RFR: 8340297: Use-after-free recognition for metaspace and class space [v4]

[Axel Boldt-Christmas]

On Wed, 20 Aug 2025 04:53:37 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> src/hotspot/share/oops/metadata.hpp line 49:
>> 
>>> 47:   static constexpr uint32_t common_prefix_mask   = BUILD_32(0xFFFF, 0);
>>> 48:   static constexpr uint32_t instance_klass_token = BUILD_32(0x3E7A, 0x100);
>>> 49:   static constexpr uint32_t array_klass_token    = BUILD_32(0x3E7A, 0x101);
>> 
>> Nit / question: Any particular reason we're building the tokens with `BUILD_32(high, low)` instead of writing out the full literal (e.g. `0x3E7A0100`)? Is it mainly for readability or are there other considerations?
>
> None whatsoever, apart from readability. I changed to use plain constants.

You can use `'` as a digit separator. If that was the readability you were looking after.
```c++
  static constexpr uint32_t common_prefix        = 0x3E7A'0000;
  static constexpr uint32_t common_prefix_mask   = 0xFFFF'0000;
  static constexpr uint32_t instance_klass_token = 0x3E7A'0101;
  static constexpr uint32_t array_klass_token    = 0x3E7A'0102;

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PR Review Comment: https://git.openjdk.org/jdk/pull/25891#discussion_r2297128203