[LLVMdev] JIT and array pointers

[dummy1 at boxpl.com]

On Sun, 2005-04-10 at 19:47 -0500, Chris Lattner wrote:
> On Mon, 11 Apr 2005, Paul wrote:
> > I'm trying to pass an array pointer to my run-time generated module, and
> > after a long time of searching for the answer, the only thing I got was
> > a headache. Basically I have a few arrays (few megabytes which will
> > sometimes be accessed as 8 / 16 / 32 / 64 bit binary / fp values), that
> > will be modified by both the generated module, and my main c program, so
> > I would like to put those into global variables. I also tried passing it
> > as a function argument (with argument type PointerType::get
> > (Type::UIntTy)), and failed miserably. I feel lost. Right now I got only
> > a small module with some simple operations (big thanks to the examples,
> > they really help a lot). Maybe its because I'm not a c++ guy, but I
> > really want to use llvm, especially because of it's nice job at
> > optimizing code. And I only need a few basic building blocks to get
> > things started.
> 
> There are many possible ways to do this, can you be a bit more specific 
> about what you're trying to do?
> 
> -Chris
> 

Here is a basic example:

============================================
unsigned int buff[4096];

int main (void)
{
  buff[0] = 1;

  // compile and execute code that will change buff[0] to 2
  Value *c0 = ConstantUInt::get (Type::UIntTy, 0);
  Value *c2 = ConstantUInt::get (Type::UIntTy, 2);

  Module *m = new Module ("test_module");
  Function *f = m->getOrInsertFunction ("test_function", Type::VoidTy,
0);
  BasicBlock *b = new BasicBlock ("entry_block", f);

  // Here is the problem.
  // I need to get the buff pointer into variable, which I can
  // pass to GetElementPtrInst
  // Value *pbuff = ??
  // -> %pbuff = external global [4096 x uint]
  Value *ptr = new GetElementPtrInst (pbuff, c0, c0, "ptr", b);
  // -> %ptr = getelementptr [4096 x uint]* %buff, int 0, int 0
  new StoreInst (c2, ptr, b); 
  // -> store uint 2, uint* %ptr
  new ReturnInst (b);

  ExistingModuleProvider *mp = new ExistingModuleProvider (m);
  ExecutionEngine *ee = ExecutionEngine::create (mp, false);

  cout << "constructed module:\n\n" << *m << "\n\n";

  vector <GenericValue> args;
  GenericValue ret = ee->runFunction (f, args);

  printf ("buff[0] is now: %d\n", buff[0]);
  return 0;
}
============================================

I've been trying to do it with GlobalVariable. It seems like a
completely basic thing, but I just can't get it to work.