llvm dev - Apr 2005 - [LLVMdev] Using LLVM for a dynamically typed language

I recently ran into the following problem.

I'm prototyping a compiler for a dynamically typed language in which
functions are first class objects. Assuming I have something like
this:

if(rand() > 5)
  i = define(x, y, z) { return x + y + z; }
else
  i = define(x, y) { return x + y; }

At this point I cannot know the type of 'i' at compile time. At
runtime 'i' is a structure that contains a type and a function
pointer. What I can't figure out is how to cast my llvm function 
pointer to an appropriate function type. I cannot know until runtime
what the type will be. Naturally I can add runtime code that will do
all appropriate checks (number of parameters, etc.) but how do I do
the actual cast?

I hope I'm being clear.

Thanks,
- Slava.

On Thu, 2005-21-04 at 09:31 -0400, Vyacheslav Akhmechet
wrote:
> At this point I cannot know the type of 'i' at compile time. At
> runtime 'i' is a structure that contains a type and a function
> pointer. What I can't figure out is how to cast my llvm function 
> pointer to an appropriate function type. I cannot know until runtime
> what the type will be. Naturally I can add runtime code that will do
> all appropriate checks (number of parameters, etc.) but how do I do
> the actual cast?
I'm not sure that I understand. LLVM permits you to cast any pointer to
any other pointer. If you are using LLVM to generate your code, this
basically has 3 steps:

1. Get a Type* for the function type you want to cast to.
2. Use a cast instruction to actually perform the type cast.

In the code I'm currently working on, I do the following to cast a
function pointer from one type to another. Hopefully the line wrapping
won't totally destroy this.

std::vector<const Type*> paramTypes;

// First parameter: a void pointer
paramTypes.push_back( PointerType::get( Type::getPrimitiveType( Type::SByteTyID
) ) );
		
FunctionType* pthreadFunctionType = FunctionType::get( 
	PointerType::get( Type::getPrimitiveType( Type::SByteTyID ) ), 
	paramTypes, false );
assert( pthreadFunctionType != NULL );

// Cast the function to the pthread function type
Value* castedFunction = new CastInst( function, 
	PointerType::get( pthreadFunctionType ), "", &terminator );



I hope this helps,

Evan Jones

Evan,

The problem is that I do not know the type of a target function at
compile time. If you consider my code example, I don't know the type
of 'i' until runtime (in fact, I can't even know a possible range of
types 'i' may assume).

Thanks,
- Slava.

On 4/21/05, Evan Jones <ejones at uwaterloo.ca>
wrote:
> On Thu, 2005-21-04 at 09:31 -0400, Vyacheslav Akhmechet wrote:
> > At this point I cannot know the type of 'i' at compile time.
At
> > runtime 'i' is a structure that contains a type and a function
> > pointer. What I can't figure out is how to cast my llvm function
> > pointer to an appropriate function type. I cannot know until runtime
> > what the type will be. Naturally I can add runtime code that will do
> > all appropriate checks (number of parameters, etc.) but how do I do
> > the actual cast?
> 
> I'm not sure that I understand. LLVM permits you to cast any pointer to
> any other pointer. If you are using LLVM to generate your code, this
> basically has 3 steps:
> 
> 1. Get a Type* for the function type you want to cast to.
> 2. Use a cast instruction to actually perform the type cast.
> 
> In the code I'm currently working on, I do the following to cast a
> function pointer from one type to another. Hopefully the line wrapping
> won't totally destroy this.
> 
> std::vector<const Type*> paramTypes;
> 
> // First parameter: a void pointer
> paramTypes.push_back( PointerType::get( Type::getPrimitiveType(
Type::SByteTyID ) ) );
> 
> FunctionType* pthreadFunctionType = FunctionType::get(
>         PointerType::get( Type::getPrimitiveType( Type::SByteTyID ) ),
>         paramTypes, false );
> assert( pthreadFunctionType != NULL );
> 
> // Cast the function to the pthread function type
> Value* castedFunction = new CastInst( function,
>         PointerType::get( pthreadFunctionType ), "",
&terminator );
> 
> I hope this helps,
> 
> Evan Jones
> 
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