RFR: 8357053: ZGC: Improved utility for ZPageAge [v3]

[Stefan Karlsson]

On Fri, 23 May 2025 12:28:45 GMT, Joel Sikström <jsikstro at openjdk.org> wrote:

>> Hello,
>> 
>> This RFE improves utility for converting to/from, iterating over and defining structures that are indexed using the `ZPageAge` type.
>> 
>> Converting to/from ZPageAge and its underlying type (uint8_t, often just uint) is currently done via using static_cast. This works fine because sane values are converted in all use cases. However, to make conversion safer (and also more readable), I propose we add a `to_zpageage` and a corresponding `untype` that checks that the conversion is valid. Such conversion methods should be used instead of calling `static_cast<uint/ZPageAge>`.
>> 
>> We currently define a value called `ZPageAgeMax`, which is defined as `static_cast<uint>(ZPageAge::old)`. The majority of places that use this value actualy use `ZPageAgeMax + 1`, which is equivalent to the number of ages. Instead, I propose we define and use a value that represents the number of possible ages, called `ZPageAgeCount`.
>> 
>> Lastly, to make iterating over ages more accessible, I propose we create an intreface of enum iterators of ZPageAge. This will also create a foundation for generating values that require a ZPageAge in the future. Since the end of the enum iterators are exclusive, I've opted to use the following value as end for the iterators:
>> 
>> constexpr ZPageAge ZPageAgeLastPlusOne = static_cast<ZPageAge>(ZPageAgeCount);
>> 
>> 
>> I see us using either this or a sentinel/dummy value at the end of the enum class, but I prefer having a value similar to `ZPageAgeLastPlusOne` over a dummy value.
>> 
>> Testing:
>> * Oracle's tier 1-4
>> * GHA
>
> Joel Sikström has updated the pull request incrementally with five additional commits since the last revision:
> 
>  - Style fix :)
>  - Added operator+/- for ZPageAge
>  - Fix include order in enumIterator.hpp
>  - Use T instead of EnumType
>  - Use ENUMERATOR_RANGE instead of ENUMERATOR_VALUE_RANGE

Looks good but I found a couple nits.

src/hotspot/share/gc/z/zAllocator.inline.hpp line 30:

> 28: 
> 29: #include "gc/z/zAddress.inline.hpp"
> 30: #include "gc/z/zPageAge.inline.hpp"

Sort order.

src/hotspot/share/utilities/enumIterator.hpp line 241:

> 239: private:
> 240: 
> 241:   struct ConstExprConstructTag {};

The above is implicitly private so this should probably be removed:
Suggestion:

  struct ConstExprConstructTag {};

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

PR Review: https://git.openjdk.org/jdk/pull/25251#pullrequestreview-2864760693
PR Review Comment: https://git.openjdk.org/jdk/pull/25251#discussion_r2104793485
PR Review Comment: https://git.openjdk.org/jdk/pull/25251#discussion_r2104801793