> Sounds good. Please let me know how your experiments go. I'm keen to
improve the Orc APIs further, so your feedback would be very welcome.
LLILC is using ORC now, and it was a remarkably smooth transition / small change
(https://github.com/dotnet/llilc/commit/47513add13980e7a32f9b0ec3d2da3db0911bca2).
The CoreCLR execution engine handles laziness (and symbol resolution) for us, so
a straightforward application of IRCompileLayer using SimpleCompiler and
ObjectLinkingLayer along the lines of the "initial" orc kaleidoscope
example covered our current functionality neatly.
There were just two spots that I found myself writing some boilerplate:
One was in constructing the TargetMachine (to pass to the SimpleCompiler
constructor). With MCJit, the EngineBuilder took care of creating the
TargetMachine, and the code to set up the EngineBuilder looked like this:
std::unique_ptr<Module> M = ..;
M->setTargetTriple(LLILC_TARGET_TRIPLE);
EngineBuilder Builder(std::move(M)); // Builder takes ownership of module
std::string ErrStr;
Builder.setErrorStr(&ErrStr);
Builder.setOptLevel(/* value depending on what the CLR EE requests */);
Builder.setTargetOptions(/* value depending on what the CLR EE requests */);
ExecutionEngine *NewEngine = Builder.create();
I noticed you used "EngineBuilder().selectTarget()" in the
kaleidoscope sample, but I couldn't do exactly that for LLILC since
we're threading the triple through the module that the builder wants to own,
and also it seemed to make sense that we shouldn't need an EngineBuilder if
we're not building an Engine, so I looked at what our Builder had been doing
and wound up with this code to instead create the TargetMachine directly:
std::string ErrStr;
const llvm::Target *TheTarget
TargetRegistry::lookupTarget(LLILC_TARGET_TRIPLE, ErrStr);
TargetOptions Options = /* value depending on what the CLR EE requests */;
CodeGenOpt::Level OptLevel = /* value depending on what the CLR EE requests
*/;
TargetMachine *TM = TheTarget->createTargetMachine(
LLILC_TARGET_TRIPLE, "", "", Options, Reloc::Default,
CodeModel::Default,
OptLevel);
This tiny amount of boilerplate that I ended up with for creating the
TargetMachine seems entirely reasonable, but going through the exercise and
looking at EngineBuilder code (which of course has several more paths than
reflected above) made me wonder how much of its logic is for building the
TargetMachine and whether pulling it out into a TargetMachineBuilder would be
useful to give ORC clients the support that EngineBuilder gives MCJit clients.
The second spot where I needed some code was getting a SymbolResolver to pass to
the ObjectLinkingLayer. In LLILC we actually don't need cross-module
symbols resolved because the CoreCLR execution engine resolves them and gives
the Jit raw addresses. So I simply defined a NullResolver like so (I felt it
was more readable this way than building an equivalent LambdaResolver):
/// \brief Stub \p SymbolResolver expecting no resolution requests
///
/// The ObjectLinkingLayer takes a SymbolResolver ctor parameter.
/// The CLR EE resolves tokens to addresses for the Jit during IL reading,
/// so no symbol resolution is actually needed at ObjLinking time.
class NullResolver : public llvm::RuntimeDyld::SymbolResolver {
public:
llvm::RuntimeDyld::SymbolInfo findSymbol(const std::string &Name) final {
llvm_unreachable("Reader resolves tokens directly to addresses");
}
llvm::RuntimeDyld::SymbolInfo
findSymbolInLogicalDylib(const std::string &Name) final {
llvm_unreachable("Reader resolves tokens directly to addresses");
}
};
It occurs to me that if any other ORC clients are in the same boat, they'll
need a resolver that look a lot like this, but I think perhaps we're in an
unusual boat.
One general comment I have is that I wish it were more easily discoverable what
the constraints are on the various template parameters in ORC code, both from
the client side (e.g. realizing that SymbolResolverPtrT should be a
naked/unique/etc pointer to RuntimeDyld::SymbolResolver or a derived class) and
the consumer side (e.g. realizing that an ObjSetHandleT needs to be moved rather
than assigned). The comment on IRCompileLayer mentions that BaseLayerT
"must implement the object layer concept", for example, but I
wasn't sure what exactly constitutes the object layer concept. I had to dig
into that one, as I went on to implement an object layer to satisfy the
CoreCLR's requirements on EH frame reporting that started this thread. I
figured that implementing the methods on ObjectLinkingLayer would be a good bet,
but approached it by starting with just addObjectSet and adding method
implementations as the compiler complained they were missing. With that
approach, I never did need to implement findSymbolIn, mapSectionAddress, or
emitAndFinalize, for reasons I haven't dug in to understand. The object
layer class I produced in this way was itself mostly boilerplate:
class ReserveSpaceLayerT {
public:
typedef LLILCJit::LoadLayerT::ObjSetHandleT ObjSetHandleT;
ReserveSpaceLayerT(LLILCJit::LoadLayerT *Loader, llilc::EEMemoryManager *MM) {
this->Loader = Loader;
this->MM = MM;
}
template <typename ObjSetT, typename MemoryManagerPtrT, typename
SymbolResolverPtrT>
ObjSetHandleT addObjectSet(const ObjSetT &Objects,
MemoryManagerPtrT MemMgr,
SymbolResolverPtrT Resolver) {
for (const auto& Obj : Objects) {
MM->reserveUnwindSpace(*Obj);
}
return Loader->addObjectSet(Objects, MemMgr, Resolver);
}
void removeObjectSet(ObjSetHandleT H) {
Loader->removeObjectSet(std::move(H));
}
orc::JITSymbol findSymbol(StringRef Name, bool ExportedSymbolsOnly) {
return Loader->findSymbol(Name, ExportedSymbolsOnly);
}
template <typename OwningMBSet>
void takeOwnershipOfBuffers(ObjSetHandleT H, OwningMBSet MBs) {
Loader->takeOwnershipOfBuffers(std::move(H), std::move(MBs));
}
private:
LLILCJit::LoadLayerT *Loader;
llilc::EEMemoryManager *MM;
};
I could imagine that abstracting this might make a useful utility -- what I
wanted was to call MM->reserveUnwindSpace on each ObjectFile as it passes
through, and otherwise forward everything on to the base layer. Perhaps a
LambdaObjectLayer that takes a function<void(const ObjectFile*)> would be
appropriate? Let me know if you'd like to see a patch along those lines;
I'm happy to contribute back, but don't have much context on what would
be useful for other clients.
Thanks
-Joseph
From: Lang Hames [mailto:lhames at gmail.com]
Sent: Monday, June 1, 2015 1:41 PM
To: Joseph Tremoulet
Cc: Russell Hadley; llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] MCJit interface question
Hi Russell, Joseph,
> I'll look into moving LLILC to ORC.
Sounds good. Please let me know how your experiments go. I'm keen to improve
the Orc APIs further, so your feedback would be very welcome.
Cheers,
Lang.
On Sat, May 30, 2015 at 11:14 AM, Joseph Tremoulet <jotrem at
microsoft.com<mailto:jotrem at microsoft.com>> wrote:
Agreed, that sounds like the best plan. I'll look into moving LLILC to ORC.
Thanks
-Joseph
From: Russell Hadley
Sent: Friday, May 29, 2015 8:13 PM
To: Lang Hames; Joseph Tremoulet
Cc: llvmdev at cs.uiuc.edu<mailto:llvmdev at cs.uiuc.edu>
Subject: RE: [LLVMdev] MCJit interface question
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> [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<mailto: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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