I>  suspect the way I'm hoping to use LLVM may be slightly
> uncommon:
>
> I'd like to use the LLVM C++ API to generate modules and
> functions on-the-fly and JIT-execute them, and then I'd
> like to add additional functions and modules in a piecemeal
> fashion, to the growing system.  Similar in this way to a
> Forth or Smalltalk image-based system where one can add,
> remove, or recompile function definitions on the fly, in
> the context of the whole image in memory.
>
> My impression so far from the LLVM C++ API is that this
> would appear to be possible - at least up to a point.
>
> One thing I'm unclear on is whether it's possible to add
> a new function and/or module to the system, which can
> reference functions in previously defined modules, without
> needing to re-generate the bitcode for the whole system.
>
>
>
> Thanks,
>
> Bill
>


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