LLVM ERROR: MCJIT::runFunction does not support full-featured argument passing - c++

Question

I have got an example from here and I faced with a run error
LLVM ERROR: Target does not support MC emission!
which I fixed it by this.
nevertheless, I still observe runtime problem:
./example 3 5
LLVM ERROR: MCJIT::runFunction does not support full-featured argument passing. Please use ExecutionEngine::getFunctionAddress and cast the result to the desired function pointer type.
main.cpp
/**
* LLVM equivalent of:
*
* int sum(int a, int b) {
* return a + b;
* }
*/
#include <llvm-c/Core.h>
#include <llvm-c/ExecutionEngine.h>
#include <llvm-c/Target.h>
#include <llvm-c/Analysis.h>
#include <llvm-c/BitWriter.h>
#include <inttypes.h>
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char const *argv[]) {
LLVMModuleRef mod = LLVMModuleCreateWithName("my_module");
LLVMTypeRef param_types[] = { LLVMInt32Type(), LLVMInt32Type() };
LLVMTypeRef ret_type = LLVMFunctionType(LLVMInt32Type(), param_types, 2, 0);
LLVMValueRef sum = LLVMAddFunction(mod, "sum", ret_type);
LLVMBasicBlockRef entry = LLVMAppendBasicBlock(sum, "entry");
LLVMBuilderRef builder = LLVMCreateBuilder();
LLVMPositionBuilderAtEnd(builder, entry);
LLVMValueRef tmp = LLVMBuildAdd(builder, LLVMGetParam(sum, 0), LLVMGetParam(sum, 1), "tmp");
LLVMBuildRet(builder, tmp);
char *error = NULL;
LLVMVerifyModule(mod, LLVMAbortProcessAction, &error);
LLVMDisposeMessage(error);
LLVMExecutionEngineRef engine;
error = NULL;
LLVMLinkInMCJIT();
LLVMInitializeNativeTarget();
LLVMInitializeNativeAsmPrinter(); // added
LLVMInitializeNativeAsmParser(); // added
if (LLVMCreateExecutionEngineForModule(&engine, mod, &error) != 0)
{
fprintf(stderr, "failed to create execution engine\n");
abort();
}
if (error)
{
fprintf(stderr, "error: %s\n", error);
LLVMDisposeMessage(error);
exit(EXIT_FAILURE);
}
if (argc < 3)
{
fprintf(stderr, "usage: %s x y\n", argv[0]);
exit(EXIT_FAILURE);
}
long long x = strtoll(argv[1], NULL, 10);
long long y = strtoll(argv[2], NULL, 10);
LLVMGenericValueRef args[] = {
LLVMCreateGenericValueOfInt(LLVMInt32Type(), x, 0),
LLVMCreateGenericValueOfInt(LLVMInt32Type(), y, 0)
};
LLVMGenericValueRef res = LLVMRunFunction(engine, sum, 2, args);
printf("%d\n", (int)LLVMGenericValueToInt(res, 0));
// Write out bitcode to file
if (LLVMWriteBitcodeToFile(mod, "sum.bc") != 0) {
fprintf(stderr, "error writing bitcode to file, skipping\n");
}
LLVMDisposeBuilder(builder);
LLVMDisposeExecutionEngine(engine);
}
Though the meessage is clear for the view point of the code author, for me as a user it is so cryptic. How can I solve this?

Answer 1

From the documentation of MCJIT::runFunction:
For MCJIT execution engines, clients are encouraged to use the "GetFunctionAddress" method (rather than runFunction) and cast the returned uint64_t to the desired function pointer type. However, for backwards compatibility MCJIT's implementation can execute 'main-like' function (i.e. those returning void or int, and taking either no arguments or (int, char*[])).
So you can't call MCJIT::runFunction (and by extension the C API's LLVMRunFunction when used with an MCJIT engine) unless the arguments array is either empty or consists of only an i32 and an i8* (in that order). Your array contains two i32s, so it does not meet those restrictions.
As stated in the documentation (and the exception message), you should instead use ExecutionEngine::getFunctionAddress (or its C wrapper LLVMGetFunctionAddress), cast the result to int (*)(int, int) and then call it as f(0, 0);.