Date: Sat, 23 Apr 2022 15:33:13 -0700 From: Mark Millard <marklmi@yahoo.com> To: joerg@bec.de, FreeBSD Hackers <freebsd-hackers@freebsd.org> Cc: jbo@insane.engineer Subject: Re: llvm & RTTI over shared libraries Message-ID: <E22DA198-862B-4B99-8E0B-E63AEBDCCF35@yahoo.com> References: <E22DA198-862B-4B99-8E0B-E63AEBDCCF35.ref@yahoo.com>
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=E2=80=A2 Joerg Sonnenberger <joerg_at_bec.de> wrote on =E2=80=A2 Date: Sat, 23 Apr 2022 21:33:04 UTC : > Am Tue, Apr 19, 2022 at 11:03:33PM -0700 schrieb Mark Millard: > > Joerg Sonnenberger <joerg_at_bec.de> wrote on > > Tue, 19 Apr 2022 21:49:44 UTC : > >=20 > > > Am Thu, Apr 14, 2022 at 04:36:24PM +0000 schrieb = jbo@insane.engineer: > > >> > After some research I seem to understand that the way that RTTI = is handled over shared library boundaries is different between GCC and = LLVM. > > >>=20 > > > I think you are running into the old problem that GCC thinks = comparing > > > types by name makes sense where as everyone else compares types by = type > > > pointer identity. > >=20 > > Seems out of date for the GCC information . . . > >=20 > > https://gcc.gnu.org/faq.html#dso reports: > >=20 > > QUOTE > > The new C++ ABI in the GCC 3.0 series uses address comparisons, = rather than string compares, to determine type equality. > > END QUOTE >=20 > Compare that with the implementation in <typeinfo>. Looking at /usr/local/lib/gcc11/include/c++/typeinfo I see: configurable, in part based on the intent for possible handling RTLD_LOCAL (when weak symbol are available). I'll quote the comments for reference . . . // Determine whether typeinfo names for the same type are merged (in = which // case comparison can just compare pointers) or not (in which case = strings // must be compared), and whether comparison is to be implemented inline = or // not. We used to do inline pointer comparison by default if weak = symbols // are available, but even with weak symbols sometimes names are not = merged // when objects are loaded with RTLD_LOCAL, so now we always use strcmp = by // default. For ABI compatibility, we do the strcmp inline if weak = symbols // are available, and out-of-line if not. Out-of-line pointer = comparison // is used where the object files are to be portable to multiple = systems, // some of which may not be able to use pointer comparison, but the // particular system for which libstdc++ is being built can use pointer // comparison; in particular for most ARM EABI systems, where the ABI // specifies out-of-line comparison. The compiler's target = configuration // can override the defaults by defining __GXX_TYPEINFO_EQUALITY_INLINE = to // 1 or 0 to indicate whether or not comparison is inline, and // __GXX_MERGED_TYPEINFO_NAMES to 1 or 0 to indicate whether or not = pointer // comparison can be used. So, to some extent, the details are choices in the likes of lang/gcc11 instead of an always-the-same rule for handling. Below gives some more idea of what __GXX_TYPEINFO_EQUALITY_INLINE and __GXX_MERGED_TYPEINFO_NAMES do for configuration. Is there a combination that matches FreeBSD's system clang++ related behavior? If yes, should the likes of lang/gcc11 be using that combination? #if !__GXX_TYPEINFO_EQUALITY_INLINE // In old abi, or when weak symbols are not supported, there can // be multiple instances of a type_info object for one // type. Uniqueness must use the _name value, not object address. . . . #else #if !__GXX_MERGED_TYPEINFO_NAMES . . . // Even with the new abi, on systems that support dlopen // we can run into cases where type_info names aren't merged, // so we still need to do string comparison. . . . #else // On some targets we can rely on type_info's NTBS being unique, // and therefore address comparisons are sufficient. . . . #endif #endif =3D=3D=3D Mark Millard marklmi at yahoo.com
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