[onert] Apply inline variable to EventWriter

[ragmani]

This commit applies inline variable to class EventWriter.

ONE-DCO-1.0-Signed-off-by: ragmani [email protected]

[hseok-oh]

@ragmani Is it OK to use static inline for mutex class variable? Doesn't it have possibility to inlining mutex object by compiler?

[ragmani]

Yes, it's perfectly fine to use static inline for a mutex class variable. The inline keyword in this context is used to ensure that the variable has a single definition across all translation units, which allows you to define it in a header file without causing multiple definition errors. It does not cause the mutex object itself to be "inlined" or optimized away in the way that function inlining might; it merely affects linkage. The mutex will still behave as expected for synchronization.

[hseok-oh]

I found sentence from your link at #14647: https://en.cppreference.com/w/cpp/language/inline

• An inline function or variable(since C++17) with external linkage (e.g. not declared static) has the following additional properties:
  • It has the same address in every translation unit.

Doesn't it mean that variable with non-external linkage (static variable) can be different address?

[hseok-oh]

I found answer at https://en.cppreference.com/w/cpp/language/storage_duration#external_linkage

• Any of the following names declared at namespace scope have external linkage, unless they are declared in an unnamed namespace
  • names of classes, their member functions, static data members (const or not), nested classes and enumerations, and functions first introduced with friend declarations inside class bodies;

[hseok-oh]

This feature is hard to understand based on compiler world...

[ragmani]

This feature is hard to understand based on compiler world...

The two types of variables(static class member or namespace-scope variable) can be declared as inline variables. If both variables associated with inline keyword are problematic, it can be expected that the compiler will generate errors about that.
I think there is no need to overthink it and we just follow the rule at https://en.cppreference.com/w/cpp/language/inline regardless of compiler world.

Because the meaning of the keyword inline for functions came to mean "multiple definitions are permitted" rather than "inlining is preferred" since C++98, that meaning was extended to variables.