[PATCH v3] libgcc: Use `-fasynchronous-unwind-tables' for LIB2_DIVMOD_FUNCS

[Kito Cheng]

Hi Maciej:

Thanks for your patch, I tried to compile all target libraries with
-O0 and got link error as you describe, it's really confusing about
the error message, __pthread_mutex_lock and __pthread_mutex_unlock are
undefined, it turns out the root cause is the __umoddi3 called
_Unwind_Resume and pull-in lots of functions, so really appreciate you
digging out this issue.

After figure out what happen and reproduce what you see, I spend times
to dig out the reason why -fexceptions -fnon-call-exceptions is set
for div and mod routines, it was introduce at many years ago[1] and no
much comment on why doing this, but I guess it's because some ISAs
might generate exception/trap when divide-by-0, and then the execution
environment will convert that to an exception, so those files/routines
need to compile with such flags.

However RISC-V never causes a trap when divide-by-0, so I believe
-fexceptions -fnon-call-exceptions could be removed safely for RISC-V,
but I am not sure it's safe for all other targets or not.

I would suggest you set LIB2_DIVMOD_EXCEPTION_FLAGS to empty or
'-fasynchronous-unwind-tables' for RISC-V port and do not change the
default one in libgcc/Makefile.in.

[1] https://github.com/gcc-mirror/gcc/commit/fc6aa0a98a0c7d10d39dd9d1485d0996b84b1860#diff-f98ad72e54bdf47d90acbfafa467d802R132


On Fri, Aug 21, 2020 at 2:46 AM Maciej W. Rozycki via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> 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.
>
> 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.
>
>         gcc/
>         * testsuite/gcc.target/arm/div64-unwinding.c: Rename to...
>         * testsuite/gcc.dg/div64-unwinding.c: ... this.
>
>         libgcc/
>         * Makefile.in [!LIB2_DIVMOD_EXCEPTION_FLAGS]
>         (LIB2_DIVMOD_EXCEPTION_FLAGS): Replace `-fexceptions
>         -fnon-call-exceptions' with `-fasynchronous-unwind-tables'.
> ---
> Hi,
>
>  No change from v2 except for the removal of the ARM parts; hence no need
> to retest.  OK to apply?
>
>   Maciej
>
> Changes from v2:
>
> - Removal of the ARM overrides removed.
>
> Changes from v1:
>
> - ChangeLog entries added.
> ---
>  gcc/testsuite/gcc.dg/div64-unwinding.c         |   25 +++++++++++++++++++++++++
>  gcc/testsuite/gcc.target/arm/div64-unwinding.c |   25 -------------------------
>  libgcc/Makefile.in                             |    2 +-
>  3 files changed, 26 insertions(+), 26 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.