The performance of the `MemorySegment::fil` can be improved by replacing the `checkAccess()` method call with calling `checkReadOnly()` instead (as the bounds of the segment itself do not need to be checked).
Also, smaller segments can be handled directly by Java code rather than transitioning to native code. Here is how the `MemorySegment::fill` performance is improved by this PR:  Operations involving 8 or more bytes are delegated to native code whereas smaller segments are handled via a switch rake. It should be noted that `Arena::allocate` is using `MemorySegment::fil`. Hence, this PR will also have a positive effect on memory allocation performance. ------------- Commit messages: - Reduce kick-in size and add test - Initial implementation Changes: https://git.openjdk.org/jdk/pull/20712/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20712&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8338967 Stats: 235 lines in 3 files changed: 231 ins; 0 del; 4 mod Patch: https://git.openjdk.org/jdk/pull/20712.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/20712/head:pull/20712 PR: https://git.openjdk.org/jdk/pull/20712
