RFR(S): 8149745: C2 should optimize long accumulations in a counted loop

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8149745/webrev.00/

This extends the code that looks for a parallel induction variable to long accumulations so in the following code:

public static long testLongAcc() {
   long acc = 0;

   for (int i = 0; i < 1000; i++) {
       acc += 8;
   }

   return acc;
}

the loop is optimized out.

What I find puzzling is that code for integer accumulations doesn’t try to estimate the benefit of turning one AddI into a more expensive MulI + AddI. Maybe it’s too unpredictable but in the case of the long accumulations would we want to check that, for instance, the long Phi has no use in the loop?

Roland.