RFR: 8252237: C2: Call to compute_separating_interferences has wrong argument order

[Vladimir Kozlov]

On Thu, 3 Dec 2020 15:25:43 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

> Looks good to me but I'm also wondering why that was never an issue and if the fix could now trigger previously hidden problems. We should at least give it a good amount of testing before integration.

compute_separating_interferences() is used only in one place and resulting `reg_degree` is only used in place to compare with `max_juint`:
https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/coalesce.cpp#L746
Due to this bug the returning value is most likely `max_juint` and we bailout coalesce of copies. As result we would have more LRG live ranges to work with but Graph still can be processed.
In short RA produce less optimal code.
I agree with @TobiHartmann we should do a lot of tiers testing for this fix because coalescing code after L755 may be not exercised as much as we want due to this bug.

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PR: https://git.openjdk.java.net/jdk/pull/1533