[LLVMdev] Please help with LLVM C++ integration

[John Criswell]

kirill havok wrote:
> Hi Gordon,
> I wrote a small example, but while running I get an error("Tied to
> execute an unknown external function"), where I am wrong?
>   
I think the problem is that some_test_func() is a C++ function, so its 
name is being mangled during compilation.

To fix it, I think you want to add a declaration telling the compiler to 
treat some_test_func() as a C function.  Something like the below will work:

extern "C" {
    int some_test_func(int);
}

int some_test_func (int) {
    <code for function here>
    ...
}

-- John T.

> Thanks in advance.
> Kirill.
>
> int some_test_func( int ){
>         std::cout << "!!!!" << std::endl;
>         return 8848;
> }
>
> int _tmain(int argc, _TCHAR* argv[]){
>
>         Module *M = new Module("test");
>         ExistingModuleProvider* MP = new ExistingModuleProvider(M);
>         ExecutionEngine* EE = ExecutionEngine::create(MP, false);
>
>         std::vector<const Type *> func_args;
>         func_args.push_back(Type::Int32Ty);
>         FunctionType * func_type = FunctionType::get(Type::Int32Ty, func_args, false);
>         Function * my_function = Function::Create( func_type,
> Function::ExternalLinkage, "some_test_func", M );
>         EE->addGlobalMapping( my_function, (void*)&some_test_func );
>
>         Function *FooF = cast<Function>(M->getOrInsertFunction("foo",
> Type::Int32Ty, (Type *)0));
>         BasicBlock * BB = BasicBlock::Create("EntryBlock", FooF);
>         Value *Ten = ConstantInt::get(Type::Int32Ty, 10);
>         CallInst *Add1CallRes = CallInst::Create(my_function, Ten,
> "some_test_func", BB);
>         Add1CallRes->setTailCall(true);
>
>         ReturnInst::Create(Add1CallRes, BB);
>
>         std::cout << "We just constructed this LLVM module:\n\n" << *M;
>         std::cout << "\n\nRunning foo: " << std::flush;
>
>         std::vector<GenericValue> noargs;
>         GenericValue gv = EE->runFunction(FooF, noargs);
>
>         std::cout << "Result: " << gv.IntVal.toStringUnsigned(10) << "\n";
>         return 0;
> }
>
>
>
> 2008/8/19, Gordon Henriksen <gordonhenriksen at me.com>:
>   
>> On Aug 19, 2008, at 13:36, kirill havok wrote:
>>
>> I got very interested in LLVM project, and I decided to start writing my own
>> scripting language based on it. By studying the documentation, I could not
>> find how to call external function, written in C. That is, I have a set of
>> functions written in C/C++, I generate code, using LLVM C++ interface, how
>> can I call(or register in machine in run-time) my external functions? Maybe
>> I just missed something. Please, give in which direction to search.
>>
>> Hi Kirill,
>>
>> Since LLVM goes to the bare metal, there is no need for an "FFI" or bindings
>> to call native code. Simply create a declaration for the function—a Function
>> with no body—and call it as any other function in your program. (Also, of
>> course, link with a library to define the symbols.) It might be instructive
>> to examine the llvm-gcc -S -emit-llvm output of a program like this:
>>
>> int MyExternalFn(int, int, int);
>>
>> int main(int, const char**, const char**) {
>>   return MyExternalFn(1, 2, 3);
>> }
>>
>>
>> Where you tweak MyExternalFn to have the desired prototype.
>>
>> To create a function declaration:
>>
>>
>> From C++, simply call Function::Create and then do not add any basic blocks.
>> From LLVM assembly, use the keyword "declare" instead of "define":
>> http://llvm.org/docs/LangRef.html#functionstructure
>>
>>
>> If you're using the JIT, you'll need to do the above, but also may need to
>> register your functions using
>> ExecutionEngine::addGlobalMapping.
>>
>> http://llvm.org/doxygen/classllvm_1_1ExecutionEngine.html#a8
>>
>>
>> Note that the C calling convention do not map directly to LLVM IR for many
>> cases, so it's best to stick to simple parameter types (pointers and ints,
>> for example) and also to verify that the LLVM FunctionTypes you use are
>> correct on each platform you support.
>>
>> Good luck!
>>
>> — Gordon
>>
>>
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>>
>>     
>
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