RFR: 8371667: Shenandoah: Re-design alloc request type enum for better efficiency and cleaner code [v7]

[Xiaolong Peng]

On Fri, 14 Nov 2025 09:42:46 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> A bit confusing still. How about this:
> 
> ```
> [x|xx|xxx|xx]
>            ^---- Requester:   
>                      00 -- mutator
>                      10 -- mutator (CDS)
>                      01 -- GC
>         ^------- Purpose:
>                      00 -- shared
>                      01 -- TLAB/GCLAB
>                      11 -- PLAB
>      ^---------- Affiliation: 
>                      00 -- YOUNG
>                      01 -- OLD
>                      11 -- OLD, promotion
> ```
> 
> Then:
> 
> ```
>   static constexpr int bit_gc_alloc         = 1 << 0;
>   static constexpr int bit_cds_alloc        = 1 << 1;
>   static constexpr int bit_lab_alloc        = 1 << 2;
>   static constexpr int bit_plab_alloc       = 1 << 3;
>   static constexpr int bit_old_alloc        = 1 << 4;
>   static constexpr int bit_promotion_alloc  = 1 << 5;
> 
>   static constexpr Type _alloc_shared              = 0;
>   static constexpr Type _alloc_tlab                = bit_lab_alloc;
>   static constexpr Type _alloc_cds                 = bit_cds_alloc;
>   static constexpr Type _alloc_shared_gc           = bit_gc_alloc;
>   static constexpr Type _alloc_shared_gc_old       = bit_gc_alloc | bit_old_alloc;
>   static constexpr Type _alloc_shared_gc_promotion = bit_gc_alloc | bit_old_alloc | bit_promotion_alloc;
>   static constexpr Type _alloc_gclab               = bit_gc_alloc | bit_lab_alloc;
>   static constexpr Type _alloc_plab                = bit_gc_alloc | bit_plab_alloc | bit_old_alloc;     
> ```

Thanks Aleksey! This is good, I have applied the change, and fixed the wrong value for _alloc_plab, it should be `bit_gc_alloc | bit_lab_alloc | bit_plab_alloc | bit_old_alloc`.

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PR Comment: https://git.openjdk.org/jdk/pull/28247#issuecomment-3540274358