Accessing Loop Variables

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1

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

2

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

3

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

4

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@apple.com
             > To
                                       Prasenjit
             10/12/2009 11:13 Chakraborty/India/IBM@IBMIN
             PM cc
                                       llvmdev@cs.uiuc.edu
                                                                   Subject
                                       Re: [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!

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