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

[Julian Waters]

On Fri, 11 Mar 2022 23:40:19 GMT, Zhiqiang Zang <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 share the same opinion that John has from the linked https://github.com/openjdk/jdk/pull/7395, to me it would be better if we would combine all these operations into a single method that can then decide on the specific optimization after analysis. This would also have the added bonus that related optimizations in the future could then leverage this framework and have an easier time implementing what they'd like to do. This may or may not require changes to C1 and the Interpreter to gather all the extra profiling information needed, I can't be the judge of that given my relative inexperience with C2.

The only thing I'm unsure about is whether all the previous optimizations before this are all scattered across C2 or whether they all share variations of the same calculation, because we could take advantage of existing code if it's the latter. @merykitty would it be feasible to search for all of them in the C2 codebase?

Should I create a JBS issue for this PR?

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