Patchwork [ARM] Don't pull in unwinder for 64-bit division routines

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Submitter Julian Brown
Date July 20, 2012, 10:15 a.m.
Message ID <20120720111527.16dea84e@octopus>
Download mbox | patch
Permalink /patch/172194/
State New
Headers show

Comments

Julian Brown - July 20, 2012, 10:15 a.m.
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.

Patch

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);
+}