Add comments as per review feedback.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@47061 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Analysis/ScalarEvolution.cpp b/lib/Analysis/ScalarEvolution.cpp
index 65cee82..4a440df 100644
--- a/lib/Analysis/ScalarEvolution.cpp
+++ b/lib/Analysis/ScalarEvolution.cpp
@@ -2525,18 +2525,26 @@
return UnknownValue;
if (AddRec->isAffine()) {
- // The number of iterations for "{n,+,1} < m", is m-n. However, we don't
- // know that m is >= n on input to the loop. If it is, the condition
- // returns true zero times. To handle both cases, we return SMAX(m, n)-n.
-
// FORNOW: We only support unit strides.
SCEVHandle One = SE.getIntegerSCEV(1, RHS->getType());
if (AddRec->getOperand(1) != One)
return UnknownValue;
- SCEVHandle Start = AddRec->getOperand(0);
- SCEVHandle End = isSigned ? SE.getSMaxExpr(RHS, Start) : (SCEVHandle)RHS;
+ // We know the LHS is of the form {n,+,1} and the RHS is some loop-invariant
+ // m. So, we count the number of iterations in which {n,+,1} < m is true.
+ // Note that we cannot simply return max(m-n,0) because it's not safe to
+ // treat m-n as signed nor unsinged due to overflow possibility.
+ // First, we get the value of the LHS in the first iteration: n
+ SCEVHandle Start = AddRec->getOperand(0);
+
+ // Then, we get the value of the LHS in the first iteration in which the
+ // above condition doesn't hold. This equals to max(m,n).
+ // FIXME (PR2003): we should have an "umax" operator as well.
+ SCEVHandle End = isSigned ? SE.getSMaxExpr(RHS,Start) : (SCEVHandle)RHS;
+
+ // Finally, we subtract these two values to get the number of times the
+ // backedge is executed: max(m,n)-n.
return SE.getMinusSCEV(End, Start);
}