Request for reviews (XS): 7039652: Performance regression after 7004547 changes

[Vladimir Kozlov]

http://cr.openjdk.java.net/~kvn/7039652/webrev

Fixed 7039652: Performance regression after 7004547 changes

In the test case initial stride is -8 so the loop is unrolled only once after 
7004547 changes moved next condition to the beginning of policy_unroll() method:

    // Check for stride being a small enough constant
    if (abs(cl->stride_con()) > (1<<3)) return false;

Before 7004547 changes that check was done at the end of policy_unroll() and the 
method returned 'true' if there were a lot 'xor' nodes in the loop regardless 
the stride check. As result the loop in the test case was unrolled twice.

The stride check serves additional purpose to limit unrolling to 16 (when 
initial stride is '1'). But as the test shows it may prevent legit unroll with 
bigger stride.

The fix is to use unrolled_count() to limit unrolling and use the stride check 
only for initial stride value. I don't understand why there is such limit on 
stride but it is for an other investigation (RFE).

I ran refworkload and don't see any affects from this fix.