[LLVMdev] array index access

[George Baah]

Thanks Guys for the information. I now understand!

On Mon, Mar 28, 2011 at 4:16 PM, Jeffrey Yasskin <jyasskin at google.com>wrote:

> On Mon, Mar 28, 2011 at 12:48 PM, George Baah <georgebaah at gmail.com>
> wrote:
> > Oh I see what you mean but I don't think that is the value that I want.
> > In the code, I know that
> > %N = load i32* %k, align 4
> > %p = getelementptr inbounds [10 x [20 x i32]]* %3, i32 0, i32 %M, i32 %N
> > So I analyze the GEP and know that %N is a pointer to an int32, which is
> the
> > array index.
>
> p->getOperand(3) will return %N as an llvm::Value(). Then you can use
> dyn_cast<llvm::ConstantInt> to see if it's a ConstantInt as opposed to
> a run-time variable (or some other flavor of constant like a
> ConstantExpr), and then you can pull the value out using getValue() or
> get{Z,S}ExtValue().
>
> > I want to get that index so I can insert a check to see if it violates
> the
> > array bounds.
> > So the final code (in pseudocode) will look like,
> > %N = load i32* %k, align 4
> > if( 0 <= %N < array-size){
> >  %p = getelementptr inbounds [10 x [20 x i32]]* %3, i32 0, i32 %M, i32 %N
> >  ...
> > }else{
> >  throw an exception.
> > }
> >
> > On Mon, Mar 28, 2011 at 3:25 PM, Duncan Sands <baldrick at free.fr> wrote:
> >>
> >> Hi George,
> >>
> >>>  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.
> >>
> >> if
> >>  %p = getelementptr inbounds [10 x [20 x i32]]* %3, i32 0, i32 %M, i32
> %N
> >> then you need an instruction
> >>  %v = load %p
> >> to get the value %v pointed to by %p.
> >>
> >> Ciao, Duncan.
> >>
> >>>
> >>> George
> >>>
> >>> On Mon, Mar 28, 2011 at 4:32 AM, Duncan Sands <baldrick at free.fr
> >>> <mailto: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.
> >>>    _______________________________________________
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> >>>
> >>>
> >>
> >
> >
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> >
> >
>
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