[LLVMdev] Newbie question: Getting info about JIT-compiled function

[Andrew Haley]

Jeff Kuskin wrote:
> Apologies if this is a FAQ.
>
> I am using the LLVM JIT facility on an x86_64 platform.  I generate
> IR for a single function using IRBuilder(), use the
> FunctionPassManager to do some optimization passes, and then call
> ExecutionEngine::getPointerToFunction() to get a native-code version
> of the function.  Everything works fine so far.
>
> Two questions:
>
> Question (1): How can I get the size of the native function?  I see
> that the information is stored in the __jitSymbolTable structure in
> JITEmitter.cpp, but I don't see any interface to query this symbol
> table.  I see AddFunctionToSymbolTable() and
> RemoveFunctionFromSymbolTable(), but not any equivalent of
> GetFunctionFromSymbolTable().  Is there a supported way to get at
> the JitSymbolEntry for a function?  If not, is there an alternate
> way to determine the size (in bytes) of the native function?

The answer seems to be "create your own memory manager and define
endFunctionBody()".  If you have a look at class MyJITMemoryManager in
http://icedtea.classpath.org/hg/icedtea6/file/0507a324ec22/ports/hotspot/src/share/vm/shark/sharkBuilder.hpp
you'll see how I did it.

> Question (2): Does LLVM have any routines for printing the
> disassembly of a native code function?  I see that lli can do this,
> but I would much prefer an LLVM library routine that I can call from
> my own code rather than having to write the bitcode to a file and
> then invoke lli.  Suggestions?

This keeps changing, but on x86 with 4.2 you do this to enable assembly
dumps:

std::vector<const char*> Args;
Args.push_back(""); // program name
Args.push_back("-debug-only=" "x86-emitter");
cl::ParseCommandLineOptions(Args.size()-1, (char**)&Args[0]);

Andrew.