[PATCH][llvm-c] Expose MC JIT
Kaylor, Andrew
andrew.kaylor at intel.com
Mon Apr 22 11:51:06 PDT 2013
I wasn’t sure the C API needed to support the case of JITing into another address space. I guess it makes sense that we’d want to make that available eventually.
I do think it would be better to have MCJIT finalize the object from getPointerToFunction(), but as I said we can’t do that until there’s another way to trigger code generation. This is part of the sloppiness I referred to earlier. There is a web of interactions going on within MCJIT and there are some constraints on the order in which things can be done. Right now MCJIT has a patchwork way of handling this, but it really needs to be cleaned up to manage things in a more intentional way.
Anyway, my immediate goal here is to figure out the simplest reasonable thing we can do to get your patch working in a sustainable way.
I don’t know if anything other than lli is relying on the current side-effects of MCJIT::getPointerToFunction(). I’d prefer to hold off on putting new things in the MCJIT interface until the aforementioned clean-up is done. I was suggesting putting the call to finalizeObject in the C API because I’d rather rush something into implementation than rush something into an interface, but depending on your timeline for making the C API I could try to bump the priority of the MCJIT state handling clean-up on my end.
-Andy
From: Filip Pizlo [mailto:fpizlo at apple.com]
Sent: Monday, April 22, 2013 11:25 AM
To: Kaylor, Andrew
Cc: David Tweed; Eric Christopher; llvm-commits at cs.uiuc.edu
Subject: Re: [PATCH][llvm-c] Expose MC JIT
Wouldn't it be better to just have MCJIT call finalizeObject in getPointerToFunction and friends, and eradicate ExecutionEngine::finalizeObject() completely?
OTOH, if I understand right and the goal of the MCJIT is to support the notion of JITing into someone else's address space, then finalizeObject() makes loads of sense: you want the client to be able to say when it happens. If that's true then it should be exposed to the C API.
-Filip
On Apr 22, 2013, at 10:57 AM, "Kaylor, Andrew" <andrew.kaylor at intel.com<mailto:andrew.kaylor at intel.com>> wrote:
It definitely won’t be necessary to expose more things through the C API. At most, things will need to be added to the implementation.
I’m wondering if we can even avoid having to put the ‘finalize’ concept into the API.
As I understand it, the normal work flow would be something like this:
1. Create a module and populate it
2. Create an execution engine for the module
3. Get a pointer to a function in the module
4. Execute the function
If that’s right, I guess MCJIT trips because while its implementation of getPointerTo[Named]Function triggers compilation it doesn’t cause permissions to be applied or invalidate the code cache. This happens because MCJIT needs to handle the case where the generated code is going to be executed in another process (and possibly on another system), so it doesn’t make assumptions about when everything is in its final place. The C API, however, could arguably make such assumptions.
What this boils down to is that somewhere between step 2 above and step 4 above we need to:
1. Generate the code
2. Apply relocations
3. Apply memory permissions
4. Invalidate the code cache
MCJIT does 1 and 2, if necessary, in response to a getPointerToFunction() call. Arguably it could also do 3 and 4 there, since in the remote case the client code isn’t going to want pointers to functions. The trouble is that there are places (such as lli) where we are using that call to trigger code emission even though we may still want to move things around before 3 and 4 happen. That wouldn’t be a problem if we exposed a function to trigger code emission directly, but we don’t right now.
However, it may be reasonable for the C API implementation to call MCJIT::finalizeObject when its getPointerToFunction equivalent is called.
That leaves us with invalidating the code cache. I don’t see any reason that the memory manager shouldn’t do that automatically when the applyPermissions function is called. I notice that currently invalidateInstructionCache is part of the SectionMemoryManager interface but not the RuntimeDyldMemoryManager, so that’s a problem already if we don’t just make it part of applyPermissions().
So what I’m thinking is that if you can add a call to MCJIT::finalizeObject in the appropriate places in the C API implementation then the FinalizeAllObjects method can be removed completely. I’ll add an invalidateInstructionCache() call in the SectionMemoryManager::applyPermissions() implementation, and that should take care of the ARM issue.
Sounds good.
Does that sound reasonable?
-Andy
From: Filip Pizlo [mailto:fpizlo at apple.com]
Sent: Monday, April 22, 2013 10:19 AM
To: Kaylor, Andrew
Cc: David Tweed; Eric Christopher; llvm-commits at cs.uiuc.edu<mailto:llvm-commits at cs.uiuc.edu>
Subject: Re: [PATCH][llvm-c] Expose MC JIT
On Apr 22, 2013, at 10:06 AM, "Kaylor, Andrew" <andrew.kaylor at intel.com<mailto:andrew.kaylor at intel.com>> wrote:
The state management in MCJIT is quite sloppy right now. I agree that invalidating the code cache is an issue that needs to be considered. It seems to me that MCJIT itself ought to be able to do that when it needs to be done if it were paying attention. At the very least, it could happen in the 'finalizeObject' method. That's somewhat tangential to the patch at hand, but it is a consideration. If there's something quick I can do to MCJIT to make this work, that's probably preferable to pushing something into the C-interface implementation. I'll give that some thought today, but if anyone else is interested I'd be happy to make it a discussion rather than just a private rumination.
Hopefully we can do this without exposing more stuff via the C API. I think that finalizeObject() should do this, but I will think about it some more.
Otherwise, my main question about the patch has to do with the nature of the C-interface API. Is that API treated as a contract that needs to be respected from release to release or are we free to tinker with it as needed? The thing that worries me is how this interface will survive the transition to multiple module support.
I believe that this is the goal of the C API, yes. It is also a goal of this patch to be forward-compatible in this way.
My patch defends against this in two ways:
1) When creating the MCJIT via the C API, I just follow the same convention as other ExecutionEngines do: you specify a module, but you can add one later. Right now calling AddModule on an MCJIT instance will crash, and the documentation tells you this. Once the MCJIT supports multiple modules, I believe that this should Just Work - you will then be able to call AddModule.
2) I don't expose finalizeObject() directly. Instead I created a new API called FinalizeAllObjects(), which requires that all modules associated with the execution engine get finalized at the time of call. My understanding is that it is safe to finalizeObject() if you've already done it before - it's currently idempotent. So if MCJIT goes multi-module, then this API will still have a well-defined behavior, and this behavior will not be different from what my patch does.
-Filip
-Andy
-----Original Message-----
From: llvm-commits-bounces at cs.uiuc.edu<mailto:llvm-commits-bounces at cs.uiuc.edu> [mailto:llvm-commits-bounces at cs.uiuc.edu] On Behalf Of David Tweed
Sent: Monday, April 22, 2013 7:33 AM
To: 'Eric Christopher'; Filip Pizlo
Cc: llvm-commits at cs.uiuc.edu<mailto:llvm-commits at cs.uiuc.edu>
Subject: RE: [PATCH][llvm-c] Expose MC JIT
Hi,
Not a comment on the general idea (which seems like a good one), but asking detail question: some architectures (such as ARM) invalidating cache entries at more times than, eg, x86. (IIRC ARM needs to invalidate the cache at the transition time between being "data" and "instructions".) It looks to me like this can be done in LLVMFinalizeAllObjects(), but I'm cc:ing someone much, much more knowledgeable about this than me... It's certainly desirable that the API provides enough points at which the MC JIT is in command that it can call all the (possibly platform specific) permissions/cache invalidation operations needed on that memory without user code needing to do it.
Other than that, the patch looks like a good patch to me (but again Andrew is the main authority).
Cheers,
Dave
-----Original Message-----
From: llvm-commits-bounces at cs.uiuc.edu<mailto:llvm-commits-bounces at cs.uiuc.edu>
[mailto:llvm-commits-bounces at cs.uiuc.edu] On Behalf Of Eric Christopher
Sent: 22 April 2013 14:22
To: Filip Pizlo
Cc: llvm-commits at cs.uiuc.edu<mailto:llvm-commits at cs.uiuc.edu>
Subject: Re: [PATCH][llvm-c] Expose MC JIT
The API and rationale seem totally reasonable to me. Up to Andy to approve though.
-eric
On Sun, Apr 21, 2013 at 7:14 PM, Filip Pizlo <fpizlo at apple.com<mailto:fpizlo at apple.com>> wrote:
OK - I have a new patch for review, which incorporates Andrew's feedback.
The patch exposes the MCJIT via the C API.
The current API uses the SectionMemoryManager by default. I plan to
expose
the ability to have the C API call back into the client's code, and
use
the
client's own memory manager, in the future. But even then, the
default
will
be SectionMemoryManager. Because this requires calling
applyPermissions(),
I also expose the ExecutionEngine::finalizeObject() method. This was
tricky
- I take it that in the future, this method will take a Module*M
parameter to specify which module to finalize. In order to not create
future confusion in the C API, I expose this as
LLVMFinalizeAllObjects() and specify the API's semantics as being that
all objects associated with the execution engine should be finalized.
The patch also exposes the NoFramePointerElim option. The manner in
which options are exposed is designed for forward compatibility; you
supply an options struct along with a size which you zero-fill prior
to
manipulating.
This is similar to the idiom I've seen used in other C APIs like
BerkeleyDB.
I considered having separate C function calls for each option, in the
style
of the ExecutionEngineBuilder API - but while that idiom feels right
to me in C++, it feels less C-like. As well, the current options
approach
exposes
not just parts of the Builder but also part of TargetOptions (namely,
NoFramePointerElim). It's also more concise in practice.
I plan to expose more innards through the LLVMMCJITCompilerOptions in
the future. I'd be happy to do more of that in one go if that was
preferred; but I thought that a baby step would be the best thing for now.
-Filip
On Apr 21, 2013, at 6:26 PM, Filip Pizlo <fpizlo at apple.com<mailto:fpizlo at apple.com>> wrote:
On Apr 15, 2013, at 10:42 AM, "Kaylor, Andrew"
<andrew.kaylor at intel.com<mailto:andrew.kaylor at intel.com>>
wrote:
OK, let me start by saying that MCJIT does take ownership of the
memory manager. It doesn't use an OwningPtr, which would make this
clear, but it does delete the pointer in its destructor. I think this
is happening because we needed some finer control over when the MM got
deleted. I
should
probably revisit this and at least add some comments to make it clear
what's
happening and why. It might not even be an issue anymore, because I
did some work a while ago to try to clean up object ownership issues.
Actually, I was just confused. MCJIT deletes MemMgr, which is always
aliased to Dyld, as far as I can tell. So I was just wrong. :-)
That said, I have been meaning for some time to break apart the JIT
and MCJIT interfaces. The fact that they are both abstracted by
ExecutionEngine
and EngineBuilder complicates that, but it really needs to be done (as
you are seeing).
For now, would it be possible to have the C-interface provide a
wrapper
that
supplies empty implementations of the irrelevant functions when
creating a memory manager for MCJIT?
I think this is sensible. I will proceed in this way.
Thanks!
-Andy
From: Filip Pizlo [mailto:fpizlo at apple.com]
Sent: Saturday, April 13, 2013 1:18 AM
To: Kaylor, Andrew
Cc: llvm-commits at cs.uiuc.edu<mailto:llvm-commits at cs.uiuc.edu>
Subject: Re: [PATCH][llvm-c] Expose MC JIT
Ah - good thing you pointed this out. I just realized that my patch
is wrong. Perhaps I can get some feedback on the best way to architect this.
Here's the problem:
- MCJIT does not take ownership of the memory manager. Hence
allocating
one
in the constructor is wrong; it'll leak when MCJIT dies. But deleting
the memory manager passed to MCJIT would be a change in behavior, and
I'm not sure if it's in line with either what existing users expect or
what was intended. Insofar as the JIT instance corresponds to
ownership of
modules,
it feels like it shouldn't also take ownership of the memory manager;
for example you might imagine wanting to throw away the MCJIT but keep
the
code
it generated and continue to use the memory manager to track it - and
eventually free it. But EngineBuilder currently claims that the
ExecutionEngine takes ownership of the JMM - I'm assuming that this is
just
wrong documentation, and that EngineBuilder's use of the same JMM
option
for
both JIT and MCJIT is just not right.
- I'd like to expose SectionMemoryManager and, eventually in a
separate patch, the ability to create custom RTDyldMemoryManagers via the C API.
I'd
prefer this to be an RTDyldMemoryManager and not a JITMemoryManager,
since the latter has a load of methods that are not relevant to MCJIT.
But EngineBuilder wants a JITMemoryManager. This would mean that the
C API would have to pass its RTDyldMemoryManager via a cast to
JITMemoryManager just so MCJIT could then use it as an
RTDyldMemoryManager again. Seems wrong. I'm assuming that the
correct long-term thing is to fix the EngineBuilder to not pass the
JMM to the MCJIT, since it's good to expose the fact that the MCJIT
actually just wants an RTDyldMemoryManager
instead.
In short, I'd like to have a separate EngineBuilder setting for the
RTDyldMemoryManager. If this is specified and you end up using the
JIT
and
not MCJIT, you get an error. If you use the MCJIT, then the
RTDyldMemoryManager option overrides the JMM option. Or something
similar.
Does that make sense?
-Filip
On Apr 12, 2013, at 5:38 PM, Filip Pizlo <fpizlo at apple.com<mailto:fpizlo at apple.com>> wrote:
Thanks for the feedback! I will try this change and see what happens.
-Filip
On Apr 12, 2013, at 5:35 PM, "Kaylor, Andrew"
<andrew.kaylor at intel.com<mailto:andrew.kaylor at intel.com>>
wrote:
Hi Filip,
I'll take a closer look at your patches on Monday, but my initial
input is that the default memory manager used should be
SectionMemoryManager rather than the DefaultJITMemoryManager.
Thanks,
Andy
From: llvm-commits-bounces at cs.uiuc.edu<mailto:llvm-commits-bounces at cs.uiuc.edu>
[mailto:llvm-commits-bounces at cs.uiuc.edu] On Behalf Of Filip Pizlo
Sent: Friday, April 12, 2013 4:49 PM
To: llvm-commits at cs.uiuc.edu<mailto:llvm-commits at cs.uiuc.edu>
Subject: Re: [PATCH][llvm-c] Expose MC JIT
Revised patches included.
I added additional ruggedizing to the LLVMCreateMCJITCompilerForModule
function, so that if it detects that the passed struct is larger than
expected, it reports an error instead of continuing.
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