RFR: 8377307: Refactor code for AOT cache pointer compression

[Johan Sjölen]

On Fri, 6 Feb 2026 12:38:25 GMT, Johan Sjölen <jsjolen at openjdk.org> wrote:

>> AOT cache uses pointer compression to store 64-bit pointers in 32 bits. Currently the compression algorithm is simply (base + narrowPtr). However, in [JDK-8376125](https://bugs.openjdk.org/browse/JDK-8376125), we may change it to (base + narrowPtr << 3).
>> 
>> This RFE changed the encoded pointer from an u4 to (thanks to @jdksjolen for the idea):
>> 
>> 
>> enum class narrowPtr : u4;
>> 
>> 
>> This allows better tracking of where the encoded pointers are stored. Also, we can distinguish between byte offsets vs encoded pointers -- they currently have the same numerical values but after [JDK-8376125](https://bugs.openjdk.org/browse/JDK-8376125), they will be different.
>> 
>> Also minor clean up in ArchiveBuilder to change some `uintx` to `size_t`, which is used by `pointer_delta()`.
>
> src/hotspot/share/cds/aotCompressedPointers.hpp line 169:
> 
>> 167: inline u4 to_u4(AOTCompressedPointers::narrowPtr narrowp) {
>> 168:   return cast_from_narrowPtr<u4>(narrowp);
>> 169: }
> 
> Seems like `cast_to_narrowPtr` could do with a `from_u4` analogue, and the only usage of the cast_to_narrowPtr can actually be replaced with that call. Maybe we don't need those globals, and the `from_` `to_` u4 variants are the only necessary ones?

BTW, you're also saving quite a few keystrokes if you make encode/decode global, but that's up to you.

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PR Review Comment: https://git.openjdk.org/jdk/pull/29590#discussion_r2773973154