From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 48) id 9F6BF3858414; Wed, 3 Jan 2024 12:38:17 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 9F6BF3858414 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org; s=default; t=1704285497; bh=NX2swIDPqjVQf7Rukq89Cu5ndvjUej2Khyjz+5tTnEk=; h=From:To:Subject:Date:In-Reply-To:References:From; b=fH6NlA7yDvIVC4tzDIh1IvXuTA3Zc7Ij2bI/x8EKJCdTkii36a59+OJot5kh/0zyY rzJzJ2kSs9idpmehTYE+AuTjdWRFXm62L6EPWZIaf0zy9kNNk4X00e3PqlGuQXEusR /YBWG7a5t+7tkNol7JmDijcq0LhgBElSAZCrO3fQ= From: "redi at gcc dot gnu.org" To: gcc-bugs@gcc.gnu.org Subject: [Bug libstdc++/113099] locale without RTTI uses dynamic_cast before gcc 13.2 or has ODR violation since gcc 13.2 Date: Wed, 03 Jan 2024 12:38:14 +0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: changed X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gcc X-Bugzilla-Component: libstdc++ X-Bugzilla-Version: 11.4.0 X-Bugzilla-Keywords: ABI, documentation X-Bugzilla-Severity: normal X-Bugzilla-Who: redi at gcc dot gnu.org X-Bugzilla-Status: RESOLVED X-Bugzilla-Resolution: FIXED X-Bugzilla-Priority: P3 X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org X-Bugzilla-Target-Milestone: 13.2 X-Bugzilla-Flags: X-Bugzilla-Changed-Fields: resolution bug_status target_milestone 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=3D113099 Jonathan Wakely changed: What |Removed |Added ---------------------------------------------------------------------------- Resolution|--- |FIXED Status|UNCONFIRMED |RESOLVED Target Milestone|--- |13.2 --- Comment #14 from Jonathan Wakely --- (In reply to andysem from comment #13) > (In reply to Jonathan Wakely from comment #12) > > (In reply to andysem from comment #11) > > > > I'm not sure what you mean by "the compiler is free to generate cod= e that takes it into account." Takes what into account? What problem does t= hat freedom cause? > > >=20 > > > I mean the compiler could move (some part of) dynamic_cast to precede= the > > > check for the standard facet. I.e. of something like this: > > >=20 > > > template< typename _Facet > > > > const _Facet* __try_use_facet(locale const& loc) > > > { > > > const size_t __i =3D _Facet::id._M_id(); > > > const locale::facet** __facets =3D __loc._M_impl->_M_facets; > > > const _Facet* __dyn_facet =3D __dynamic_cast< const _Facet* > > > >(__facets[__i]); > > >=20 > > > // checks for every standard facet type > > > if (__is_same(_Facet, ...)) > > > return static_cast(__facets[__i]); > > >=20 > > > return __dyn_facet; > > > } > >=20 > > But why? Maybe I'm being slow but I still don't understand what problem= is > > being solved here. > >=20 > > Also this would defeat the optimization that avoids the unnecessary ove= rhead > > of dynamic_cast for standard facets. >=20 > I have seen gcc sometimes reorder code like this (i.e. move code from und= er > a branch before it), presumably to improve ILP or prefetch data, I'm not > sure. Yes, that defeats the branch, if it is used as an optimization, and= I > had to prevent this by adding compiler fences in those cases. Granted, in= my > case it happened with inlined code, but I imagine LTO makes it possible to > perform similar code transformations with out-of-line code as well. Building libstdc++ with LTO is unsupported (and doesn't happen when building GCC, it requires making custom changes to the build process, which again is unsupported). That's a non-issue. > I'm not saying this is what actually happens. I'm just pointing out that > even the code that is apparently unreachable may influence codegen and ca= use > ODR issues. I'm still not seeing an actual problem here, just speculation. The compiler knows that `if (__is_same(T,T))` is true, and will not move unreachable code before it unless that code has no side effects. And if it = has no side effects, there's no problem. Since GCC 13.2 there is no change in semantics or observable side effects w= hen the explicit instantiations for std::use_facet (which are compiled with -fr= tti) are used, or the function is inlined (which might be compiled with -fno-rtt= i). For the gcc-11 and gcc-12 branches would could suppress the explicit instantiation declarations for -fno-rtti, so that the extern template definitions do not try to use dynamic_cast on a type that has no RTTI. But = then we'd need to backport (at least part of) the PR 103755 changes to use static_cast for standard facets. Otherwise std::use_facet and std::has_facet would always fail, even for the standard facets which are guaranteed to be present. I don't think we want to do that on the stable branches, so I think this is WONTFIX for GCC 13.1 and earlier, and FIXED for 13.2 and later.=