[LLVMdev] Using LLVM to serialize object state -- and performance

[Kaylor, Andrew]

I'm not sure I have a clear picture of what you're JIT'ing.  If any of the JIT'ed functions call other JIT'ed functions, it may be difficult to find all the dependencies of a functions and recreate them correctly on a subsequent load.  Even if the JIT'ed functions only call non-JIT'ed functions, I think you'd need some confidence that the address of the called functions wasn't being moved.

It's possible that what you're considering would work, but I don't think it's a scenario that the JIT intends to support.

It would be possible, however, to use the MCJIT engine and cache its results.  It requires some modifications to the MCJIT engine but nothing major (I know because my team has a patch in the works to do this, but it's blocked by some other things at the moment).  MCJIT generates complete object images and then uses RuntimeDyld to load them.  If you had a hook to save the generated object, you could use RuntimeDyld directly to load it later.  There are other ways to generate the object image (i.e. without MCJIT), but I'm not sure it would be easier.

You basically just need to grab the Buffer that MCJIT::emitObject() has after it calls PM.run() and Buffer->flush() but before it passes it to Dyld.loadObject().  If you prefer, you could copy what MCJIT does and move it somewhere in your own code.  There's not a lot to it. 

-Andy


-----Original Message-----
From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of Paul J. Lucas
Sent: Friday, October 26, 2012 4:17 PM
To: llvmdev at cs.uiuc.edu List
Subject: [LLVMdev] Using LLVM to serialize object state -- and performance

I have a legacy C++ application that constructs a tree of C++ objects (an iterator tree to implement a query language).  I am trying to use LLVM to "serialize" the state of this tree to disk for later loading and execution (or "compile" it to disk, if you prefer).

Each of the C++ iterator objects now has a codegen() member function that adds to the LLVM code of an llvm::Function.  The LLVM code generated is a sequence of instructions to set up the arguments for and call the constructor of each C++ object.  (I am using C "thunks" that provide a C API to LLVM to make C++ class constructor calls.)  Hence, all the LLVM code taken together into a single "reconstitute" function are mostly a sequence of "call" instructions with a few "store" and "getelementptr" instructions here and there -- fairly straight-forward LLVM code.

I then write out the LLVM IR code to disk and, at some later time, read it back in with ParseIR(), do getPointerToFunction(), execute that function, and the C++ iterator tree has been reconstituted.

This all works, but the JIT compile step is *slow*.  For a sequence of about 8000 LLVM instructions (most of which are "call"), it takes several seconds to execute.

It occurred to me that I don't really want JIT compiling.  I really want to compile the LLVM code to machine code and write that to disk so that when I read it back, I can just run it. The "reconstitute" function is only ever run once per query invocation, so there's no benefit from JIT compiling it since it will never be run a second or subsequent time.

Questions:

* Is what I'm doing with LLVM a "reasonable" thing to do with LLVM?
* If so, how can I speed it up?  By generating machine code?  If so, how?

I've looked at the source for llc, but that apparently only generates assembly source code, not object code.

- Paul


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