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
