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0 pointspjmlp1y ago0 comments

Manual reference counting to be more precise.

When a GC-managed language uses reference counting as implementation algorith, the compiler might be able to optimize the reference counting to only occur when it is unavoidable or too costly to reason about, in a similar vein to bounds checking.

When using library types for reference counting, there is no way for the compiler to implement such optimizations, unless the types are somehow tagged with compiler intrisics so that they could apply the same kind of optimizations.

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ninkendo1y ago· 2 in thread

> When using library types for reference counting, there is no way for the compiler to implement such optimizations

Aren’t refcounts in shared_ptr done as part of the copy constructor and destructor? It seems like vanilla copy elision would count as optimizing away the refcounts.

pjmlpOP1y ago

Copy constructor, destructor, move constructor, copy constructor.

All of them have to do counter booking, value count and accessors, also possibly handle weak_ptr booking.

Plus how it actually works isn't part of ISO C++, so you're betting your luck on how a specific C++ is actually going to implement it, and possible issues when compiler versions change, same compiler or to another one.

ninkendo1y ago

C++17 has copy elision as part of the standard now, so you can be guaranteed of the scenarios where C++ is required to outright skip the copy/move/destruction, leaving no code paths left for any refcounts. It’s not everything, there’s still plenty of optimizations on the table, but copy elision should eliminate a ton of refcount bookkeeping.

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