[LLVMdev] MCJit interface question

[Russell Hadley]

Hey Joseph,

What Lang said made me wonder.  Is it the right time for us (LLILC) to move to ORC?  The long term plan was to go there but this could be our forcing function.

-R

From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of Lang Hames
Sent: Friday, May 29, 2015 2:23 PM
To: Joseph Tremoulet
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] MCJit interface question

Hi Joseph,

There are several reasons that a client might want to access the object before it's loaded, so a general API like #2 seems like the way to go.

To support this in MCJIT you can add this to the event listener API. Orc clients can already do this by adding a custom object-file layer.

- Lang.



On Fri, May 29, 2015 at 9:05 AM, Joseph Tremoulet <jotrem at microsoft.com<mailto:jotrem at microsoft.com>> wrote:
Hi,

I think I need to make a small change to the MCJit interface, and would like some feedback on what the most appropriate option would be.

I'm working on LLILC (a jit for the CoreCLR built on MCJit, which creates one module for each MSIL method, containing the main function and zero or more EH handler functions extracted from the MSIL method).  The CoreCLR requires the jit to notify it of the size of the unwind info descriptors for each function in the module before reserving the memory it will be loaded into.  So we need a call (or calls) from the Jit/loader into the MemoryManager that runs at-or-before reserveAllocationSpace, is conceptually similar to registerEHFrames in that it's reserving EH frames, but that additionally needs to separate out the per-function information.

A few options come to mind:

1.       Add a needsToReserveEHFrames callback on MemoryManager (to parallel needsToReserveAllocationSpace), and a reserveEHFrames callback (parallel to registerEHFrames) that the loader would use to notify the memory manager if needsToReserveEHFrames() is true.  This seems at a high-level the most straightforward fit for the LLILC requirement, but I don't know if for other targets it would even be possible to move the identification of EH frames (which depends on the 'LocalSections' computed in loadObjectImpl) to before calling reserveAllocationSpace.  I also don't know if that would be an undesirable "tight coupling" of RuntimeDyld with CoreCLR's particular interface. (and note that this is really two options, in that the code to separate out the per-function EH frame contribution could go in either the client memory manager or in the loader.)

2.       Add a notification similar to NotifyObjectEmitted, but which runs just before the call to Dyld.loadObject.  Something like NotifyObjectPreLoaded.  The LLILC-specific MemoryManager could use this hook to scan the object::ObjectFile and pass whatever it needs to the CoreCLR.  This seems like a decent option to me, but I don't know if it would be considered a bad loss of encapsulation to passs out the object::ObjectFile in the state where it's been 'created' but not yet 'loaded'.

3.       Similar to #2, the existing reserveAllocationSpace callback on MemoryManager could simply take an additional parameter which is the object::ObjectFile.  This would be a more minimal change than #2 in terms of how much surface area it adds to the MCJit interface, but also a more invasive change in that all MemoryManager implementations would need to be updated with the reserveAllocationSpace signature change (though they could just ignore the new parameter).

4.       We could avoid needing to crack the object file for this information altogether; MCJit could add a hook where a client could insert passes into the PassManager used in emitObject; LLILC could attach a pass that would consult the MachineModuleInfo, where this information could be stored (it's similar to what's stored in the WinEHFuncInfos hanging off the MMI today).  But adding hooks for client passes might be opening a can of worms…

My inclination would be #2 or #3, but I would love some feedback on which of the tradeoffs seem most agreeable (and/or what options I've overlooked).

Thanks
-Joseph


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