[LLVMdev] Function pointers

[anders.alexandersson at student.htu.se]

Ok, thanks for the tip! I am still learning the details of the syntax of
llvm...

Anders

----- Original Message ----- 
From: "Chris Lattner" <sabre at nondot.org>
To: <llvmdev at cs.uiuc.edu>
Sent: Friday, April 02, 2004 5:53 PM
Subject: Re: Re: Re: Re: [LLVMdev] Function pointers


> On Fri, 2 Apr 2004, Anders Alexandersson wrote:
>
> > OK, I solved it all ( so far :) ), mixing in some load-instructions and
> > called on the result of that, which worked.
> >
> > Here is the skeleton-code:
> >
> > %kernel = type {  int ()*  }
> >
> > int puts_kernel(){...}
> >
> > ; main()
> >
> > %theKernel = malloc %kernel
> > %puts_kernelPTR = getelementptr %kernel* %theKernel, long 1, ubyte 0
> >
> > store int ()* %puts_kernel, int ()** %puts_kernelPTR
> > %tmp.11 = load int ()** %puts_kernelPTR
> >
> > %tmp.2 = call int %tmp.11()
> >
> > free %kernel* %theKernel
>
> Yup, that's close.  You should be able to use an alloca instruction.  The
> difference between alloca and malloc is that alloca is typically
> substantially faster than malloc, but the memory allocated by it is
> automatically freed when the function it is in returns.  In this case, it
> shouldn't matter.
>
> The bigger problem is that you are accessing a piece of memory that has
> not been allocated:
>
>    %theKernel = malloc %kernel
>    %puts_kernelPTR = getelementptr %kernel* %theKernel, long 1, ubyte 0
>
> The 'long 1' operand of the getelementptr instruction tells it to step to
> the 1th kernel element.  In terms of C, we have something like this:
>
>   kernel *K = malloc (...)
>   K[1].field =
>
> If you use 'long 0' things should work fine.
>
> -Chris
>
>
>
> > -----Original Message-----
> > From: Anders Alexandersson <anders.alexandersson at student.htu.se>
> > To: llvmdev at cs.uiuc.edu
> > Date: Fri, 02 Apr 2004 13:38:30 +0200
> > Subject: Re: Re: Re: [LLVMdev] Function pointers
> >
> > I solved that one by substituting alloca into malloc, and forwarded the
problem to actually calling the %puts_kernel function via my newly created
function pointer.
> >
> > This works:
> >  %tmp.1 = call int %puts_kernel()
> >
> > This:
> >  %tmp.2 = call int %puts_kernelPTR()
> >
> > issues error:
> > "Reference to an invalid definition: 'puts_kernelPTR' of type 'int ()
*'"
> >
> > How do I call the function using the function pointer?
> >
> > Anders
> >
> > -----Original Message-----
> > From: Anders Alexandersson <anders.alexandersson at student.htu.se>
> > To: llvmdev at cs.uiuc.edu
> > Date: Fri, 02 Apr 2004 10:07:50 +0200
> > Subject: Re: Re: [LLVMdev] Function pointers
> >
> > So, here comes the next question:)
> >
> > Now the program compiles, but crashes when run, "Segmentation fault".
> >
> >
> > %kernel = type {  int ()*  }
> >
> > int %puts_kernel()
> > {
> >    ret int 0
> > }
> >
> >
> > int %main() {
> >
> >    %theKernel = alloca %kernel
> >
> >    %puts_kernelPTR = getelementptr %kernel* %theKernel, long 1, ubyte 0
> >    store int ()* %puts_kernel, int ()** %puts_kernelPTR
> >
> >    ret int 0
> > }
> >
> > I want to learn how to achieve the mechanisms in this C code:
> >
> > #include <stdio.h>
> >
> > int DoIt(void);
> > int DoIt(void){
> >  return 0;
> > }
> >
> > int main()
> > {
> >         int (*pt2Function) ();
> >
> >         int a = DoIt();
> >         printf("a is: %i\n", a);
> >
> >         pt2Function = DoIt;
> >
> >         int b = pt2Function();
> >         printf("b is: %i\n", b);
> > }
> >
> > Grateful for hints!
> >
> > Anders
> >
> >
> >
> > -----Original Message-----
> > From: Chris Lattner <sabre at nondot.org>
> > To: llvmdev at cs.uiuc.edu
> > Date: Thu, 1 Apr 2004 09:51:48 -0600 (CST)
> > Subject: Re: [LLVMdev] Function pointers
> >
> > On Thu, 1 Apr 2004 anders.alexandersson at student.htu.se wrote:
> >
> > > Yes! :-)
> > > What would I do without you folks?!
> >
> > :)
> >
> > > I am the one trying to build a Ruby compiler prototype, so be prepared
for
> > > more questions...:)
> >
> > Sounds great!
> >
> > -Chris
> >
> > > ----- Original Message -----
> > > From: "Chris Lattner" <sabre at nondot.org>
> > > To: <llvmdev at cs.uiuc.edu>
> > > Sent: Wednesday, March 31, 2004 5:49 PM
> > > Subject: Re: [LLVMdev] Function pointers
> > >
> > >
> > > On Wed, 31 Mar 2004, Anders Alexandersson wrote:
> > >
> > > > Hello out there!
> > > >
> > > > I am trying to build a table of 2 function poiters by using an
array:
> > > >
> > > > %kernel = alloca [ 2 x int ()* ]
> > > >
> > > > I try to access the first pointer to be able to store location of my
first
> > > function like this:
> > > >
> > > > %function_pointer_1 = getelementptr [2 x int ()* ]* %kernel, long 0,
long
> > > 0
> > > >
> > > > store int ()* %function_1, int ()* %function_pointer_1
> > >
> > > You just need an extra * here:      ^
> > >
> > > You're storing a pointer through a pointer to pointers... :)
> > >
> > > -Chris
> > >
> > > >
> > > > But, I get the error message from llvm-asm:
> > > > llvm-as: 1.ll:40: Can't store 'int () *' into space of type 'int
()'!
> > > >
> > > > What is my mistake?
> > > >
> > > > Grateful for hints
> > > > Anders
> > > >
> > > > ----------------------------------------------------------------
> > > > Anders Alexandersson
> > > > Masters student at the special year of Software Engineering, HTU
> > > Trollh4an
> > > > E-mail: anders.alexandersson at student.htu.se
> > > >
> > > > _______________________________________________
> > > > LLVM Developers mailing list
> > > > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > > > http://mail.cs.uiuc.edu/mailman/listinfo/llvmdev
> > > >
> > >
> > > -Chris
> > >
> > >
> >
> > -Chris
> >
> >
>
> -Chris
>
> -- 
> http://llvm.cs.uiuc.edu/
> http://www.nondot.org/~sabre/Projects/
>
>
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