llvm dev - Oct 2009 - [LLVMdev] Accessing Loop Variables

Hi,
      How do I access the loop variables in a loop.

for(i = 0; i < N; i++)
      for(j = 0; j < M; j++)
            A[i][j+k] = i + j;

Is there anyway for me to know that in A[i][j+k], i & j are loop variables
whereas k is not!

Regards,

Prasenjit Chakraborty
Performance Modeling and Analysis
IBM Systems & Technology Lab

Hi,

You can use the getInductionVariable() on each loop,and maintain a smallvect
of all induction variables that you have come across.
Also remember to run some passes to hoist loop independent entities out of
the loop.

Cheers

Dhruv

On Mon, Oct 12, 2009 at 6:46 AM, Prasenjit Chakraborty
<cprasenj at in.ibm.com>wrote:
>
> Hi,
>      How do I access the loop variables in a loop.
>
> for(i = 0; i < N; i++)
>      for(j = 0; j < M; j++)
>            A[i][j+k] = i + j;
>
> Is there anyway for me to know that in A[i][j+k], i & j are loop
variables
> whereas k is not!
>
> Regards,
>
> Prasenjit Chakraborty
> Performance Modeling and Analysis
> IBM Systems & Technology Lab
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>


-- 
School of Electrical and Computer Engineering
Georgia Institute of Technology

(M)  +1 770 827 9264
Personal Email : choudharydhruv at gmail.com
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On Oct 12, 2009, at 3:46 AM, Prasenjit Chakraborty wrote:
>
> Hi,
>      How do I access the loop variables in a loop.
>
> for(i = 0; i < N; i++)
>      for(j = 0; j < M; j++)
>            A[i][j+k] = i + j;
>
> Is there anyway for me to know that in A[i][j+k], i & j are loop  
> variables
> whereas k is not!
The ScalarEvolution analysis can help here. If you've already identified
i, j, and k yourself, you can call getSCEV on them and use the
isLoopInvariant and hasComputableLoopEvolution member functions.

ScalarEvolution doesn't yet help in identifying the indices used in
each dimension of a multi-dimensional array reference in the way that a
traditional dependence analysis would want to see them, though that's
a future goal.

Dan

I have not identified i, j or k. The whole point is I want to know that
there are variables i & j which are loop variables.

Calling getInductionVariab;le() returns NULL, reason being I don't see any
PHINode in the loop.

Regards,

Prasenjit Chakraborty
Performance Modeling and Analysis
IBM Systems & Technology Lab


                                                                           
             Dan Gohman                                                    
             <gohman at apple.com
             >                                                          To 
                                       Prasenjit                           
             10/12/2009 11:13          Chakraborty/India/IBM at IBMIN         
             PM                                                         cc 
                                       llvmdev at cs.uiuc.edu                 
                                                                   Subject 
                                       Re: [LLVMdev] Accessing Loop        
                                       Variables                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           





On Oct 12, 2009, at 3:46 AM, Prasenjit Chakraborty wrote:
>
> Hi,
>      How do I access the loop variables in a loop.
>
> for(i = 0; i < N; i++)
>      for(j = 0; j < M; j++)
>            A[i][j+k] = i + j;
>
> Is there anyway for me to know that in A[i][j+k], i & j are loop
> variables
> whereas k is not!
The ScalarEvolution analysis can help here. If you've already identified
i, j, and k yourself, you can call getSCEV on them and use the
isLoopInvariant and hasComputableLoopEvolution member functions.

ScalarEvolution doesn't yet help in identifying the indices used in
each dimension of a multi-dimensional array reference in the way that a
traditional dependence analysis would want to see them, though that's
a future goal.

Dan