[LLVMdev] Cast Pointer Address to Functions

[Xin Tong]

I have a function address held in an uint64_t. I would like to cast
the function address to a function prototype and create a call to the
function in LLVM. How could I do this ?

Thanks

Xin
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[Bill Wendling]

Using uintptr_t is the way to hold pointer values. However, I seem to recall there being an exception for function addresses.

-bw

[Xin Tong]

In fact i want to cast a uint64_t to a function address and make LLVM
generate IR to call it.

i.e. in LLVM IR, i want to generate something like this.

the uint64_t function prototype function args
call 0xAABBCC (void () (int int)) (1 , 2)

Thanks

Xin

[Caldarale, Charles R]

> From: llvmdev...@cs.uiuc.edu [mailto:llvmdev...@cs.uiuc.edu] On Behalf Of Xin Tong
> Subject: [LLVMdev] Cast Pointer Address to Functions

> I have a function address held in an uint64_t. I would like to cast
> the function address to a function prototype and create a call to the
> function in LLVM. How could I do this ?

This is what works for us:

std::vector<Type*> margs;
FunctionType* fType;
PointerType* pm3_routine;
Value* m3func;
Value* result;

margs.resize(1);
margs[0] = I64;
fType = FunctionType::get(I32, margs, false);
pm3_routine = PointerType::get(fType, 0);

m3func = Builder.CreateIntToPtr(I64_Const((uint64_t)pFunc), pm3_routine, "pm3func");
result = Builder.CreateCall(m3func, parameter, "m3func");

- Chuck


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[Xin Tong]

Interestingly I have something similar. but does not seem to work.

This one works.
Value *FuncCast = EIR->CreateIntToPtr(FuncAddr, FuncTyPntr, "FuncName");


This one does not work. it gives me an instruction with badref.
CastInst *FuncCast = CastInst::Create(Instruction::IntToPtr,
FuncAddr, FuncTyPntr);

Anyone knows why ?

Thanks

Xin