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

[Roland Westrelin]

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

> 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.

One issue with such an approach is that it's then required to update all code that pattern match an expression with SubX node to now expect an (AddX (NegX ..) ..). PhaseIdealLoop::is_scaled_iv_plus_offset() for instance. And maybe there are not that many instances of code that require updating, but it's still needed to proceed carefully and double check, possibly update some code, write tests that verify the change does what's intended etc. So what appears relatively simple is going to be more work than expected and the risk associated with the change is higher than one might think. I doubt that's worth it.

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