[LLVMdev] JIT and array pointers

[Chris Lattner]

On Mon, 11 Apr 2005, dummy1 at boxpl.com wrote:
>> There are many possible ways to do this, can you be a bit more specific
>> about what you're trying to do?

> Here is a basic example:

Ah, ok, I see what you're trying to do.  Below is some *pseudo* code for 
the basic idea:

> ============================================
> 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");

Here, create an LLVM GlobalVariable 'BuffGV' of the appropriate type, and 
insert it into m:

GlobalVariable *BuffGV = new GlobalVariable([4096 x uint], false, 
GlobalVariable::ExternalLinkage, 0, "Buff", m);

>  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]

    pbuff = BuffGV;

>  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);

Tell the EE that buffGV -> buff:

ee->addGlobalMapping(BuffGV, buff);

Then you should be good to go!

-Chris

>  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.
>
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>

-Chris

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