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build system: stop .eh_frame generation

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From Rich Felker:

By default, modern GCC generates DWARF2 debug/unwind tables in the
.eh_frame section of the object files/binaries. This adds significant
bloat (as much as 15%) to the size of the busybox binary, including
the portion mapped/loaded into memory at runtime (possibly a big issue
for NOMMU targets), and the section is not strippable with the strip
command due to being part of the loaded program text.

I've since done some further checking - both testing and asking the
GCC developers about it - and it seems the solution is to add to the
CFLAGS -fno-unwind-tables and -fno-asynchronous-unwind-tables. If
debugging is disabled, this will prevent GCC from outputting DWARF2
tables entirely. But since busybox builds with -g by default, the
interesting case is what happens then. I originally thought these
options would break debugging, but they don't; instead, they tell GCC
to output the DWARF2 tables in the .debug_frame section instead of
the newish .eh_frame section (used for exception handling). With these
options added, busybox_unstripped is still fully debuggable, and the
final busybox binary loses the 15% bloat factor from the DWARF2
tables.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
This commit is contained in:
Denys Vlasenko 2012-09-04 11:46:44 +02:00
parent b7812ce0f7
commit 9b80b903c7
1 changed files with 3 additions and 0 deletions
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3
Makefile.flags
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@ -53,6 +53,9 @@ CFLAGS += $(call cc-option,-fno-builtin-strlen -finline-limit=0 -fomit-frame-poi
CFLAGS += $(call cc-option,-fno-guess-branch-probability,)
CFLAGS += $(call cc-option,-funsigned-char -static-libgcc,)
CFLAGS += $(call cc-option,-falign-functions=1 -falign-jumps=1 -falign-labels=1 -falign-loops=1,)
# Defeat .eh_frame bloat (gcc 4.6.3 x86-32 defconfig: 20% smaller busybox binary):
CFLAGS += $(call cc-option,-fno-unwind-tables,)
CFLAGS += $(call cc-option,-fno-asynchronous-unwind-tables,)
# FIXME: These warnings are at least partially to be concerned about and should
# be fixed..