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

[Hal Finkel]

----- Original Message -----
> From: "Revital1 Eres" <ERES at il.ibm.com>
> To: "Lang Hames" <lhames at gmail.com>
> Cc: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
> Sent: Monday, July 27, 2015 1:17:52 AM
> Subject: Re: [LLVMdev] Help with using LLVM to re-compile hot functions at	run-time
> 
> 
> Hi Again,
> 
> I'm a little confused regarding what is the exact Orc's functions I
> should use
> in order to save the functions code in a code cache so it could be
> later
> replaced with different versions of it and I appreciate your help.
> 
> Just a reminder I want to dynamically recompile the program based on
> profile
> collected at the run-time. I would like to start executing the
> program from
> the code-cache and at some point be able to replace a function body
> with it's
> new compiled version; this can be done by replacing the entry in the
> function
> code with a trampoline to It's new version so that future calls to it
> will
> call the new version code.
> 
> Does the CompileOnDemandLayer executes the program from a code cache
> and holds pointers to the code of the functions it executes? I am
> compiling for Power machine.
> Is there a target specific pieces that I should implement for making
> Orc work on Power?

There is code in lib/ExecutionEngine/Orc/OrcTargetSupport.cpp that is currently only implemented for x86_64 that is necessary in order to make the lazy compilation work (triggering compilation only when a function is first called, etc.). I'll let someone else comment on the rest of the details...

 -Hal

> 
> Thanks again,
> 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: 20/07/2015 08:41 PM
> Subject: Re: [LLVMdev] Help with using LLVM to re-compile hot
> functions at run-time
> 
> 
> 
> 
> Hi Revital,
> 
> The CompileOnDemand layer is used by the lazy bitcode JIT in the lli
> tool. You can find the code in llvm/tools/lli/OrcLazyJIT.* .
> 
> Cheers,
> Lang.
> 
> 
> On Mon, Jul 20, 2015 at 2:32 AM, Revital1 Eres < ERES at il.ibm.com >
> wrote:
> 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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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory