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

[Revital1 Eres]

Hello Lang,

Thanks for your answer.

I am now looking for an example of the usage of CompileOnDemandLayer. Is 
there an example available for that (could not find one in llvm/examples)?

Thanks,
Revital



From:   Lang Hames <lhames at gmail.com>
To:     Revital1 Eres/Haifa/IBM at IBMIL
Cc:     LLVM Developers Mailing List <llvmdev at cs.uiuc.edu>
Date:   10/07/2015 12:10 AM
Subject:        Re: [LLVMdev] Help with using LLVM to re-compile hot 
functions at run-time



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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