RFR: 8278917: Use Prev Bitmap for recording evac failed objects [v4]

[Stefan Johansson]

On Tue, 21 Dec 2021 08:02:09 GMT, Hamlin Li <mli at openjdk.org> wrote:

>> I did this first, just removing the `G1VerifyBitmaps` condition. But this will cause a notable regression for regions with very few marked objects. I'm not certain that regression is to bad, but I did not see anything suggesting that clearing the whole bitmap afterwards would be quicker in any other scenario either. So went with the bit-by-bit approach.
>
> I assume mem write is more expensive (or at least not more efficient) than mem read, so the performance changes make sense to me.
> 
> And a comment about the observation of the regression will be helpful in the future for others who think this might be a optimization point.

Added a few lines about this.

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PR: https://git.openjdk.java.net/jdk/pull/6867