[PATCH] libgcc: Use `-fasynchronous-unwind-tables' for LIB2_DIVMOD_FUNCS

[Maciej W. Rozycki]

Complement commit b932f770f70d ("x86_64 frame unwind info"), SVN r46374, 
<https://gcc.gnu.org/ml/gcc-patches/2001-10/msg00860.html>, and replace 
`-fexceptions -fnon-call-exceptions' with `-fasynchronous-unwind-tables' 
in LIB2_DIVMOD_FUNCS compilation flags so as to provide unwind tables 
for the affected functions while not pulling the unwinder proper, which 
is not required here.

Remove the ARM overrides accordingly, retaining the hook infrastructure 
however, and make the ARM test case a generic one.

Beyond saving program space it fixes a RISC-V glibc build error due to 
unsatisfied `malloc' and `free' references from the unwinder causing 
link errors with `ld.so' where libgcc has been built at -O0.
---
Hi,

 As discussed: <https://gcc.gnu.org/pipermail/gcc/2020-July/233120.html>.  

 This has been regression-tested with all the GCC compiler testsuites with 
the `x86_64-linux' native configuration, which in particular means the 
moved ARM test case scored the UNSUPPORTED result.  I have no access to a 
non-Linux configuration right now, so I cannot verify this test case, but 
in principle I expect it to work across the relevant targets (and the 
irrelevant ones can be excluded as they are discovered).

 OK to apply then?  It may make sense to backport this fix too to the 
active release branches; please let me know if to do so.

 NB the original commit referred appears to contain more than just the 
corresponding mailing list posting; it looks like several patches were 
folded together before comitting, so this is as good as you can get.

  Maciej
---
 gcc/testsuite/gcc.dg/div64-unwinding.c         |   25 +++++++++++++++++++++++++
 gcc/testsuite/gcc.target/arm/div64-unwinding.c |   25 -------------------------
 libgcc/Makefile.in                             |    2 +-
 libgcc/config/arm/t-bpabi                      |    5 -----
 libgcc/config/arm/t-netbsd-eabi                |    5 -----
 5 files changed, 26 insertions(+), 36 deletions(-)

gcc-libgcc-divmod-asynchronous-unwind-tables.diff
Index: gcc/gcc/testsuite/gcc.dg/div64-unwinding.c
===================================================================
--- /dev/null
+++ gcc/gcc/testsuite/gcc.dg/div64-unwinding.c
@@ -0,0 +1,25 @@
+/* Performing a 64-bit division should not pull in the unwinder.  */
+
+/* { dg-do run { target { { ! *-*-linux* } && { ! *-*-uclinux* } } } } */
+/* { dg-skip-if "load causes weak symbol resolution" { vxworks_kernel } } */
+/* { 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);
+}
Index: gcc/gcc/testsuite/gcc.target/arm/div64-unwinding.c
===================================================================
--- gcc.orig/gcc/testsuite/gcc.target/arm/div64-unwinding.c
+++ /dev/null
@@ -1,25 +0,0 @@
-/* Performing a 64-bit division should not pull in the unwinder.  */
-
-/* { dg-do run { target { { ! *-*-linux* } && { ! *-*-uclinux* } } } } */
-/* { dg-skip-if "load causes weak symbol resolution" { vxworks_kernel } } */
-/* { 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);
-}
Index: gcc/libgcc/Makefile.in
===================================================================
--- gcc.orig/libgcc/Makefile.in
+++ gcc/libgcc/Makefile.in
@@ -533,7 +533,7 @@ 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
+LIB2_DIVMOD_EXCEPTION_FLAGS := -fasynchronous-unwind-tables
 endif
 
 # Build LIB2_DIVMOD_FUNCS.
Index: gcc/libgcc/config/arm/t-bpabi
===================================================================
--- gcc.orig/libgcc/config/arm/t-bpabi
+++ gcc/libgcc/config/arm/t-bpabi
@@ -13,8 +13,3 @@ LIB2ADDEH = $(srcdir)/config/arm/unwind-
 
 # Add the BPABI names.
 SHLIB_MAPFILES += $(srcdir)/config/arm/libgcc-bpabi.ver
-
-# 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/libgcc/config/arm/t-netbsd-eabi
===================================================================
--- gcc.orig/libgcc/config/arm/t-netbsd-eabi
+++ gcc/libgcc/config/arm/t-netbsd-eabi
@@ -11,8 +11,3 @@ LIB2ADDEH =
 
 # Add the BPABI names.
 SHLIB_MAPFILES += $(srcdir)/config/arm/libgcc-bpabi.ver
-
-# 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