From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 2181) id 6721B3858D28; Sun, 23 Jan 2022 22:49:38 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 6721B3858D28 MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="utf-8" From: Jonathan Wakely To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r12-6826] libstdc++: Fix aliasing violation in std::shared_ptr [PR104019] X-Act-Checkin: gcc X-Git-Author: Jonathan Wakely X-Git-Refname: refs/heads/master X-Git-Oldrev: 2da90ad39bf8fa9ee287e040d1f4411cb7a2e7ed X-Git-Newrev: 2d8a9ad4a922e3248d0f6c60a6331be6f47dc435 Message-Id: <20220123224938.6721B3858D28@sourceware.org> Date: Sun, 23 Jan 2022 22:49:38 +0000 (GMT) X-BeenThere: libstdc++-cvs@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libstdc++-cvs mailing list List-Unsubscribe: , List-Archive: List-Help: List-Subscribe: , X-List-Received-Date: Sun, 23 Jan 2022 22:49:38 -0000 https://gcc.gnu.org/g:2d8a9ad4a922e3248d0f6c60a6331be6f47dc435 commit r12-6826-g2d8a9ad4a922e3248d0f6c60a6331be6f47dc435 Author: Jonathan Wakely Date: Fri Jan 21 12:08:20 2022 +0000 libstdc++: Fix aliasing violation in std::shared_ptr [PR104019] The non-atomic store that sets both reference counts to zero uses a type-punned pointer, which has undefined behaviour. We could use memset to write 8 bytes, but we don't actually need it to be a single store anyway. No other thread can observe the values, that's why it's safe to use non-atomic stores in the first place. So we can just set each count to zero. With -fstore-merging (which is enabled by default at -O2) GCC produces the same code for this as for memset or the type punned store. Clang does that store merging even at -O1. libstdc++-v3/ChangeLog: PR libstdc++/104019 * include/bits/shared_ptr_base.h (_Sp_counted_base<>::_M_release): Set members to zero without type punning. Diff: --- libstdc++-v3/include/bits/shared_ptr_base.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/libstdc++-v3/include/bits/shared_ptr_base.h b/libstdc++-v3/include/bits/shared_ptr_base.h index 5b8f84b65be..b2f955b41f7 100644 --- a/libstdc++-v3/include/bits/shared_ptr_base.h +++ b/libstdc++-v3/include/bits/shared_ptr_base.h @@ -340,7 +340,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION // 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; + _M_weak_count = _M_use_count = 0; _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_use_count); _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count); _M_dispose();