Add a comment

Author: fiigii

llvm/lib/Transforms/Scalar/StraightLineStrengthReduce.cpp

@@ -701,6 +701,13 @@ void StraightLineStrengthReduce::setBasisAndDeltaFor(Candidate &C) {
 
       assert(DT->dominates(Basis->Ins, C.Ins));
       Value *Delta = getDelta(C, *Basis, K);
+      
+      // IndexDelta rewrite is not always profitable, e.g.,
+      // X = B + 8 * S
+      // Y = B + S,
+      // rewriting Y to X - 7 * S is probably a bad idea.
+      // So, we need to check if the rewrite form's computation efficiency 
+      // is better than the original form.
       if (K == Candidate::IndexDelta &&
           !C.isProfitableRewrite(Delta, Candidate::IndexDelta))
         return false;