RFR: 8363998: Implement Compressed Class Pointers for 32-bit [v3]

[Andrew Haley]

On Mon, 28 Jul 2025 06:31:00 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> 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 happe...
>
> Thomas Stuefe has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove stray include

src/hotspot/share/oops/compressedKlass.cpp line 56:

> 54:   // Klass pointers here, coding needs to be revised.
> 55:   // We keep one page safety zone free to guard against size_t overflows on 32-bit. In practice
> 56:   // this is irrelevant, since these upper address space parts are not user-addressable on

Suggestion:

  // this is irrelevant because these upper address space parts are not user-addressable on

Reason: correlation is not causation.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26491#discussion_r2237134166