[LLVMdev] Disabling some JIT optimization?

[Capps, Jim]

Keno,

With your suggestion, I think I have figured out exactly how to do what I need.   It will take a day or two of testing before I know for sure if it working properly, but it certainly looks promising.

Thanks,
Jim

-----Original Message-----
From: Keno Fischer [mailto:kfischer at college.harvard.edu] 
Sent: Monday, August 18, 2014 3:53 PM
To: Capps, Jim
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Disabling some JIT optimization?

You can set what CPU model to generate when you call EngineBuilder::selectTarget. If you're not calling that directly yet, you might have to mull it out of the call to create:

auto TM = eb.selectTarget(...)
eb.create(TM)

hope that helps.

On Mon, Aug 18, 2014 at 4:39 PM, Capps, Jim <James.Capps at hp.com> wrote:
> I am currently trying to use LLVM (version 3.2) to generate code for a 
> cluster where each node in the cluster will be running a 64-bit X86 
> chip, but *not* necessarily the same version of the X86 chip 
> throughout the cluster.  I am specifying that I want 
> Position-Independent Code produced because the generated machine code 
> may get moved (before it is executed) to a different logical address that where it is built by the JIT Compiler.
> Furthermore, the generated machine code may get moved to a different 
> node in the cluster before it is executed.
>
> Yes, I know that version 3.2 is not the latest and, yes, I know that I 
> should be using MCJIT rather than JIT, but from my reading thus far on 
> version 3.4 and on MCJIT, I don’t think it will affect the answer to my
> question.   If I am wrong, I would be happy to be corrected.    [When I
> started the project, version 3.2 was the latest available (and JIT 
> looked easier to use) … and I haven’t yet had sufficient reason to 
> upgrade.]
>
> I have been telling the JIT compiler to use the Default level of code 
> optimization.  My problem is that I believe I have found that the JIT 
> compiler is doing chip-specific code optimization to make the code run as
> fast as possible for the chip which is running the compiler.    However, the
> generated machine code may get moved to a different version of the chip when
> the code gets moved to a different node in the cluster.   I believe I can
> get generic X86 code generated by using an optimization level of NONE, 
> but that may hurt performance too much.
>
> So, my question is:  Is there an option (using either JIT or MCJIT) 
> where I can specify that I want the generated code to be generic 
> enough to run on any version of a 64-bit X86 chip … without turning off all optimization?
>
> Thanks,
> Jim Capps
>
>
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