Induction variable identification?

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1

Hi,
How does LLVM identify induction variables of a loop?
Is the algorithm based on SSA graphs?

I have a complicated loop and I need to do some analysis around it.
Can anyone please point me to source of identification part?

2

If you have a canonicalized IV, you can use Loop::getCanonicalInductionVariable() declared in “include/llvm/Analysis/LoopInfo.h”.

Otherwise you probably need to start from function simplifyLoopIVs() in “lib/Transforms/Utils/SimplifyIndVar.cpp”. This method looks at all phi nodes in the header of the loop, but later on in the process, it skips some of them. For example, when we reach SimplifyIndvar::simplifyUsers(), in the beginning of this function we check if the IV data type isSCEVable() or not. If not, we immediately return.

Hope that helps

Ehsan

3

Hi Madhur,

If you're looking to solely _analyze_ induction variables, maybe you
should try leveraging LLVM's "Scalar Evolution" framework? See
include/llvm/Analysis/ScalarEvolution.h for the docs or the
IndVarSimplify pass for examples.

Thanks,
-- Sanjoy

4

To be clear, I was suggesting to read the code in simplifyLoopIVs() in “lib/Transforms/Utils/SimplifyIndVar.cpp” to see how it identifies IVs. (as opposed to calling the function to change the code).

5

Thanks this helps!