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

[Axel Boldt-Christmas]

On Mon, 13 Nov 2023 01:13:10 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> src/hotspot/share/runtime/lockStack.cpp line 82:
>> 
>>> 80:       if (VM_Version::supports_recursive_lightweight_locking()) {
>>> 81:         oop o = _base[i];
>>> 82:         for (;i < top - 1; i++) {
>> 
>> Nit: space after first `;`
>
> 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.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16606#discussion_r1390816671