Hi,
This (old!) patch avoids uselessly pulling in the unwinder for 64-bit
division routines. I last posted it here:
http://gcc.gnu.org/ml/gcc-patches/2009-10/msg01618.html
Other people have noticed the same issue, e.g.:
http://gcc.gnu.org/ml/gcc-help/2011-03/msg00187.html
Note that we take special care already for the case where
division-by-zero throws an exception: we tailcall the division-by-zero
handler at the head of the 64-bit division helper routines, so never
create a stack frame which needs to be unwound. On ARM, division by
zero can trapped by defining magic __aeabi_[il]div0 functions. See
here:
http://gcc.gnu.org/ml/gcc-patches/2009-10/msg01661.html
Unfortunately, despite that, throwing an exception from the
division-by-zero handler functions doesn't appear to work at present --
either with or without the current patch. I'm not intending to tackle
that now (see attached program -- which exits with "terminate called
after throwing an instance of 'int'" for me). I'm no C++ expert, so I
might have done something dumb :-).
Anyway: this revised version of the patch removes the strange libgcc
Makefile-fragment changes, the equivalent of which have since been
incorporated into mainline GCC now anyway, so the patch is somewhat
more straightforward than it was previously.
The patch allows a trivial program using 64-bit division (on bare metal)
to shrink quite considerably:
$ arm-none-eabi-size div64
text data bss dec hex filename
13120 2404 276 15800 3db8 div64
$ arm-none-eabi-size div64-unpatched
text data bss dec hex filename
20064 2404 276 22744 58d8 div64-unpatched
OK to apply?
Thanks,
Julian
ChangeLog
libgcc/
* config.host (arm*-*-linux-*eabi, arm*-*-uclinux*eabi)
(arm*-*-eabi*, arm*-*-symbianelf*, arm*-*-rtemseabi*): Add
arm/t-divmod-ef to tmake_file.
* Makefile.in (LIB2_DIVMOD_EXCEPTION_FLAGS): Default to -fexceptions
-fnon-call-exceptions if not defined.
($(lib2-divmod-o), $(lib2-divmod-s-o)): Use above.
* config/arm/t-divmod-ef: New file.
gcc/testsuite/
* gcc.target/arm/div64-unwinding.c: New test.
#include <cstdio>
long long a = 100, b = 0;
extern "C" long long
__aeabi_ldiv0 (long long retval)
{
throw 0;
}
int bad_division (void)
{
return a / b;
}
int main(int argc, char *argv[])
{
try
{
bad_division ();
}
catch (int e)
{
fprintf (stderr, "caught division by zero\n");
return 0;
}
fprintf (stderr, "didn't catch exception\n");
return 1;
}
Index: libgcc/config.host
===================================================================
--- libgcc/config.host (revision 189656)
+++ libgcc/config.host (working copy)
@@ -317,7 +317,7 @@ arm*-*-linux*) # ARM GNU/Linux with EL
tmake_file="${tmake_file} arm/t-arm t-fixedpoint-gnu-prefix"
case ${host} in
arm*-*-linux-*eabi)
- tmake_file="${tmake_file} arm/t-elf arm/t-bpabi arm/t-linux-eabi t-slibgcc-libgcc"
+ tmake_file="${tmake_file} arm/t-elf arm/t-bpabi arm/t-linux-eabi arm/t-divmod-ef t-slibgcc-libgcc"
tm_file="$tm_file arm/bpabi-lib.h"
unwind_header=config/arm/unwind-arm.h
;;
@@ -331,7 +331,7 @@ arm*-*-uclinux*) # ARM ucLinux
tmake_file="${tmake_file} t-fixedpoint-gnu-prefix"
case ${host} in
arm*-*-uclinux*eabi)
- tmake_file="${tmake_file} arm/t-bpabi"
+ tmake_file="${tmake_file} arm/t-bpabi arm/t-divmod-ef"
tm_file="$tm_file arm/bpabi-lib.h"
unwind_header=config/arm/unwind-arm.h
;;
@@ -344,7 +344,7 @@ arm*-*-ecos-elf)
extra_parts="$extra_parts crti.o crtn.o"
;;
arm*-*-eabi* | arm*-*-symbianelf* | arm*-*-rtemseabi*)
- tmake_file="${tmake_file} arm/t-arm arm/t-elf t-fixedpoint-gnu-prefix"
+ tmake_file="${tmake_file} arm/t-arm arm/t-elf arm/t-divmod-ef t-fixedpoint-gnu-prefix"
tm_file="$tm_file arm/bpabi-lib.h"
case ${host} in
arm*-*-eabi* | arm*-*-rtemseabi*)
Index: libgcc/Makefile.in
===================================================================
--- libgcc/Makefile.in (revision 189656)
+++ libgcc/Makefile.in (working copy)
@@ -497,18 +497,24 @@ libgcc-s-objects += $(patsubst %,%_s$(ob
endif
endif
+ifeq ($(LIB2_DIVMOD_EXCEPTION_FLAGS),)
+# Provide default flags for compiling divmod functions, if they haven't been
+# set already by a target-specific Makefile fragment.
+LIB2_DIVMOD_EXCEPTION_FLAGS := -fexceptions -fnon-call-exceptions
+endif
+
# Build LIB2_DIVMOD_FUNCS.
lib2-divmod-o = $(patsubst %,%$(objext),$(LIB2_DIVMOD_FUNCS))
$(lib2-divmod-o): %$(objext): $(srcdir)/libgcc2.c
$(gcc_compile) -DL$* -c $< \
- -fexceptions -fnon-call-exceptions $(vis_hide)
+ $(LIB2_DIVMOD_EXCEPTION_FLAGS) $(vis_hide)
libgcc-objects += $(lib2-divmod-o)
ifeq ($(enable_shared),yes)
lib2-divmod-s-o = $(patsubst %,%_s$(objext),$(LIB2_DIVMOD_FUNCS))
$(lib2-divmod-s-o): %_s$(objext): $(srcdir)/libgcc2.c
$(gcc_s_compile) -DL$* -c $< \
- -fexceptions -fnon-call-exceptions
+ $(LIB2_DIVMOD_EXCEPTION_FLAGS)
libgcc-s-objects += $(lib2-divmod-s-o)
endif
Index: libgcc/config/arm/t-divmod-ef
===================================================================
--- libgcc/config/arm/t-divmod-ef (revision 0)
+++ libgcc/config/arm/t-divmod-ef (revision 0)
@@ -0,0 +1,4 @@
+# On ARM, specifying -fnon-call-exceptions will needlessly pull in
+# the unwinder in simple programs which use 64-bit division. Omitting
+# the option is safe.
+LIB2_DIVMOD_EXCEPTION_FLAGS := -fexceptions
Index: gcc/testsuite/gcc.target/arm/div64-unwinding.c
===================================================================
--- gcc/testsuite/gcc.target/arm/div64-unwinding.c (revision 0)
+++ gcc/testsuite/gcc.target/arm/div64-unwinding.c (revision 0)
@@ -0,0 +1,24 @@
+/* Performing a 64-bit division should not pull in the unwinder. */
+
+/* { dg-do run } */
+/* { dg-options "-O0" } */
+
+#include <stdlib.h>
+
+long long
+foo (long long c, long long d)
+{
+ return c/d;
+}
+
+long long x = 0;
+long long y = 1;
+
+extern int (*_Unwind_RaiseException) (void *) __attribute__((weak));
+
+int main(void)
+{
+ if (&_Unwind_RaiseException != NULL)
+ abort ();;
+ return foo (x, y);
+}
