8283175: Add equals/hashcode to MemorySegment

[mcimadamore]

When looking at the API, we realized that MemorySegment is the only class not to expose equals/hashcode (MemoryLayout, MemoryAddress and MemorySession do). This makes it difficult to e.g. use a MemorySegment as a map key.

This patch adds equals/hashcode; the behavior is that of a shallow comparison - e.g. equals makes sure that the two segments point to the same memory region and have same spatial and temporal bounds. Read-only status is ignored by the comparison (on the basis that the segment is still pointing to the same memory region). And, since comparing memory session uses MemorySession::equals, segments backed by different "views" of the same session will also turn out to be equal (which also makes sense).

To perform a deeper comparison, clients can go through the MemorySegment::mismatch path, but I don't think that's a useful implementation for equals/hashCode (even though that's how byte buffers behave - but then, byte buffers do not provide a mismatch method).

When fixing this I realized that the equals method for FunctionDescriptor did not take into account the variable arity index, so I've fixed that too (and also tweaked the toString method to take that into account as well). Doing this revealed an issue in some of the calling convention tests, where we passed a variable arity descriptor as the expected descriptor, where in reality the calling convention "flattens" the variable arity descriptor into a specialized, fixed-arity one.

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Progress

• Change must not contain extraneous whitespace
• Change must be properly reviewed

Issue

• JDK-8283175: Add equals/hashcode to MemorySegment

Reviewers

• Athijegannathan Sundararajan (@sundararajana - Committer)

Reviewing

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[mlbridge]

Webrevs

• 01: Full - Incremental (a80a8674)
• 00: Full (74e260e7)

[sundararajana]

LGTM

[openjdk]

@mcimadamore This change now passes all automated pre-integration checks.

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8283175: Add equals/hashcode to MemorySegment

Reviewed-by: sundar

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[mcimadamore]

/integrate

[openjdk]

Going to push as commit 5b15ef7.

[openjdk]

@mcimadamore Pushed as commit 5b15ef7.

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