[PATCH] Remove loop variant range check when induction variable is strictly increasing

[Philip Reames]

Hi atrick, sanjoy, nlewycky,

Given a range check on an induction variable, we can convert that range check to a loop invariant check against the starting value of the induction variable if the check would fail on the first iteration or not at all.  This creates a check which is easily unswitched and thus effectively removes the range check from within the loop entirely.

Given C code:
for(int i = M; i < N; i++) // i is known not to overflow
  if (i < 0) break;
  a[i] = 0;
}
This transformation produces:
for(int i = M; i < N; i++)
  if (M < 0) break;
  a[i] = 0;
}
Which can be unswitched into:
if (M < 0) break;
for(int i = M; i < N; i++)
  a[i] = 0;
}

I'm not entirely sure this is done in the best place.  I couldn't really find a more natural fit for it, but the bit of code walking the use of each candidate induction variable didn't really seem elegant.  Suggestions on a better way to structure this are very welcome.

http://reviews.llvm.org/D9784

Files:
  lib/Transforms/Utils/SimplifyIndVar.cpp
  test/Transforms/IndVarSimplify/strictly-increasing.ll

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