[LLVMdev] ScalarEvolution: Suboptimal handling of globals

[Vaivaswatha N]

Hi,
For the program below, where "incr" and "Arr" are globals
=================================
int incr;
float Arr[1000];

int foo ()
{
  float x = 0;
  int newInc = incr+1;
  for (int i = 0; i < 1000; i++) {
    for (int j = 0; j < 1000; j += incr) {
        x += (Arr[i] + Arr[j]);
    }
  }
  return x;
}
=================================

The SCEV expression computed for the variable "j" is %j.0 = phi i32 [ 0, %for.body ], [ %add8, %for.inc ]
  -->  %j.0             Exits: <<Unknown>>As is evident, this isn't a useful computation.

Whereas if I use the variable newInc to be the increment for "j", i.e., "j += newInc" in the inner loop, the computed SCEV is  %j.0 = phi i32 [ 0, %for.body ], [ %add8, %for.inc ]
  -->  {0,+,(1 + %0)}<nsw><%for.cond1> 
where %0 here is a LoadInst for "incr". In this situation, the analysis is working as expected to compute the add-recurrence for "j". 

I would've expected a similar computation for the first case too, something like {0,+,%0}<nsw><%for.cond1> 
Where %0 would be the LoadInst for "incr". 
Is this a bug or is there something more involved that causes the deficiency?
Thanks,- Vaivaswatha
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