[foreign-preview] RFR: 8282070: Drop workaround from memory segment implementation [v2]

[Paul Sandoz]

On Fri, 18 Feb 2022 10:14:34 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Following some preliminary experiments (see [1]) it is now finally time to drop the various workarounds we have in the memory segment implementation. This patch removes two workarounds:
>> 
>> * special treatment for "small" segments (e.g. segments whose size fits in an int) - see `MemorySegment::isSmall`; these segments were special cased so that bound checks would only involve `int` comparisons. This has now been fixed in JDK-8276116, and special treatment is no longer required.
>> 
>> * special shortcuts to avoid extra alignment checks. Since when generating var handles from a layout we check most of the alignment constraints (except those involving the base address), in the past we have added a fastpath for these var handles, as C2 had trouble hoisting alignment checks outside of the loops. This has now been fixed in JDK-8277850 and is no longer required.
>> 
>> I have run benchmarks before and after and got identical results. Some benchmarkks in `ParallelSum` got a boost, but those always have noise, due to multi-threading.
>
> Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Address review comments:
>   * inlined Utils::scaleOffset into callsites
>   * drop redundant check from AbstractMemorySegment::checkBounds

Marked as reviewed by psandoz (Committer).

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PR: https://git.openjdk.java.net/panama-foreign/pull/644