RFR: JDK-8300080: offset_of for GCC/Clang exhibits undefined behavior and is not always a compile-time constant

[Kim Barrett]

On Thu, 12 Jan 2023 20:28:50 GMT, Justin King <jcking at openjdk.org> wrote:

> The implementation of `offset_of` for GCC/Clang only deals with types are aligned to 16 bytes or less, if they are more, such as `zCollectedHeap` the behavior is undefined. UBSan also suggests that `offset_of` is not always a compile time constant, as the stack trace came from the dynamic loader during library loading. This patch changes `offset_of` to use `offsetof` and disables the warning `invalid-offsetof` for the JVM.

Please don't make this change.  We are intentionally not using offsetof.  See JDK-8239357.

A change to the existing code would be to use `alignof(klass)` to align the space buffer, once use of
`alignof` is approved.  That would fix the alignment problem.

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PR: https://git.openjdk.org/jdk/pull/11978