From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-vs1-xe32.google.com (mail-vs1-xe32.google.com [IPv6:2607:f8b0:4864:20::e32]) by sourceware.org (Postfix) with ESMTPS id 55023384A07D; Thu, 10 Jun 2021 14:41:23 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 55023384A07D Received: by mail-vs1-xe32.google.com with SMTP id x13so1980301vsf.11; Thu, 10 Jun 2021 07:41:23 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=QGyMWNBcgjFyq+7cFSJ5ao/TjApn4d9zD74kouL8wIo=; b=aCKJ/951xu9cOicE1jgzQ+6wwaQgQ2MnRuih9eRZMQpQ32m/f6z5pj8YLHUo/rwUGc GzGIxHWeDfu6VMdbcUv4N88kjetgiZMY/h8SsK6GRROgVGuW8YG2P1+1smdhPYznb/Pc nVKWDVjggHLJX5KVHHhq777iT+sq6vnPg7JgcgUiwbYEuw6iskWS59d23zDzk6Gx5C/B K4dRmsU9B0zA2zcn+MRTaI+eqfDtG4rfwZR7mHebb+H8ekwJkqwThfYfpVTuLtmX+fwW yZynTw4xnxun5pTgFcnXa32NBBd0J+67CgGvCKCEMpX91uHNfRq15uyCmAN04HjLNsjF rj0g== X-Gm-Message-State: AOAM531jxVGIq01+Ck6vUBETkNsQ9CS/KNIQ7UaSlROcNt1U5FqYje41 9kD1/3Wu2eRGG54U4XlBn/RAvAURpsQoA05kZlQnySgb X-Google-Smtp-Source: ABdhPJzwedBrOpagVyigllSh3FpHoC985Ts2OlZ6CCSpliQkcJBGgnOR1UKmhhBygAUQIkOM3ONKD0RvQJWpcWKSb4Q= X-Received: by 2002:a05:6102:2f4:: with SMTP id j20mr5151569vsj.51.1623336082449; Thu, 10 Jun 2021 07:41:22 -0700 (PDT) MIME-Version: 1.0 References: In-Reply-To: From: Maged Michael Date: Thu, 10 Jun 2021 10:41:11 -0400 Message-ID: Subject: Re: [PATCH] libstdc++: Skip atomic instructions in _Sp_counted_base::_M_release when both counts are 1 To: libstdc++@gcc.gnu.org Cc: gcc-patches@gcc.gnu.org, Jonathan Wakely X-Spam-Status: No, score=-9.3 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_FROM, GIT_PATCH_0, HTML_MESSAGE, KAM_NUMSUBJECT, KAM_SHORT, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org Content-Type: text/plain; charset="UTF-8" X-Content-Filtered-By: Mailman/MimeDel 2.1.29 X-BeenThere: libstdc++@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libstdc++ mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 10 Jun 2021 14:41:26 -0000 Would appreciate any comments on this proposed patch. Thanks, Maged On Thu, Dec 17, 2020 at 3:49 PM 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++ 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? > > > 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, > would you please create a bugzilla account for me ( > https://gcc.gnu.org/bugzilla/) using my gmail address. Thank you. > > > 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). > > > Maged > > > > 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) || > > + !__atomic_always_lock_free(sizeof(_Atomic_word), 0) || > > + sizeof(long long) < (2 * sizeof(_Atomic_word)) || > > + sizeof(long long) > (sizeof(void*))) > > + { > > + _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))))) > > + { > > + // 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) > > >