[code-reflection] RFR: Use memory segment

[Maurizio Cimadamore]

On Wed, 26 Feb 2025 12:49:29 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> No - `allocateFrom` creates a new off-heap segment when invoked on an `Arena`. `Arena` is a segment allocator that always allocates off-heap. The `allocateFrom` functions are explicitly designed for use cases when you want to allocate and copy existing data on top of the new allocated segment -- in which case the FFM knows that zeroing the allocated segment is redundant. I think that should be used here.
>
>> No - `allocateFrom` creates a new off-heap segment when invoked on an `Arena`. `Arena` is a segment allocator that always allocates off-heap. The `allocateFrom` functions are explicitly designed for use cases when you want to allocate and copy existing data on top of the new allocated segment -- in which case the FFM knows that zeroing the allocated segment is redundant. I think that should be used here.
> 
> Note: what I'm pointing out is that, performance-wise, `allocateFrom` is not the same as `allocate` + `copyFrom`. Internally, `allocateFrom` will still create a temporary on-heap view of the byte array (w/o allocating a new heap array though) -- but that will be passed to an allocation routine that does allocation + copy w/o zeroring. If you call `allocate` zeroing will happen no matter what.

And... final note -- ByteBuffer doesn't have an `allocateFrom` variant, so zeroing is unavoidable there -- but with segments we can do better.

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PR Review Comment: https://git.openjdk.org/babylon/pull/328#discussion_r1971531526