RFR: 8281453: New optimization: convert `~x` into `-1-x` when `~x` is used in an arithmetic expression [v9]

[Zhiqiang Zang]

On Tue, 27 Sep 2022 18:55:18 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> Thanks @merykitty  So do you mean we want to do the transformation only when ~a has one use, i.e., `outcnt() == 1`? This makes sense to me.
>> 
>> However I still don't get why it should be done in `XOrNode`. Could you shed light on this?
>> 
>>> Consider there are 2 statements:
>>> 
>>> ```
>>> int x = ~a + b;
>>> int y = ~a | c;
>>> ```
>>> 
>>> Your transformation would duplicate the `~a` as it is transformed into `(-a - 1)` in the calculation of `x` but not in the calculation of `y`.
>>> 
>>> Thanks.
>
>> So do you mean we want to do the transformation only when ~a has one use, i.e., `outcnt() == 1`
> 
> Yes, you can iterate through the uses of the node to be more general
> 
>> However I still don't get why it should be done in XOrNode
> 
> Because this seems logically to be a transformation of the not node, not of the add or sub node, and putting the transformation there may hurt maintainability.
> 
> Thanks.

@merykitty Sounds good! I'm implementing this in XorNode.

I came a question on `outcnt()`, I included in `XorINode::Ideal`

if (phase->type(in2) == TypeInt::MINUS_1) {
  std::cerr << "!!!outcnt() = " << outcnt() << std::endl;
  if (outcnt() == 1) {
    int user_op = unique_out()->Opcode();
    if (user_op == Op_AddI || user_op == Op_SubI) {
      return new SubINode(in2, in1);
    }
  }
}

but I found tests failed, for example

@Test
@IR(failOn = {IRNode.SUB, IRNode.XOR, IRNode.ADD})
// Checks -1 - ~x => x
public int test25(int x) {
    return -1 - ~x;
}

After some debugging, I found the cause was `outcnt()` is `0` so the transformation did not happen at all (the IR printed stills follows the original structure of `-1-~x`). I don't understand why `outcnt()` is `0` not `1` since `~x` is used by `-1 - ~x`. Appreciate it if you can shed some light on this. Thank you.

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PR: https://git.openjdk.org/jdk/pull/7376