RFR(XS): 8211451: ~2.5% regression on compression benchmark starting with 12-b11

[Roland Westrelin]

>> The test for equality is turned into an inequality check only if init <
>> limit is known and stride == 1 (or a similar condition for stride =
>> -1). In that case i += stride can't overflow.
>
> Where this is done?

  // Now we need to canonicalize loop condition.
  if (bt == BoolTest::ne) {
    assert(stride_con == 1 || stride_con == -1, "simple increment only");
    // 'ne' can be replaced with 'lt' only when init < limit.
    if (stride_con > 0 && init_t->_hi < limit_t->_lo)
      bt = BoolTest::lt;
    // 'ne' can be replaced with 'gt' only when init > limit.
    if (stride_con < 0 && init_t->_lo > limit_t->_hi)
      bt = BoolTest::gt;
  }


>> If the equality check is kept, i+stride can overflow only if init >
>> limit and that would be an infinite loop anyway.
>
> This argument does not hold. You are converting loop into counted loop for which loop optimizations 
> assume that it is not infinite.

So it's not an infinite loop. It's a very long running loop. What I'm
saying is if there's an overflow of i+stride then init > limit and
converting it to a counted loop doesn't make it run for a different
number of iterations.

Roland.