[PATCH v2] libgcc: Use `-fasynchronous-unwind-tables' for LIB2_DIVMOD_FUNCS

Tue Aug 18 22:17:50 GMT 2020

On 06/08/2020 14:04, Maciej W. Rozycki via Gcc-patches 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.
> 
> 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.
> 
> 	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'.
> 	* config/arm/t-bpabi (LIB2_DIVMOD_EXCEPTION_FLAGS): Remove
> 	variable.
> 	* config/arm/t-netbsd-eabi (LIB2_DIVMOD_EXCEPTION_FLAGS):
> 	Likewise.

>From a quick glance, I'm not convinced this is right for Arm, since the
Arm unwind format does not support anything other than call-based
exceptions.  How did you test it?

R.

> ---
> Hi,
> 
>  I realised we still use handwritten ChangeLog entries (I got confused 
> with now different policies each of the various pieces of the GNU 
> toolchain has), so here's v2 of the change with a fix for that problem 
> being the only update.
> 
>  Also I have since run verification with the `riscv64-linux-gnu' target 
> and the ilp32d multilib as more representative for the change being made.
> No problems were observed, although the now enabled test case scored:
> 
> UNSUPPORTED: gcc.dg/div64-unwinding.c
> 
> of course with the target failing the `! *-*-linux*' condition.
> 
>  Given that for the `riscv64-linux-gnu' target and the ilp32d multilib 
> glibc currently fails to link against libgcc.a built at -O0 I first ran 
> reference testing with target libraries built at -O2, but comparing that 
> to change-under-test -O2 results revealed another issue with GCC target 
> libraries built at -O0 causing link failures across testsuites, namely 
> libgcov.a referring atomic primitives where libatomic.a has not been 
> linked in.  I haven't figured out yet if the issue is in libgcov, the 
> testsuite or the specs.  Examples of failures:
> 
> .../bin/riscv64-linux-gnu-ld: .../gcc/testsuite/g++/../../lib32/ilp32d/libgcov.a(_gcov_indirect_call_profiler_v4.o): in function `__gcov_topn_values_profiler_body': .../libgcc/libgcov-profiler.c:116: undefined reference to `__atomic_fetch_add_8'
> .../bin/riscv64-linux-gnu-ld: .../libgcc/libgcov-profiler.c:129: undefined reference to `__atomic_fetch_add_8'
> .../bin/riscv64-linux-gnu-ld: .../libgcc/libgcov-profiler.c:150: undefined reference to `__atomic_fetch_sub_8'
> collect2: error: ld returned 1 exit status
> compiler exited with status 1
> FAIL: g++.dg/other/pr55650.C  -std=gnu++98 (test for excess errors)
> 
> There were some odd Fortran failures too, with test cases failing to link, 
> making the results difficult to interpret.  Therefore I decided to arrange 
> for a special build with first stage GCC built with its target libraries 
> at -O2, so that first stage glibc builds, and then second stage GCC built 
> with its target libraries at -O0 and second stage glibc omitted.  That 
> removed the extra Fortran failures regardless of whether this change has 
> been applied or not, but we may consider looking overall into why a full 
> `riscv64-linux-gnu' build at -O0 has regressions against -O2 at least in 
> the ilp32d multilib.
> 
>  Meanwhile, OK to apply?
> 
>   Maciej
> 
> 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 +-
>  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
>