On Fri, May 10, 2024 at 03:59:25PM -0400, Jason Merrill wrote: > > 2024-05-09 Jakub Jelinek <ja...@redhat.com> > > Jason Merrill <ja...@redhat.com> > > > > PR lto/113208 > > * cp-tree.h (maybe_optimize_cdtor): Remove. > > * decl2.cc (tentative_decl_linkage): Call maybe_make_one_only > > for implicit instantiations of maybe in charge ctors/dtors > > declared inline. > > (import_export_decl): Don't call maybe_optimize_cdtor. > > (c_parse_final_cleanups): Formatting fixes. > > * optimize.cc (can_alias_cdtor): Adjust condition, for > > HAVE_COMDAT_GROUP && DECL_ONE_ONLY && DECL_WEAK return true even > > if not DECL_INTERFACE_KNOWN. > > > --- gcc/cp/optimize.cc.jj 2024-04-25 20:33:30.771858912 +0200 > > +++ gcc/cp/optimize.cc 2024-05-09 17:10:23.920478922 +0200 > > @@ -220,10 +220,8 @@ can_alias_cdtor (tree fn) > > gcc_assert (DECL_MAYBE_IN_CHARGE_CDTOR_P (fn)); > > /* Don't use aliases for weak/linkonce definitions unless we can put > > both > > symbols in the same COMDAT group. */ > > - return (DECL_INTERFACE_KNOWN (fn) > > - && (SUPPORTS_ONE_ONLY || !DECL_WEAK (fn)) > > - && (!DECL_ONE_ONLY (fn) > > - || (HAVE_COMDAT_GROUP && DECL_WEAK (fn)))); > > + return (DECL_WEAK (fn) ? (HAVE_COMDAT_GROUP && DECL_ONE_ONLY (fn)) > > + : (DECL_INTERFACE_KNOWN (fn) && !DECL_ONE_ONLY (fn))); > > Hmm, would > > (DECL_ONE_ONLY (fn) ? HAVE_COMDAT_GROUP > : (DECL_INTERFACE_KNOWN (fn) && !DECL_WEAK (fn))) > > make sense instead? I don't think DECL_WEAK is necessary for COMDAT.
I think it isn't indeed necessary for COMDAT, although e.g. comdat_linkage will not call make_decl_one_only if !flag_weak. But I think it is absolutely required for the alias cdtor optimization in question, because otherwise it would be an ABI change. Consider older version of GCC or some other compiler emitting _ZN6vectorI12QualityValueEC1ERKS1_ and _ZN6vectorI12QualityValueEC2ERKS1_ symbols not as aliases, each in their own comdat groups, so .text._ZN6vectorI12QualityValueEC1ERKS1_ in _ZN6vectorI12QualityValueEC1ERKS1_ comdat group and .text._ZN6vectorI12QualityValueEC2ERKS1_ in _ZN6vectorI12QualityValueEC2ERKS1_ comdat group. And then comes GCC with the above patch without the DECL_WEAK check in there, and decides to use alias, so _ZN6vectorI12QualityValueEC1ERKS1_ is an alias to _ZN6vectorI12QualityValueEC2ERKS1_ and both live in .text._ZN6vectorI12QualityValueEC2ERKS1_ section in _ZN6vectorI12QualityValueEC5ERKS1_ comdat group. If you mix TUs with this, the linker can keep one of the section sets from the _ZN6vectorI12QualityValueEC1ERKS1_ and _ZN6vectorI12QualityValueEC2ERKS1_ and _ZN6vectorI12QualityValueEC5ERKS1_ comdat groups. If there is no .weak for the symbols, this will fail to link, one can emit it either the old way or the new way but never both, it is part of an ABI. While with .weak, mixing it is possible, worst case one gets some unused code in the linked binary or shared library. Of course the desirable case is that there is no mixing and there is no unused code, but if it happens, no big deal. Without .weak it is a big deal. Jakub