RFR: 8328626: GenShen: Combine old generation surplus/deficit fields into a single balance field [v2]

[Y. Srinivas Ramakrishna]

On Mon, 25 Mar 2024 17:13:35 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> src/hotspot/share/gc/shenandoah/shenandoahGenerationalHeap.cpp line 242:
>> 
>>> 240:     // curtailed if the budget is restricted.
>>> 241:     old_region_deficit = MIN2(old_region_deficit, max_old_region_xfer);
>>> 242:     old_generation()->set_region_balance(0 - checked_cast<ssize_t>(old_region_deficit));
>> 
>> The code in the if and the else branches is highly isomorphic. I have the feeling you may be able to fold them together, but I am not sure how clean it will be. Also if you start with `ssize_t` for some of the operands that feed into the surplus calculation (or balance if you will, but we can call it surplus wit the understanding that a negative surplus is a deficit), then you may be able to avoid some of the later casts and the signing of unsigned quantities, including the `(cast)x`, `-(cast)x` or `0-(cast)x`constructs above.
>
> I agree the code in the branches is similar, but the APIs we have for managing capacity for a generation will ultimately make us chose old or young for the source or destination of the transfer based on whether `old_region_balance` is positive or negative.

Yes, I was thinking that the generation sizer could do that based on the sign of the balance.
But I realize that might not necessarily simplify the code, as you seem to have discovered. OK, thanks for checking.

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/410#discussion_r1538161758