RFR: 8352075: Perf regression accessing fields [v23]

[Radim Vansa]

On Mon, 9 Jun 2025 12:16:38 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> From what I could find, strict alignment checking must be explicitly enabled an aarch64. x86_64 does not require alignment either. In both cases, there might be a performance penalty.
>
> Once I turned on hard signals for these unaligned accesses to find some performance problems (and I think I was debugging something on sparc).  The OS handles these signals silently but it does/can cause performance loss.  There must be a way to write this without all this C style casting with C++ syntax or a special memcpy.  It would still be performant for field access even if the array was copied as a byte stream.  There must be a more readable way to do this.

What's wrong about `memcpy`, or rather the builtin version? Naturally I could write a for cycle copying the bytes, and rely on the compiler to optimize that out anyway, but I think that this makes the intention clear.

If the handling was done through OS, I guess that the penalty would be actually quite severe. I could have tested the previous version on aarch64 e.g. in AWS, though now there's no casting of pointers anymore.

When we have a final version, I could set up a build in AWS and report performance data from there.

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PR Review Comment: https://git.openjdk.org/jdk/pull/24847#discussion_r2135764757