RFR: 8354954: Typed static memory for late initialization of static class members in Hotspot [v2]

[Johan Sjölen]

> Hi,
> 
> This PR introduces a `StableValue<T>` which is sized and aligned identically to a `T`, with the difference that a `StableValue<T>` needs to be explicitly instantiated.
> 
> Dynamic static initalization in C++ leads to unpredictable bugs as there is no defined order in which objects will be initialized, to the degree that 'static initialization fiasco' is a term used. In the code I've worked on in Hotspot we resolve this by having an initialization function, and instead of having static members of `T` we have `T*` instead and use `malloc` in order to gain the memory for the objects.
> 
> This is fine, however when working with certain code you may want to allocate before NMT is initialized. This leads to you having to use raw malloc. This is probably something you find out after having used `os::malloc` and already had a strange crash when building your code.
> 
> That's why I'd like to have `StableValue<T>`. It let's you avoid the whole `malloc` thing, and we overload `->` to make it behave as if it is actually a `T`. We add in a simple checker in debug mode that checks whether the memory has been initialized before using it.
> 
> In the code I've switched two members to be of `StableValue` instead. One is the malloc case above, the second (MemBaseline) is one where I got a bug while developing. The bug occurred because I  changed the initializer of `MemBaseline` without knowing that it was dynamic-static-allocated, and the exact change I made caused weird crashes (because of initialization order issues).
> 
> This solution is quite practical to me, but I wanted to know what others think.

Johan Sjölen has updated the pull request incrementally with one additional commit since the last revision:

  Rename to ptr()

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/24689/files
  - new: https://git.openjdk.org/jdk/pull/24689/files/58a4164d..f402ad3a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24689&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24689&range=00-01

  Stats: 4 lines in 2 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/24689.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24689/head:pull/24689

PR: https://git.openjdk.org/jdk/pull/24689