Linking LLVM JIT code to external C++ functions

问题:

I'm writing a LLVM scripting engine that JIT compiles scripting code in a custom language. My problem is that I'm unable to call external functions (even the C99 erf() function is failing).

For example if I extern "C" the erf function,

extern "C" double erft(double x){
return erf(x);
}

and create a function with external linkage

std::vector<const Type*> Double1(1,Type::getDoubleTy(getGlobalContext()));
FunctionType *FT = FunctionType::get(Type::getDoubleTy(getGlobalContext()),Double1,false);
Function *erft = Function::Create(FT,Function::ExternalLinkage,"erft",TheModule);

get the following error message when running my script with erft(0.0) :

LLVM ERROR: Program used external function 'erft' which could not be resolved!

Doing the mapping manually,

void ExecutionEngine::addGlobalMapping( const GlobalValue *  erfF, void *  erft); 

will get me the following error:

declaration of `void llvm::ExecutionEngine::addGlobalMapping(const llvm::GlobalValue*, void*)' outside of class is not definition

Obviously I'm doing something very wrong. Any help would be much appreciated

回答1:

Assuming you haven't disabled it (by calling EE->DisableSymbolSearching()) then LLVM will use dlsym() to find the symbols in the JIT program itself. Depending on your platform, that might mean that you need to build your JIT with -fPIC, or that it might not be available at all (such as on Windows).

Aside from automatic symbol searching, you can always register the individual functions yourself using EE->addGlobalMapping(GV, &function) where GV = the llvm::Function* function declaration that matches the native function you're calling. In your case with ertf() that's:

EE->addGlobalMapping(erft, &::erft);

Note that you named the global function erft() and the local variable erft, hence the "::". Please pick different names next time!

回答2:

This might be happening because you forgot to add the "libm" depedency, try using:

[your module]->addLibrary("m");

See here for more information about the Module::addLibrary().

回答3:

I don't know llvm, but this make no sense:

void ExecutionEngine::addGlobalMapping( const GlobalValue *  erfF, void *  erft); 

That defines a new function in C++. What you need to do is somehow register your function with LLVM. Defining that function is like trying to add new methods to the LLVM classes, not what you want to do.