RFR: 8354954: Typed static memory for late initialization of static class members in Hotspot [v11]

[Stefan Karlsson]

On Wed, 16 Apr 2025 17:37:16 GMT, Johan Sjölen <jsjolen at openjdk.org> wrote:

>> src/hotspot/share/nmt/memoryFileTracker.cpp line 129:
>> 
>>> 127: bool MemoryFileTracker::Instance::initialize(NMT_TrackingLevel tracking_level) {
>>> 128:   if (tracking_level == NMT_TrackingLevel::NMT_off) return true;
>>> 129:   new (_tracker.as()) MemoryFileTracker(tracking_level == NMT_TrackingLevel::NMT_detail);
>> 
>> Maybe you could add an `init` function that forwards the constructor arguments, with an extra check to see if the memory has already been initialized:
>> 
>> 
>>   template<typename...As>
>>   void init(As&&...args) {
>>     assert(is_death_pattern(), "StaticArea already initialized");
>>     new (as()) T(std::forward<As>(args)...);
>>   }
>> 
>> 
>> Suggestion:
>> 
>>   _tracker.init(tracking_level == NMT_TrackingLevel::NMT_detail);
>
> I think that's a good idea. Unfortunately, move semantics and Rvalue references are currently undecided in the style guide, so we can't write this exact code. We can still do
> 
> ```c++
> template<typename... As>
> void init(As&... args) {
>     assert(is_death_pattern(), "StaticArea already initialized");
>     new (as()) T(args...);
> }
> 
> 
> Which is pretty good.

FWIW, the current implementation doesn't allow me to do:

struct Thing {
  Thing(int value) {}
};
...
Defered<Thing> _deferred;
...
_deferred.initialize(1);

I have to write last piece as:

int temp = 1;
_deferred.initialize(temp);

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PR Review Comment: https://git.openjdk.org/jdk/pull/24689#discussion_r2156365079