llvm dev - Oct 2014 - [LLVMdev] How to call a pointer that points to a C function

I have a pointer to a function that I need to invoke without going
through llvm::Module::getOrInsertFunction. This example does not work:

static int add(int x, int y);
llvm::Value *one, *two;

llvm::Constant* addfn
   = llvm::ConstantInt::get(JB->getIntPtrTy(DataLayout), (intptr_t)add);
llvm::Type* args[] = { Int32Ty, Int32Ty };
llvm::FunctionType* ftype = llvm::FunctionType::get(Int32Ty, args);
addfn = llvm::ConstantExpr::getPointerCast(addfn, ftype);
CreateCall(addfn, one, two);


---

Is there a way? Or must I call getOrInsertFunction?

Before anyone asks why I am doing this, it is because I have maybe 200
functions with the same signature that I would like JIT a call to. I
would like to avoid calling getOrInsertFunction 200 times. I know
exactly the address of the functions.

Thanks.

There is at least one error here, possibly two.

On 27 Oct 2014, at 07:46, C K Tan <cktanx at gmail.com> wrote:
> I have a pointer to a function that I need to invoke without going
> through llvm::Module::getOrInsertFunction. This example does not work:
> 
> static int add(int x, int y);
> llvm::Value *one, *two;
> 
> llvm::Constant* addfn
>   = llvm::ConstantInt::get(JB->getIntPtrTy(DataLayout), (intptr_t)add);
> llvm::Type* args[] = { Int32Ty, Int32Ty };
> llvm::FunctionType* ftype = llvm::FunctionType::get(Int32Ty, args);
The type that you want is not the function type, but the function *pointer*
type.  ftype->getPointerTo() should give you the type that you need.
> addfn = llvm::ConstantExpr::getPointerCast(addfn, ftype);
I don't remember if this will just do casts between pointers or if it can
also create inttoptrs, but you might need to be more explicit and ask for an
inttoptr.

David

Does anyone have a sample code on calling a c function thru pointer? Thanks.
> On Oct 27, 2014, at 1:37 AM, David Chisnall <David.Chisnall at
cl.cam.ac.uk> wrote:
> 
> There is at least one error here, possibly two.
> 
>> On 27 Oct 2014, at 07:46, C K Tan <cktanx at gmail.com> wrote:
>> 
>> I have a pointer to a function that I need to invoke without going
>> through llvm::Module::getOrInsertFunction. This example does not work:
>> 
>> static int add(int x, int y);
>> llvm::Value *one, *two;
>> 
>> llvm::Constant* addfn
>>  = llvm::ConstantInt::get(JB->getIntPtrTy(DataLayout),
(intptr_t)add);
>> llvm::Type* args[] = { Int32Ty, Int32Ty };
>> llvm::FunctionType* ftype = llvm::FunctionType::get(Int32Ty, args);
> 
> The type that you want is not the function type, but the function *pointer*
type.  ftype->getPointerTo() should give you the type that you need.
> 
>> addfn = llvm::ConstantExpr::getPointerCast(addfn, ftype);
> 
> I don't remember if this will just do casts between pointers or if it
can also create inttoptrs, but you might need to be more explicit and ask for an
inttoptr.
> 
> David