[LLVMdev] Help with using LLVM to re-compile hot functions at run-time

[Lang Hames]

Hi Revital,

LLVM does have an IR interpreter, but I don't think it's maintained well
(or possibly at all). The interpreter is also not designed to interact with
the LLVM JITs.

We generally encourage people to just JIT LLVM IR, rather than interpreting
it. For the use-case you have described, you could JIT IR with no
optimizations to begin with, then re-JIT hot functions at a higher level.

The Orc JIT APIs (LLVM's newer JIT APIs) were written with this kind of
use-case in mind, and are probably a better fit for this than MCJIT. There
is no built-in hot-function detection or recompilation yet, but I think
this would be *fairly* easy to write in terms of Orc's callback API.

Cheers,
Lang.


On Thu, Jul 9, 2015 at 4:19 AM, Revital1 Eres <ERES at il.ibm.com> wrote:

> Hello,
>
> I am new to LLVM and a I appreciate your help with the following:
>
> I want to run the LLVM IR through virtual machine (LLVM interpreter?) and
> jit
> compile the hot functions (using MCJIT).
>
> This task will require amongst other identifying the hot functions and
> having a
> code cache that should be patched with the native code of the functions
> after
> they are jitted.
>
> I've read so far about MCJIT and lli however I have not seen that the LLVM
> interpreter can be used as a VM the way I was looking for; meaning
> execute the code one instruction at a time; have a profiling mode to
> identify hot functions and call jit to compile the hot functions.
>
> I appreciate any advice/starting points for this project.
>
> Thanks,
> Revital
>
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