diff options
| -rw-r--r-- | libcxx/src/memory.cpp | 30 |
1 files changed, 29 insertions, 1 deletions
diff --git a/libcxx/src/memory.cpp b/libcxx/src/memory.cpp index 15e947fc0ff..b9a741b23f4 100644 --- a/libcxx/src/memory.cpp +++ b/libcxx/src/memory.cpp @@ -96,7 +96,35 @@ __shared_weak_count::__release_shared() _NOEXCEPT void __shared_weak_count::__release_weak() _NOEXCEPT { - if (decrement(__shared_weak_owners_) == -1) + // NOTE: The acquire load here is an optimization of the very + // common case where a shared pointer is being destructed while + // having no other contended references. + // + // BENEFIT: We avoid expensive atomic stores like XADD and STREX + // in a common case. Those instructions are slow and do nasty + // things to caches. + // + // IS THIS SAFE? Yes. During weak destruction, if we see that we + // are the last reference, we know that no-one else is accessing + // us. If someone were accessing us, then they would be doing so + // while the last shared / weak_ptr was being destructed, and + // that's undefined anyway. + // + // If we see anything other than a 0, then we have possible + // contention, and need to use an atomicrmw primitive. + // The same arguments don't apply for increment, where it is legal + // (though inadvisable) to share shared_ptr references between + // threads, and have them all get copied at once. The argument + // also doesn't apply for __release_shared, because an outstanding + // weak_ptr::lock() could read / modify the shared count. + if (__libcpp_atomic_load(&__shared_weak_owners_, _AO_Aquire) == 0) + { + // no need to do this store, because we are about + // to destroy everything. + //__libcpp_atomic_store(&__shared_weak_owners_, -1, _AO_Release); + __on_zero_shared_weak(); + } + else if (decrement(__shared_weak_owners_) == -1) __on_zero_shared_weak(); } |
