[LLVMdev] array index access

[Jeffrey Yasskin]

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.
>>>    _______________________________________________
>>>    LLVM Developers mailing list
>>>    LLVMdev at cs.uiuc.edu <mailto:LLVMdev at cs.uiuc.edu>
>>> http://llvm.cs.uiuc.edu
>>>    http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>
>>>
>>
>
>
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