RFR: 8371667: Shenandoah: Re-design alloc request type enum for better efficiency and cleaner code [v2]

[Aleksey Shipilev]

On Wed, 12 Nov 2025 16:11:00 GMT, Xiaolong Peng <xpeng at openjdk.org> wrote:

>> Current alloc request type enum:
>> 
>>   enum Type {
>>     _alloc_shared, // Allocate common, outside of TLAB
>>     _alloc_shared_gc, // Allocate common, outside of GCLAB/PLAB
>>     _alloc_cds, // Allocate for CDS
>>     _alloc_tlab, // Allocate TLAB
>>     _alloc_gclab, // Allocate GCLAB
>>     _alloc_plab, // Allocate PLAB
>>     _ALLOC_LIMIT
>>   };
>> 
>> With current design, we have to use switch statement in multiple places resulting in unnecessary branches, for instance the function is_mutator_alloc:
>> 
>> 
>>   inline bool is_mutator_alloc() const {
>>     switch (_alloc_type) {
>>       case _alloc_tlab:
>>       case _alloc_shared:
>>       case _alloc_cds:
>>         return true;
>>       case _alloc_gclab:
>>       case _alloc_plab:
>>       case _alloc_shared_gc:
>>         return false;
>>       default:
>>         ShouldNotReachHere();
>>         return false;
>>     }
>>   }
>> 
>> 
>> 
>> In PR, I have re-designed the enum to make the function like is_mutator_alloc much simpler by making the values of the enum follow two simple rules:
>> 1. Smaller value for mutator alloc, larger value for gc alloc; GC alloc types are always greater than any of mutator alloc types.
>> 2. Odd for lab, even number for non-lab
>> 
>> Three functions have been simplified to one-line impl w/o branches in machine code:
>> 
>> 
>>   inline bool is_mutator_alloc() const {
>>     return _alloc_type <= _alloc_shared;
>>   }
>> 
>>   inline bool is_gc_alloc() const {
>>     return _alloc_type >= _alloc_shared_gc;
>>   }
>> 
>>   inline bool is_lab_alloc() const {
>>     return (_alloc_type & 1) == 1;
>>   }
>> 
>> 
>> I didn't check compiled assemble code  of hotspot, in instead, I wrote similar/equivalent code and compile with gcc for comparison using godbolt.org:  
>> 
>> bool is_lab_alloc(int alloc_type) {
>>     return (alloc_type & 1) == 1;
>> }
>> 
>> bool is_lab_alloc_switch(int alloc_type) {
>>     switch (alloc_type) {
>>         case 0:
>>         case 2: 
>>         case 4:
>>           return false;
>>         case 1:
>>         case 3:
>>         case 5:
>>           return true;
>>         default:
>>           throw "Should not reach here";
>> 
>>     }
>> }
>> 
>> x86_64 assembly code (https://godbolt.org/z/h7xfz8PaT):
>> 
>> is_lab_alloc(int):
>>         push    rbp
>>         mov     rbp, rsp
>>         mov     DWORD PTR [rbp-4], edi
>>         mov     eax, DWORD PTR [rbp-4]
>>         and     eax, 1
>>         and     eax, 1
>>         pop     rbp
>>         ret
>> .LC0:
>>         .string "Should not reach here"
>> is_lab_allo...
>
> Xiaolong Peng has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Refactor alloc type with bit masks

Now that we are doing this... I do wonder if we want to fold `_is_promotion` and `_affiliation` into the same bitset?

src/hotspot/share/gc/shenandoah/shenandoahAllocRequest.hpp line 178:

> 176: 
> 177:   inline bool is_mutator_alloc() const {
> 178:     return !is_gc_alloc();

Inline this, to check the `bit_gc_alloc` directly.

src/hotspot/share/gc/shenandoah/shenandoahAllocRequest.hpp line 182:

> 180: 
> 181:   inline bool is_gc_alloc() const {
> 182:     return (_alloc_type & bit_gc_alloc) == bit_gc_alloc;

Check for `!= 0` in these. It is microscopically more efficient.

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

PR Review: https://git.openjdk.org/jdk/pull/28247#pullrequestreview-3454438588
PR Review Comment: https://git.openjdk.org/jdk/pull/28247#discussion_r2519075032
PR Review Comment: https://git.openjdk.org/jdk/pull/28247#discussion_r2519052973