[LLVMdev] SCEV and induction variable identification

[Paul Vario]

Hi Fellows,

    The goal is to find the induction variable for a loop, where the
induction variable increments with the multiplication, division or shift
operations, like this one:

sz = 8;
do {

  ... ...

   sz = sz / 2;
} while (sz)

    Is SCEV capable of detecting the induction variable 'sz' in this case?
The code snippet I am using to solve the problem is

for each basic-block in a loop
    for each instruction J in a basic block
          if ( J is a PHINode) {
              const SCEV *S = SE->getSCEV(J);
              const SCEVAddRecExpr *SARE = dyn_cast<SCEVAddRecExpr>(S);

              if (SARE) {
                 const Loop *CurLoop = SARE->getLoop();

                 if (CurLoop == L) {
                     /* => J is the induction variable*/
                }
              }
            }

     SCEVAddRecExpr is said to be able to handle any polynomial recurrence
on the trip count of the loop. However, for my sample program, The dyn_cast
result SARE is always NULL ... hence getting me to think that SCEV is not
capable of handling cases like i=i/2, which contradicts what was said in
the LLVM API documentation. Thanks.

Best,
Paul
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