Hi Maciej: I just found the mail thread about div mod with -fnon-call-exceptions, I think keeping the default LIB2_DIVMOD_EXCEPTION_FLAGS unchanged should be the best way to go.
Non-call exceptions and libcalls https://gcc.gnu.org/legacy-ml/gcc/2001-06/msg01108.html Non-call exceptions and libcalls Part 2 https://gcc.gnu.org/legacy-ml/gcc/2001-07/msg00402.html On Tue, Aug 25, 2020 at 9:41 AM Kito Cheng <kito.ch...@gmail.com> wrote: > > 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.
