On Thu, 17 Dec 2020 at 20:51, Maged Michael wrote: > > Please find a proposed patch for _Sp_counted_base::_M_release to skip the > two atomic instructions that decrement each of the use count and the weak > count when both are 1. I proposed the general idea in an earlier thread ( > https://gcc.gnu.org/pipermail/libstdc++/2020-December/051642.html) and got > useful feedback on a draft patch and responses to related questions about > multi-granular atomicity and alignment. This patch is based on that > feedback. > > > I added a check for thread sanitizer to use the current algorithm in that > case because TSAN does not support multi-granular atomicity. I'd like to > add a check of __has_feature(thread_sanitizer) for building using LLVM. I > found examples of __has_feature in libstdc++
There are no uses of __has_feature in libstdc++. We do use __has_builtin (which GCC also supports) and Clang's __is_identifier (which GCC doesn't support) to work around some weird semantics of __has_builtin in older versions of Clang. > but it doesn't seem to be > recognized in shared_ptr_base.h. Any guidance on how to check > __has_feature(thread_sanitizer) in this patch? I think we want to do something like this in include/bits/c++config #if __SANITIZE_THREAD__ # define _GLIBCXX_TSAN 1 #elif defined __has_feature # if __has_feature(thread_sanitizer) # define _GLIBCXX_TSAN 1 # endif #endif Then in bits/shared_ptr_base.h #if _GLIBCXX_TSAN _M_release_orig(); return; #endif > GCC generates code for _M_release that is larger and more complex than that > generated by LLVM. I'd like to file a bug report about that. Jonathan, Is this the same issue as https://gcc.gnu.org/PR101406 ? > would you please create a bugzilla account for me ( > https://gcc.gnu.org/bugzilla/) using my gmail address. Thank you. Done (sorry, I didn't notice the request in this mail until coming back to it to review the patch properly). > > > Information about the patch: > > - Benefits of the patch: Save the cost of the last atomic decrements of > each of the use count and the weak count in _Sp_counted_base. Atomic > instructions are significantly slower than regular loads and stores across > major architectures. > > - How current code works: _M_release() atomically decrements the use count, > checks if it was 1, if so calls _M_dispose(), atomically decrements the > weak count, checks if it was 1, and if so calls _M_destroy(). > > - How the proposed patch works: _M_release() loads both use count and weak > count together atomically (when properly aligned), checks if the value is > equal to the value of both counts equal to 1 (e.g., 0x100000001), and if so > calls _M_dispose() and _M_destroy(). Otherwise, it follows the original > algorithm. > > - Why it works: When the current thread executing _M_release() finds each > of the counts is equal to 1, then (when _lock_policy is _S_atomic) no other > threads could possibly hold use or weak references to this control block. > That is, no other threads could possibly access the counts or the protected > object. > > - The proposed patch is intended to interact correctly with current code > (under certain conditions: _Lock_policy is _S_atomic, proper alignment, and > native lock-free support for atomic operations). That is, multiple threads > using different versions of the code with and without the patch operating > on the same objects should always interact correctly. The intent for the > patch is to be ABI compatible with the current implementation. > > - The proposed patch involves a performance trade-off between saving the > costs of two atomic instructions when the counts are both 1 vs adding the > cost of loading the combined counts and comparison with two ones (e.g., > 0x100000001). > > - The patch has been in use (built using LLVM) in a large environment for > many months. The performance gains outweigh the losses (roughly 10 to 1) > across a large variety of workloads. > > > I'd appreciate feedback on the patch and any suggestions for checking > __has_feature(thread_sanitizer). N.B. gmail completely mangles patches unless you send them as attachments. > diff --git a/libstdc++-v3/include/bits/shared_ptr_base.h > b/libstdc++-v3/include/bits/shared_ptr_base.h > > index 368b2d7379a..a8fc944af5f 100644 > > --- a/libstdc++-v3/include/bits/shared_ptr_base.h > > +++ b/libstdc++-v3/include/bits/shared_ptr_base.h > > @@ -153,20 +153,78 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION > > if (!_M_add_ref_lock_nothrow()) > > __throw_bad_weak_ptr(); > > } > > > bool > > _M_add_ref_lock_nothrow() noexcept; > > > void > > _M_release() noexcept > > { > > +#if __SANITIZE_THREAD__ > > + _M_release_orig(); > > + return; > > +#endif > > + if (!__atomic_always_lock_free(sizeof(long long), 0) || The line break should come before the logical operator, not after. This makes it easier to see which operator it is, because it's at a predictable position, not off on the right edge somewhere. i.e. if (!__atomic_always_lock_free(sizeof(long long), 0) || !__atomic_always_lock_free(sizeof(_Atomic_word), 0) || sizeof(long long) < (2 * sizeof(_Atomic_word)) || sizeof(long long) > (sizeof(void*))) But I think I'd like to see this condition expressed differently anyway, see below. > > + !__atomic_always_lock_free(sizeof(_Atomic_word), 0) || > > + sizeof(long long) < (2 * sizeof(_Atomic_word)) || Shouldn't this be != instead of < ? On a big endian target where sizeof(long long) > sizeof(_Atomic_word) loading two _Atomic_word objects will fill the high bits of the long long, and so the (1LL + (1LL << (8 * 4))) calculation won't match what got loaded into the long long. > > + sizeof(long long) > (sizeof(void*))) This is checking the alignment, right? I think we can do so more reliably, and should comment it. I think I'd like to see this condition defined as a number of constexpr booleans, with comments. Maybe: constexpr bool __lock_free = __atomic_always_lock_free(sizeof(long long), 0) && __atomic_always_lock_free(sizeof(_Atomic_word), 0); constexpr bool __double_word = sizeof(long long) == 2 * sizeof(_Atomic_word); // The ref-count members follow the vptr, so are aligned to alignof(void*). constexpr bool __aligned = alignof(long long) <= alignof(void*); if _GLIBCXX17_CONSTEXPR (__lock_free && __double_word && __aligned) { _M_release_double_width_cas(); return; } else { // ... original body of _M_release(); } > + { > > + _M_release_orig(); > > + return; > > + } > > + _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_use_count); > > + _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count); > > + if (__atomic_load_n((long long*)(&_M_use_count), __ATOMIC_ACQUIRE) > > + == (1LL + (1LL << (8 * sizeof(_Atomic_word))))) This should use __CHAR_BIT__ instead of 8. > > + { > > + // Both counts are 1, so there are no weak references and > > + // we are releasing the last strong reference. No other > > + // threads can observe the effects of this _M_release() > > + // call (e.g. calling use_count()) without a data race. > > + *(long long*)(&_M_use_count) = 0; > > + _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_use_count); > > + _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count); > > + _M_dispose(); > > + _M_destroy(); > > + } > > + else > > + { > > + if ((__gnu_cxx::__exchange_and_add(&_M_use_count, -1) == 1)) > > + { > > + _M_release_last_use(); > > + } > > + } > > + } > > + > > + void > > + __attribute__ ((noinline)) > > + _M_release_last_use() noexcept > > + { > > + _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_use_count); > > + _M_dispose(); > > + if (_Mutex_base<_Lp>::_S_need_barriers) > > + { > > + __atomic_thread_fence (__ATOMIC_ACQ_REL); > > + } > > + _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count); > > + if (__gnu_cxx::__exchange_and_add_dispatch(&_M_weak_count, > > + -1) == 1) > > + { > > + _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count); > > + _M_destroy(); > > + } > > + } > > + > > + void > > + _M_release_orig() noexcept > > + { > > // Be race-detector-friendly. For more info see bits/c++config. > > _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_use_count); > > if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, -1) == 1) > > { > > _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_use_count); > > _M_dispose(); > > // There must be a memory barrier between dispose() and > destroy() > > // to ensure that the effects of dispose() are observed in the > > // thread that runs destroy(). > > // See http://gcc.gnu.org/ml/libstdc++/2005-11/msg00136.html > > @@ -279,20 +337,27 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION > > _Sp_counted_base<_S_single>::_M_release() noexcept > > { > > if (--_M_use_count == 0) > > { > > _M_dispose(); > > if (--_M_weak_count == 0) > > _M_destroy(); > > } > > } > > > + template<> > > + inline void > > + _Sp_counted_base<_S_mutex>::_M_release() noexcept > > + { > > + _M_release_orig(); > > + } > > + > > template<> > > inline void > > _Sp_counted_base<_S_single>::_M_weak_add_ref() noexcept > > { ++_M_weak_count; } > > > template<> > > inline void > > _Sp_counted_base<_S_single>::_M_weak_release() noexcept > > { > > if (--_M_weak_count == 0)