RFR: 8363998: Implement Compressed Class Pointers for 32-bit [v4]

[Thomas Stuefe]

> We plan to remove the uncompressed Klass pointer mode (`-UseCompressedClassPointers`) soon. Uncompressed Klass pointers are deprecated for JDK 25, and are planned to be removed for JDK 26. For background and motivation, please refer to this discussion [1] and the description and comments in this deprecation CSR [2].
> 
> A significant roadblock for removing `-UseCompressedClassPointers` had been the ongoing existence of 32-bit. 32-bit relies on uncompressed Klass pointers. 
> 
> Now, some of us think that 32-bit ports are on their way out [3]. We already removed support for x86 with JEP 503 in JDK 25, but we still have arm32 and zero 32-bit. The usefulness of the latter is arguable, seeing that the build is broken more often than not. However, even if we do rid ourselves of 32-bit eventually, that won't happen quickly: further discussions are needed, and then we will need a proper JEP with a deprecation phase for at least one release, probably more. That is too slow - we'd like to get rid of `-UseCompressedClassPointers` a lot sooner than that.
> 
> So we need to find a way to support `+UseCompressedClassPointers` on 32-bit.
> 
> -------
> 
> This patch adds support for `+UseCompressedClassPointers` on 32-bit in a minimally invasive way that keeps technical debt low. The code is small, clearly marked, and can be removed cleanly once we completely remove 32-bit support in two or three releases.
> 
> How to do this? Most is straightforward: pointers are 32-bit, so they are already "compressed". We set up narrow Klass encoding such that `base = NULL` and `shift = 0`; the entire 32-bit address space is therefore our encoding range. We now behave exactly as we would behave on 64-bit in "unscaled" encoding mode, with encoding and decoding being no-ops.
> 
> We also don't change the object layout; header stays the same - Klass is stored in the second word of the header as before, regardless of whether you interpret the word as `narrowKlass` or `Klass*`.
> 
> We also don't need to make many changes in terms of storage. Today, on 32-bit systems, Klass structures live inside the non-Klass metaspace regions, which are scattered arbitrarily throughout the address space. On 64-bit, we need a class space to confine Klass structures to the encoding range; here, where the encoding range encompasses the entire address space, we don't need a class space. Note that should we ever consider supporting some form of compact object headers on 32-bit - god forbid that ever happens - we need to revise this design...

Thomas Stuefe has updated the pull request incrementally with one additional commit since the last revision:

  since to because
  
  Co-authored-by: Andrew Haley <aph-open at littlepinkcloud.com>

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/26491/files
  - new: https://git.openjdk.org/jdk/pull/26491/files/cdd14445..377ee7bd

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26491&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26491&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/26491.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26491/head:pull/26491

PR: https://git.openjdk.org/jdk/pull/26491