RFR: JDK-8304820: Statically allocate ObjectSynchronizer mutexes [v4]

[Axel Boldt-Christmas]

On Wed, 29 Mar 2023 02:41:32 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Justin King has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Remove unnecessary align_up
>>   
>>   Signed-off-by: Justin King <jcking at google.com>
>
> src/hotspot/share/runtime/synchronizer.cpp line 253:
> 
>> 251: 
>> 252: // Static storage for an array of PlatformMutex.
>> 253: alignas(PlatformMutex) static uint8_t _inflation_locks[inflation_lock_count()][sizeof(PlatformMutex)];
> 
> Sorry I am still not understanding why this can't just be:
> 
> alignas(PlatformMutex) static PlatformMutex _inflation_locks[inflation_lock_count()];
> 
> ??? (And is the alignas even needed?)

`alignas` is not needed in your example. 

I guess we do not do this because we want to control the initialization of these objects and not rely on C++ dynamic initialization. 

You could solve this in different ways. Some containing struct, which was rejected in the #13143. Or rewriting PlatformMutex to be constant initializable. 

Having a byte blob as backing storage does seem like the simplest least intrusive solution.

When we move to C++17 then this could be a candidate use for `std::optional`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/13160#discussion_r1151783335