[foreign-memaccess+abi] RFR: 8300201: When storing MemoryAddress.ofLong(0x0000000080000000L), MemorySegment.get is not equal to MemorySegment.set because of the expanded sign [v2]

[Per Minborg]

On Mon, 23 Jan 2023 17:51:31 GMT, Gavin Ray <duke at openjdk.org> wrote:

> Sorry to follow on here, but a question related to this -- you can use the upper 16 bits of a 64-bit pointer for pointer tagging, and also the lower 3-bits if it's on a word-aligned boundary.
> 
> This is really useful for building things like atomic tagged pointers, where you can store an ABA-like counter in the 16 bits and use the 3 bits for whatever userdata tag you want:
> 
> ```
> // A word-aligned, atomic tagged pointer.
> // Uses both the upper 16 bits for storage, and the lower 3 bits for tagging.
> //
> //   64                48                32                16
> // 0xXXXXXXXXXXXXXXXX  0000000000000000  0000000000000000  0000000000000XXX
> //   ^                 ^                                                ^
> //   |                 |                                                +-- Tag (3 bits)
> //   |                 +-- Pointer (48 bits)
> //   +-- Counter (16 bits)
> ```
> 
> Is this possible with `MemoryAddress` and bitmasks currently in Java 🤔
> 
> (Or does an optimization like that even make sense on the JVM?)

On some systems, mapped memory segments appear as negative longs and thus, is at least using bit 63.

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PR: https://git.openjdk.org/panama-foreign/pull/774