RFR: 8293416: ZGC: Set mark bit with unconditional atomic ops [v3]

[Albert Mingkun Yang]

On Thu, 8 Sep 2022 08:15:28 GMT, hev <duke at openjdk.org> wrote:

>> **Summary**
>> Support to set ZGC mark bit with unconditional atomic ops.
>> 
>> **Motivation**
>> ZGC currently modify mark-bitmap by a conditional atomic operation (cmpxchg). This way is not optimal, which will retry the loop when cmpxchg fails.
>> 
>> **Description**
>> First, This patch-set add an new unconditional atomic operation: Atomic::fetch_and_or, which is implemented in different ways for different CPU architectures:
>> 
>> * Exclusive access: Non-nested loop
>> 
>> 
>> retry:
>>   ll old_val, addr
>>   or new_val, old_val, set_val
>>   sc new_val, addr
>>   beq retry
>> 
>> 
>> * Atomic access: One instruction
>> 
>> 
>> ldset old_val, set_val, addr
>> 
>> 
>> * Generic: Fallback to cmpxchg or use c++ __atomic_fetch_or
>> 
>> **Testing**
>> * jtreg tests
>> * benchmark tests
>
> hev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:
> 
>  - ZGC: Set mark bit with unconditional atomic ops
>  - BitMap: Set bit with unconditional atomic ops
>  - Atomic: Add bitset functions

src/hotspot/share/gc/z/zBitMap.inline.hpp line 67:

> 65:     inc_live = false;
> 66:     return false;
> 67:   }

The fast/slow paths are almost identical. I believe some numbers would be nice to justify the presence of the fast path.

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

PR: https://git.openjdk.org/jdk/pull/10182