From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 48) id D88F03847718; Wed, 3 Apr 2024 15:41:11 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org D88F03847718 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org; s=default; t=1712158871; bh=ZbhFDEo/+CCWdOVJIfZjcYSu0kTja1oeWT+y+i44B7g=; h=From:To:Subject:Date:In-Reply-To:References:From; b=HKjYluSWJxGi5RYSYFFn+nDPtfc9w1aR143aepEjrP49SZlSWKfCFCdbDRQL657Ct 0KvbqVgbnJ/ieydBEUMIrxxVB1b6gNUJTXufsL7anqk/pvy/IrqWD3x4DoTFjkGOJ4 ld1XQz7HASBu/fXKVmni/3A1Y5terHJtIQLngtnI= From: "jakub at gcc dot gnu.org" To: gcc-bugs@gcc.gnu.org Subject: [Bug c++/114536] wrong constant evaluation of std::bit_cast for bit fields Date: Wed, 03 Apr 2024 15:41:08 +0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: changed X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gcc X-Bugzilla-Component: c++ X-Bugzilla-Version: 13.2.1 X-Bugzilla-Keywords: accepts-invalid X-Bugzilla-Severity: normal X-Bugzilla-Who: jakub at gcc dot gnu.org X-Bugzilla-Status: NEW 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: cc Message-ID: In-Reply-To: References: 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 List-Id: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114536 Jakub Jelinek changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |jakub at gcc dot gnu.org, | |jason at gcc dot gnu.org, | |redi at gcc dot gnu.org --- Comment #7 from Jakub Jelinek --- Adjusted testcase: namespace std { template constexpr T bit_cast (const F& f) noexcept { return __builtin_bit_cast (T, f); } } struct A { unsigned char a : 7; }; struct B { unsigned char b; }; constexpr unsigned char c =3D __builtin_bit_cast (B, A{1}).b; constexpr unsigned char d =3D std::bit_cast (A{1}).b; This shows that we diagnose correctly the c case: pr114536.C:12:58: error: accessing uninitialized member =E2=80=98B::b=E2=80= =99 12 | constexpr unsigned char c =3D __builtin_bit_cast (B, A{1}).b; | ~~~~~~~~~^ but don't diagnose when the builtin call is wrapped in another call (std::bit_cast). The reason for that is: /* The result of a constexpr function must be completely initialized. However, in C++20, a constexpr constructor doesn't necessarily have to initialize all the fields, so we don't clear CONSTRUCTOR_NO_CLEARING in order to detect reading an unitialized object in constexpr instead of value-initializing it. (reduced_constant_expression_p is expected = to take care of clearing the flag.) */ if (TREE_CODE (result) =3D=3D CONSTRUCTOR && (cxx_dialect < cxx20 || !DECL_CONSTRUCTOR_P (fun))) clear_no_implicit_zero (result); hunk in cxx_eval_call_expression. This is done there since PR80829 for the nested CONSTRUCTOR_NO_CLEARING, an= d on the outermost since r7-4090-gf64e0c029c452c9fc508adebf18d0ceb3ffdc066. If it is UB to return not completely initialized aggregate, shouldn't clear_no_implicit_zero actually diagnose it if CONSTRUCTOR_NO_CLEARING is s= et and not all ctor elements are initialized?=