RFR: 8338912: CDS: Segmented roots array [v5]

[Aleksey Shipilev]

On Tue, 10 Sep 2024 00:42:12 GMT, Ioi Lam <iklam at openjdk.org> wrote:

> Diff of the map files confirms this:
> 
> ```
> - heap_root_segments.roots_count:         2628
> - heap_root_segments.seg_max_size_elems:  1048576
> - heap_root_segments.seg_max_size_bytes:  131070
> ....
> 131c131
> < 0x0000000000000300:   0010000000000a44 0000020a0001fffe 0000000000000002 000000000000f022   
> ---
> > 0x0000000000000300:   0010000000000a44 000001e20001fffe 0000000000000002 000000000000f022   
> 
>     2628 = 0x00000a44
>  1048576 = 0x00100000
>   131080 = 0x00001ff2
> ```
> 
> The 0x000001e2 is garbage.

Whoa, that looks fragile! It feels safer to memset(0) the entire header then? Whack-a-mole-ing the alignment paddings every time we add a field is not convenient.

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PR Comment: https://git.openjdk.org/jdk/pull/20858#issuecomment-2339970672