RFR: 8283094: Add Ideal transformation: x + (con - y) -> (x - y) + con

[Zhiqiang Zang]

On Sat, 12 Mar 2022 07:44:09 GMT, Quan Anh Mai <duke at openjdk.java.net> wrote:

>> Hello,
>> 
>> `x + (con - y)  -> (x - y) + con` is a widely seen pattern; however it is missing in current implementation, which prevents some obvious constant folding from happening, such as `x + (1 - y) + 2` will be not optimized at all, rather than into `x - y + 3`.
>> 
>> This pull request adds this transformation.
>
> I think I have a better idea for this. During idealisation, we will transform every subtraction into an addition with the negation of the second operand, pushing the negation down the computation graph. This will simplify numerous rules we have with subtractions. Addition presents nice commutative and associative properties so the graph transformation during idealisation with be much easier. Before matching, we may have another strength reduction phase which will push the negations up the computation graph, transform the computations to be as compact as possible.
> 
> This problem seems a lot like the problem we face in #7700 when eagerly strength reducing operations hurt our ability to perform other transformations by complicating the elements inside the graph. I believe this solution would simplify our arithmetic transformations and allow better numeric calculations. Similar to what has been discussed in #7395 , a more general solution would be much more valuable than trying to match every possible combinations.
> 
> Thank you very much.

@merykitty Thank you for your quick response. I agree a general solution is better and I also think addition is generally easier to optimize than subtraction. However considering this case, I noticed there are some existing transformation that does not follow this: for example https://github.com/openjdk/jdk/blob/5c408c1410e15087f735a815b7edc716d514b1b3/src/hotspot/share/opto/addnode.cpp#L307, which seems doing exactly the opposite thing.

Does the transformation hold all the time?
>During idealisation, we will transform every subtraction into an addition with the negation of the second operand, pushing the negation down the computation graph.

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PR: https://git.openjdk.java.net/jdk/pull/7795