RFR: 8319797: Recursive lightweight locking: Runtime implementation [v3]

[David Holmes]

On Mon, 13 Nov 2023 09:20:26 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

>> Though rather than walk the lockstack twice can't we just change the check below to something like:
>> 
>> if (VM_Version::supports_recursive_lightweight_locking() && i != j - 1) {
>>   assert(_base[i] != _base[j], "entries must be unique: %s", msg);
>> }
>
> Not sure I understand. 
> 
> The change here is that [for every object A, there exists no other object B such that A == B] is changed to [for every contiguous run of object A, there exists no other object B outside that run such that A == B]. Both of these checks are `O(n^2)` in the worst case, and the second is is `O(n)` in the best case. 
> 
> I think you have describe the algorithm you envision. Or maybe you want to change what property we are asserting.

I thought the issue was that without recursion support we must never find an A and B such that A == B. But with recursion support we can allow A == B if they are adjacent in the lock-stock. So that is what the check is performing, but apparently in two loops, where I was suggesting it can perhaps be done in a single loop.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/16606#discussion_r1391770153