From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 48) id A617E3858D28; Wed, 16 Mar 2022 00:46:54 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org A617E3858D28 From: "pdimov at gmail dot com" To: gcc-bugs@gcc.gnu.org Subject: [Bug libstdc++/104945] New: std::hash ignores the top 32 bits when size_t is 32 bit Date: Wed, 16 Mar 2022 00:46:54 +0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: new X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gcc X-Bugzilla-Component: libstdc++ X-Bugzilla-Version: 12.0 X-Bugzilla-Keywords: X-Bugzilla-Severity: normal X-Bugzilla-Who: pdimov at gmail dot com X-Bugzilla-Status: UNCONFIRMED X-Bugzilla-Resolution: X-Bugzilla-Priority: P3 X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org X-Bugzilla-Target-Milestone: --- X-Bugzilla-Flags: X-Bugzilla-Changed-Fields: bug_id short_desc product version bug_status bug_severity priority component assigned_to reporter target_milestone Message-ID: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/ Auto-Submitted: auto-generated MIME-Version: 1.0 X-BeenThere: gcc-bugs@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-bugs mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 16 Mar 2022 00:46:54 -0000 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104945 Bug ID: 104945 Summary: std::hash ignores the top 32 bits when size_t is 32 bit Product: gcc Version: 12.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: libstdc++ Assignee: unassigned at gcc dot gnu.org Reporter: pdimov at gmail dot com Target Milestone: --- A benchmark in Boost.Unordered uses uint64_t keys of the form `i << 40`: https://github.com/boostorg/unordered/blob/33f81fd49039bccd1aa3dfd5a29ef607= 3b93009c/benchmark/uint64.cpp#L65 which leads to pathological behavior with std::unordered_map in 32 bit mode, because std::hash ignores the top 32 bits: https://godbolt.org/z/PncKbT7aq I'm not entirely sure whether this would be considered a bug, but decided t= hat it's worth reporting. Ideally, std::hash ought to take into account all bit= s of the integral input, instead of truncating it to size_t.=