RFR: 8340171: CDS: Enhance bitmap truncation [v2]

[Aleksey Shipilev]

On Tue, 17 Sep 2024 19:31:34 GMT, Matias Saavedra Silva <matsaave at openjdk.org> wrote:

>  Since the pointer rich objects are now at the beginning, does that mean that the number of leading zeros is now very small? We might be losing the benefit of truncating if it's only a few bits.

Maybe. I think we keep the leading truncation to cater for native ptrs bitmap, the objects with which are logically "in the middle". My first iteration flipped leading-truncate to trailing-truncate completely, and footprint regressed a bit, I assumed because of that. So I opted to make a more general leading- and trailing-truncate to avoid surprises in future.

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PR Comment: https://git.openjdk.org/jdk/pull/21010#issuecomment-2357613144