llvm dev - May 2004 - [LLVMdev] Problems with getelementptr

Hello!

I'm having trouble with pointer traversing. I have a design as follows: 
class -> map -> classFunctions

Starting with a pointer to the class, I want to get a pointer to a classFunction
via a pointer to the map.

I can't get that function pointer!

How shall I think to get the traversing right (see code below)? Is it something
with the fact that I am using only pointers in my structs?

Best regards
Anders

; My user defined function
int %puts_kernel(sbyte* %string)
{
        %tmp.0 = call int (sbyte*, ...)* %printf( sbyte* %string )
        ret int 0

}

;the map and the class. The class is called "Kernel".
"myKernelMap" = type {int (sbyte*)*}
"Kernel" = type {"myKernelMap"*}

;Allocating...OK
"myKernelMapInstance" = malloc "myKernelMap"
%myKernel = malloc "Kernel"

;Assigning the map to the class...OK
"myKernelMapPTR" = getelementptr "Kernel"* %myKernel, long
0, ubyte 0
store "myKernelMap"* "myKernelMapInstance",
"myKernelMap"** "myKernelMapPTR"

; Try to get pointer to the functionPointer in the map, NOT OK! :-(
%puts_kernelPTR = getelementptr "Kernel"* %myKernel, long 0, ubyte 0,
long 0, ubyte 0
store int (sbyte*)* %puts_kernel, int (sbyte*)** %puts_kernelPTR

;Want to call the function with a string
%tmp.11 = load int (sbyte*)** %puts_kernelPTR
%result = call int %tmp.11(sbyte* %string)

----------------------------------------------------------------
Anders Alexandersson
Masters student at the special year of Software Engineering, HTU Trollhättan
E-mail: anders.alexandersson at student.htu.se

On Mon, 3 May 2004, Anders Alexandersson wrote:
> Hello!
>
> I'm having trouble with pointer traversing. I have a design as follows:
> class -> map -> classFunctions
>
> Starting with a pointer to the class, I want to get a pointer to a
> classFunction via a pointer to the map.
Okay...
> I can't get that function pointer!
>
> How shall I think to get the traversing right (see code below)? Is it
> something with the fact that I am using only pointers in my structs?
>
> Best regards
> Anders
>
> ; My user defined function
> declare int %puts_kernel(sbyte* %string)
>
> ;the map and the class. The class is called "Kernel".
> "myKernelMap" = type {int (sbyte*)*}
Okay, this declares a structure that has a single pointer-to-function
element in it.
> "Kernel" = type {"myKernelMap"*}
This is a single element structure with a pointer to the map.
> ;Allocating...OK
> "myKernelMapInstance" = malloc "myKernelMap"
> %myKernel = malloc "Kernel"
Okay, these are of type %myKernelMap and %Kernel, respectively.
> ;Assigning the map to the class...OK
> "myKernelMapPTR" = getelementptr "Kernel"* %myKernel,
long 0, ubyte 0
This gives you a pointer to the '%myKernelMap*' in the Kernel type.
> store "myKernelMap"* "myKernelMapInstance",
"myKernelMap"** "myKernelMapPTR"
And this stores the pointer as you would expect.
> ; Try to get pointer to the functionPointer in the map, NOT OK! :-(
> %puts_kernelPTR = getelementptr "Kernel"* %myKernel, long 0,
ubyte 0, long 0, ubyte 0
Okay, here you're getting into trouble.  myKernel is of type Kernel, which
doesn't contain a pointer to a function.  In your case, you need to
actually emit a load to get to the structure in question, something like
this:

%t1 = getelementptr "Kernel"* %myKernel, long 0, ubyte 0
%t2 = load "myKernelMap"**
%puts_kernelPTR = getelementptr "myKernelMap"*, long 0, ubyte 0
> store int (sbyte*)* %puts_kernel, int (sbyte*)** %puts_kernelPTR
This should now work.
> ;Want to call the function with a string
> %tmp.11 = load int (sbyte*)** %puts_kernelPTR
> %result = call int %tmp.11(sbyte* %string)
This should also work.

-Chris

-- 
http://llvm.cs.uiuc.edu/
http://www.nondot.org/~sabre/Projects/

Thanks for yet another patient and professional answer! :)

Actually, I solved it some time after I posted the message, just the way you
suggested. Good to know that it is your recommendation as well.

Thanks again!

Anders

----- Original Message ----- 
From: "Chris Lattner" <sabre at nondot.org>
To: <llvmdev at cs.uiuc.edu>
Sent: Monday, May 03, 2004 7:15 PM
Subject: Re: [LLVMdev] Problems with getelementptr

> On Mon, 3 May 2004, Anders Alexandersson wrote:
>
> > Hello!
> >
> > I'm having trouble with pointer traversing. I have a design as
follows:
> > class -> map -> classFunctions
> >
> > Starting with a pointer to the class, I want to get a pointer to a
> > classFunction via a pointer to the map.
>
> Okay...
>
> > I can't get that function pointer!
> >
> > How shall I think to get the traversing right (see code below)? Is it
> > something with the fact that I am using only pointers in my structs?
> >
> > Best regards
> > Anders
> >
> > ; My user defined function
> > declare int %puts_kernel(sbyte* %string)
> >
> > ;the map and the class. The class is called "Kernel".
> > "myKernelMap" = type {int (sbyte*)*}
>
> Okay, this declares a structure that has a single pointer-to-function
> element in it.
>
> > "Kernel" = type {"myKernelMap"*}
>
> This is a single element structure with a pointer to the map.
>
> > ;Allocating...OK
> > "myKernelMapInstance" = malloc "myKernelMap"
> > %myKernel = malloc "Kernel"
>
> Okay, these are of type %myKernelMap and %Kernel, respectively.
>
> > ;Assigning the map to the class...OK
> > "myKernelMapPTR" = getelementptr "Kernel"*
%myKernel, long 0, ubyte 0
>
> This gives you a pointer to the '%myKernelMap*' in the Kernel type.
>
> > store "myKernelMap"* "myKernelMapInstance",
"myKernelMap"**
"myKernelMapPTR"
>
> And this stores the pointer as you would expect.
>
> > ; Try to get pointer to the functionPointer in the map, NOT OK! :-(
> > %puts_kernelPTR = getelementptr "Kernel"* %myKernel, long 0,
ubyte 0,
long 0, ubyte 0
>
> Okay, here you're getting into trouble.  myKernel is of type Kernel,
which
> doesn't contain a pointer to a function.  In your case, you need to
> actually emit a load to get to the structure in question, something like
> this:
>
> %t1 = getelementptr "Kernel"* %myKernel, long 0, ubyte 0
> %t2 = load "myKernelMap"**
> %puts_kernelPTR = getelementptr "myKernelMap"*, long 0, ubyte 0
>
> > store int (sbyte*)* %puts_kernel, int (sbyte*)** %puts_kernelPTR
>
> This should now work.
>
> > ;Want to call the function with a string
> > %tmp.11 = load int (sbyte*)** %puts_kernelPTR
> > %result = call int %tmp.11(sbyte* %string)
>
> This should also work.
>
> -Chris
>
> -- 
> http://llvm.cs.uiuc.edu/
> http://www.nondot.org/~sabre/Projects/
>
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