llvm dev - Mar 2011 - [LLVMdev] array index access

Hi Everyone,
  I am trying to get the first index into this two-dimensional array, that
is *5.*

%4 = getelementptr inbounds [10 x [20 x i32]]* %3, i32 0, i32 *5*
*
*
I can iterate over the GEP and get the types of the operands but not
the values.

How do I get the value (5)?
Thanks.

George
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Hi George,
>    I am trying to get the first index into this two-dimensional array, that
is *5.*
not sure what you mean, but if you think of your array as being a 10 x 20 matrix
then to get a pointer to element M,N you would do:
   getelementptr inbounds [10 x [20 x i32]]* %3, i32 0, i32 %M, i32 %N
>
> %4 = getelementptr inbounds [10 x [20 x i32]]* %3, i32 0, i32 *5*
> *
> *
> I can iterate over the GEP and get the types of the operands but not the
values.
>
> How do I get the value (5)?
getelementptr only does pointer arithmetic, i.e. using it you can get a pointer
to an array element.  But to get the element itself you need to use a load
instruction to load the value pointed to by the pointer.

Ciao, Duncan.

Hi Duncan,
 For this example,
  getelementptr inbounds [10 x [20 x i32]]* %3, i32 0, i32 %M, i32 %N
I am trying to retrieve %N and instrument the program to see if the value
pointed to by  %N exceeds the array bound (10). I assume that
%N will be associated with a load instruction. I have searched through
all the tutorials etc. and still have no clue as to how to do this, that
is,
get the value and then instrument the code.

George

On Mon, Mar 28, 2011 at 4:32 AM, Duncan Sands <baldrick at free.fr> wrote:
> Hi George,
>
> >    I am trying to get the first index into this two-dimensional array,
> that is *5.*
>
> not sure what you mean, but if you think of your array as being a 10 x 20
> matrix
> then to get a pointer to element M,N you would do:
>   getelementptr inbounds [10 x [20 x i32]]* %3, i32 0, i32 %M, i32 %N
>
> >
> > %4 = getelementptr inbounds [10 x [20 x i32]]* %3, i32 0, i32 *5*
> > *
> > *
> > I can iterate over the GEP and get the types of the operands but not
the
> values.
> >
> > How do I get the value (5)?
>
> getelementptr only does pointer arithmetic, i.e. using it you can get a
> pointer
> to an array element.  But to get the element itself you need to use a load
> instruction to load the value pointed to by the pointer.
>
> Ciao, Duncan.
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
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