[LLVMdev] array index access

[George Baah]

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.
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>
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>>
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>>
>>
>>
>
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