Disabling some JIT optimization?

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?

Jim Capps

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(...)

hope that helps.


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.