Ping! <http://gcc.gnu.org/ml/gcc-patches/2014-09/msg00403.html>
> From: Hans-Peter Nilsson <h...@axis.com> > Date: Thu, 4 Sep 2014 23:42:28 +0200 > This adds an option to allow programs and libraries built > *without* inhibit_libc to stay compatible with system libraries > (really: libgcc_s.so.1) built *with* inhibit_libc, at the cost > of the registration. As mentioned, that's a one-way > compatibility barrier. > > While it's nice to avoid the overhead of a function call at > DSO/program initialization time, using eh-registry isn't that > much of overhead until an exception is thrown: most of the > execution-time overhead will come from the additional > symbol-table-entries due to the registry calls (typically 2 in > all libraries and programs, as weak references) and calls to > thread-locking as the (main) part of its work-load. Besides the > symbol-table-entries the footprint size is typically the code for > if (__register_frame_info) > __register_frame_info (__EH_FRAME_BEGIN__, &object); > and the corresponding deregistration and the static object > struct (6 or 7 pointers). (As mentioned, the library-part of > the eh-registry support is present either way.) So, a low-cost > compatibility path is to always call __register_frame_info, > despite favorable conditions for phdr usage. Here's a patch to > optionally do that, controlled by a configure-time option > tentatively called --enable-explicit-exception-frame-registration > (subject to bikeshedding if only for the length). Note > that there is a cost when an exception *is* thrown, the dreaded > sorting of FDE:s. There seems to be some obvious room for > improvement though, as the same information is available > *without* sorting through the PT_GNU_EH_FRAME header entry for > the same file. > > Tested with no regressions after fixing g++.old-deja/g++.eh/badalloc1.C > (see <http://gcc.gnu.org/ml/gcc-patches/2014-09/msg00115.html>) for > native x86_64-linux before compared to with patch/with patch and > --enable-explicit-exception-frame-registration/. > > Ok to commit? > > libgcc: > * crtstuff.c [EH_FRAME_SECTION_NAME && !USE_PT_GNU_EH_FRAME]: > Sanity-check USE_EH_FRAME_REGISTRY_ALWAYS against > EH_FRAME_SECTION_NAME, emit error if unsane. > (USE_EH_FRAME_REGISTRY): Let USE_EH_FRAME_REGISTRY_ALWAYS > override USE_PT_GNU_EH_FRAME. > * Makefile.in (FORCE_EXPLICIT_EH_REGISTRY): New > variable for substituted force_explicit_eh_registry. > (CRTSTUFF_CFLAGS): Add FORCE_EXPLICIT_EH_REGISTRY. > * configure.ac (explicit-exception-frame-registration): > New AC_ARG_ENABLE. > * configure: Regenerate. > > Index: libgcc/Makefile.in > =================================================================== > --- libgcc/Makefile.in (revision 214759) > +++ libgcc/Makefile.in (working copy) > @@ -50,6 +50,8 @@ target_noncanonical = @target_noncanonic > # The rules for compiling them should be in the t-* file for the machine. > EXTRA_PARTS = @extra_parts@ > > +FORCE_EXPLICIT_EH_REGISTRY = @force_explicit_eh_registry@ > + > extra-parts = libgcc-extra-parts > > # Multilib support variables. > @@ -283,7 +285,7 @@ INTERNAL_CFLAGS = $(CFLAGS) $(LIBGCC2_CF > CRTSTUFF_CFLAGS = -O2 $(GCC_CFLAGS) $(INCLUDES) $(MULTILIB_CFLAGS) -g0 \ > -finhibit-size-directive -fno-inline -fno-exceptions \ > -fno-zero-initialized-in-bss -fno-toplevel-reorder -fno-tree-vectorize \ > - -fno-stack-protector \ > + -fno-stack-protector $(FORCE_EXPLICIT_EH_REGISTRY) \ > $(INHIBIT_LIBC_CFLAGS) > > # Extra flags to use when compiling crt{begin,end}.o. > Index: libgcc/configure.ac > =================================================================== > --- libgcc/configure.ac (revision 214759) > +++ libgcc/configure.ac (working copy) > @@ -262,6 +262,22 @@ no) > ;; > esac > > +AC_ARG_ENABLE([explicit-exception-frame-registration], > + [AC_HELP_STRING([--enable-explicit-exception-frame-registration], > + [register exception tables explicitly at module start, for use > + e.g. for compatibility with installations without PT_GNU_EH_FRAME > support])], > +[ > +force_explicit_eh_registry= > +if test "$enable_explicit_exception_frame_registration" = yes; then > + if test "$enable_sjlj_exceptions" = yes; then > + AC_MSG_ERROR([Can't enable both of --enable-sjlj-exceptions > + and --enable-explicit-exception-frame-registration]) > + fi > + force_explicit_eh_registry=-DUSE_EH_FRAME_REGISTRY_ALWAYS > +fi > +]) > +AC_SUBST([force_explicit_eh_registry]) > + > AC_LIB_PROG_LD_GNU > > AC_MSG_CHECKING([for thread model used by GCC]) > Index: libgcc/crtstuff.c > =================================================================== > --- libgcc/crtstuff.c (revision 214709) > +++ libgcc/crtstuff.c (working copy) > @@ -131,7 +131,12 @@ call_ ## FUNC (void) > \ > # define USE_PT_GNU_EH_FRAME > #endif > > -#if defined(EH_FRAME_SECTION_NAME) && !defined(USE_PT_GNU_EH_FRAME) > +#ifdef USE_EH_FRAME_REGISTRY_ALWAYS > +# ifndef EH_FRAME_SECTION_NAME > +# error "Can't use explicit exception-frame-registration without > EH_FRAME_SECTION_NAME" > +# endif > +#endif > +#if defined(EH_FRAME_SECTION_NAME) && (!defined(USE_PT_GNU_EH_FRAME) || > defined(USE_EH_FRAME_REGISTRY_ALWAYS)) > # define USE_EH_FRAME_REGISTRY > #endif > #if defined(EH_FRAME_SECTION_NAME) && EH_TABLES_CAN_BE_READ_ONLY > > brgds, H-P >
