Re: [PATCH RFC] c++: use __cxa_call_terminate for MUST_NOT_THROW [PR97720]

[Jonathan Wakely <jwakely@redhat.com>]

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On Wed, 24 May 2023 at 19:56, Jason Merrill via Libstdc++ <
libstdc++@gcc.gnu.org> wrote:

> Middle-end folks: any thoughts about how best to make the change described
> in
> the last paragraph below?
>
> Library folks: any thoughts on the changes to __cxa_call_terminate?
>

I see no harm in exporting it (with the adjusted signature). The "looks
standard but isn't" name is a little unfortunate, but not a big deal.


>
> -- 8< --
>
> [except.handle]/7 says that when we enter std::terminate due to a throw,
> that is considered an active handler.  We already implemented that properly
> for the case of not finding a handler (__cxa_throw calls __cxa_begin_catch
> before std::terminate) and the case of finding a callsite with no landing
> pad (the personality function calls __cxa_call_terminate which calls
> __cxa_begin_catch), but for the case of a throw in a try/catch in a
> noexcept
> function, we were emitting a cleanup that calls std::terminate directly
> without ever calling __cxa_begin_catch to handle the exception.
>
> A straightforward way to fix this seems to be calling __cxa_call_terminate
> instead.  However, that requires exporting it from libstdc++, which we have
> not previously done.  Despite the name, it isn't actually part of the ABI
> standard.  Nor is __cxa_call_unexpected, as far as I can tell, but that one
> is also used by clang.  For this case they use __clang_call_terminate; it
> seems reasonable to me for us to stick with __cxa_call_terminate.
>
> I also change __cxa_call_terminate to take void* for simplicity in the
> front
> end (and consistency with __cxa_call_unexpected) but that isn't necessary
> if
> it's undesirable for some reason.
>
> This patch does not fix the issue that representing the noexcept as a
> cleanup is wrong, and confuses the handler search; since it looks like a
> cleanup in the EH tables, the unwinder keeps looking until it finds the
> catch in main(), which it should never have gotten to.  Without the
> try/catch in main, the unwinder would reach the end of the stack and say no
> handler was found.  The noexcept is a handler, and should be treated as
> one,
> as it is when the landing pad is omitted.
>
> The best fix for that issue seems to me to be to represent an
> ERT_MUST_NOT_THROW after an ERT_TRY in an action list as though it were an
> ERT_ALLOWED_EXCEPTIONS (since indeed it is an exception-specification).
> The
> actual code generation shouldn't need to change (apart from the change made
> by this patch), only the action table entry.
>
>         PR c++/97720
>
> gcc/cp/ChangeLog:
>
>         * cp-tree.h (enum cp_tree_index): Add CPTI_CALL_TERMINATE_FN.
>         (call_terminate_fn): New macro.
>         * cp-gimplify.cc (gimplify_must_not_throw_expr): Use it.
>         * except.cc (init_exception_processing): Set it.
>         (cp_protect_cleanup_actions): Return it.
>
> gcc/ChangeLog:
>
>         * tree-eh.cc (lower_resx): Pass the exception pointer to the
>         failure_decl.
>         * except.h: Tweak comment.
>
> libstdc++-v3/ChangeLog:
>
>         * libsupc++/eh_call.cc (__cxa_call_terminate): Take void*.
>         * config/abi/pre/gnu.ver: Add it.
>
> gcc/testsuite/ChangeLog:
>
>         * g++.dg/eh/terminate2.C: New test.
> ---
>  gcc/cp/cp-tree.h                     |  2 ++
>  gcc/except.h                         |  2 +-
>  gcc/cp/cp-gimplify.cc                |  2 +-
>  gcc/cp/except.cc                     |  5 ++++-
>  gcc/testsuite/g++.dg/eh/terminate2.C | 30 ++++++++++++++++++++++++++++
>  gcc/tree-eh.cc                       | 16 ++++++++++++++-
>  libstdc++-v3/libsupc++/eh_call.cc    |  4 +++-
>  libstdc++-v3/config/abi/pre/gnu.ver  |  7 +++++++
>  8 files changed, 63 insertions(+), 5 deletions(-)
>  create mode 100644 gcc/testsuite/g++.dg/eh/terminate2.C
>
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index a1b882f11fe..a8465a988b5 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -217,6 +217,7 @@ enum cp_tree_index
>         definitions.  */
>      CPTI_ALIGN_TYPE,
>      CPTI_TERMINATE_FN,
> +    CPTI_CALL_TERMINATE_FN,
>      CPTI_CALL_UNEXPECTED_FN,
>
>      /* These are lazily inited.  */
> @@ -358,6 +359,7 @@ extern GTY(()) tree cp_global_trees[CPTI_MAX];
>  /* Exception handling function declarations.  */
>  #define terminate_fn                   cp_global_trees[CPTI_TERMINATE_FN]
>  #define call_unexpected_fn
>  cp_global_trees[CPTI_CALL_UNEXPECTED_FN]
> +#define call_terminate_fn
> cp_global_trees[CPTI_CALL_TERMINATE_FN]
>  #define get_exception_ptr_fn
>  cp_global_trees[CPTI_GET_EXCEPTION_PTR_FN]
>  #define begin_catch_fn
>  cp_global_trees[CPTI_BEGIN_CATCH_FN]
>  #define end_catch_fn                   cp_global_trees[CPTI_END_CATCH_FN]
> diff --git a/gcc/except.h b/gcc/except.h
> index 5ecdbc0d1dc..378a9e4cb77 100644
> --- a/gcc/except.h
> +++ b/gcc/except.h
> @@ -155,7 +155,7 @@ struct GTY(()) eh_region_d
>      struct eh_region_u_must_not_throw {
>        /* A function decl to be invoked if this region is actually
> reachable
>          from within the function, rather than implementable from the
> runtime.
> -        The normal way for this to happen is for there to be a CLEANUP
> region
> +        The normal way for this to happen is for there to be a TRY region
>          contained within this MUST_NOT_THROW region.  Note that if the
>          runtime handles the MUST_NOT_THROW region, we have no control over
>          what termination function is called; it will be decided by the
> diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
> index 216a6231d6f..853b1e44236 100644
> --- a/gcc/cp/cp-gimplify.cc
> +++ b/gcc/cp/cp-gimplify.cc
> @@ -343,7 +343,7 @@ gimplify_must_not_throw_expr (tree *expr_p, gimple_seq
> *pre_p)
>    gimple *mnt;
>
>    gimplify_and_add (body, &try_);
> -  mnt = gimple_build_eh_must_not_throw (terminate_fn);
> +  mnt = gimple_build_eh_must_not_throw (call_terminate_fn);
>    gimple_seq_add_stmt_without_update (&catch_, mnt);
>    mnt = gimple_build_try (try_, catch_, GIMPLE_TRY_CATCH);
>
> diff --git a/gcc/cp/except.cc b/gcc/cp/except.cc
> index 91a5e049860..b04eb00d220 100644
> --- a/gcc/cp/except.cc
> +++ b/gcc/cp/except.cc
> @@ -64,6 +64,9 @@ init_exception_processing (void)
>    tmp = build_function_type_list (void_type_node, ptr_type_node,
> NULL_TREE);
>    call_unexpected_fn
>      = push_throw_library_fn (get_identifier ("__cxa_call_unexpected"),
> tmp);
> +  call_terminate_fn
> +    = push_library_fn (get_identifier ("__cxa_call_terminate"), tmp,
> NULL_TREE,
> +                      ECF_NORETURN | ECF_COLD | ECF_NOTHROW);
>  }
>
>  /* Returns an expression to be executed if an unhandled exception is
> @@ -76,7 +79,7 @@ cp_protect_cleanup_actions (void)
>
>       When the destruction of an object during stack unwinding exits
>       using an exception ... void terminate(); is called.  */
> -  return terminate_fn;
> +  return call_terminate_fn;
>  }
>
>  static tree
> diff --git a/gcc/testsuite/g++.dg/eh/terminate2.C
> b/gcc/testsuite/g++.dg/eh/terminate2.C
> new file mode 100644
> index 00000000000..1c69dab95f8
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/eh/terminate2.C
> @@ -0,0 +1,30 @@
> +// PR c++/97720
> +// { dg-do run }
> +
> +// Test that there is an active exception when we reach the terminate
> handler.
> +
> +#include <exception>
> +#include <cstdlib>
> +
> +void bad_guy() throw() {
> +  try { throw 0; }
> +  catch (float) { }
> +  // Don't catch int.
> +}
> +
> +void level1() {
> +  bad_guy();
> +  throw "dead code";
> +}
> +
> +void my_term()
> +{
> +  try { throw; }
> +  catch(...) { std::exit(0); }
> +}
> +
> +int main() {
> +  std::set_terminate (my_term);
> +  try { level1(); }
> +  catch (int) { }
> +}
> diff --git a/gcc/tree-eh.cc b/gcc/tree-eh.cc
> index 934209d205f..e8ceff36cc6 100644
> --- a/gcc/tree-eh.cc
> +++ b/gcc/tree-eh.cc
> @@ -3382,8 +3382,22 @@ lower_resx (basic_block bb, gresx *stmt,
>               lab = gimple_block_label (new_bb);
>               gsi2 = gsi_start_bb (new_bb);
>
> +             /* Handle failure fns that expect either no arguments or the
> +                exception pointer.  */
>               fn = dst_r->u.must_not_throw.failure_decl;
> -             x = gimple_build_call (fn, 0);
> +             if (TYPE_ARG_TYPES (TREE_TYPE (fn)) != void_list_node)
> +               {
> +                 tree epfn = builtin_decl_implicit (BUILT_IN_EH_POINTER);
> +                 src_nr = build_int_cst (integer_type_node, src_r->index);
> +                 x = gimple_build_call (epfn, 1, src_nr);
> +                 tree var = create_tmp_var (ptr_type_node);
> +                 var = make_ssa_name (var, x);
> +                 gimple_call_set_lhs (x, var);
> +                 gsi_insert_after (&gsi2, x, GSI_CONTINUE_LINKING);
> +                 x = gimple_build_call (fn, 1, var);
> +               }
> +             else
> +               x = gimple_build_call (fn, 0);
>               gimple_set_location (x, dst_r->u.must_not_throw.failure_loc);
>               gsi_insert_after (&gsi2, x, GSI_CONTINUE_LINKING);
>
> diff --git a/libstdc++-v3/libsupc++/eh_call.cc
> b/libstdc++-v3/libsupc++/eh_call.cc
> index 3cfc71a4ef1..2bec4e83a7d 100644
> --- a/libstdc++-v3/libsupc++/eh_call.cc
> +++ b/libstdc++-v3/libsupc++/eh_call.cc
> @@ -36,8 +36,10 @@ using namespace __cxxabiv1;
>  // terminate.
>
>  extern "C" void
> -__cxa_call_terminate(_Unwind_Exception* ue_header) throw ()
> +__cxa_call_terminate(void* ue_header_in) throw ()
>  {
> +  _Unwind_Exception* ue_header
> +    = reinterpret_cast<_Unwind_Exception*>(ue_header_in);
>
>    if (ue_header)
>      {
> diff --git a/libstdc++-v3/config/abi/pre/gnu.ver
> b/libstdc++-v3/config/abi/pre/gnu.ver
> index 768cd4a4a6c..a2e5f3b4e74 100644
> --- a/libstdc++-v3/config/abi/pre/gnu.ver
> +++ b/libstdc++-v3/config/abi/pre/gnu.ver
> @@ -2841,6 +2841,13 @@ CXXABI_1.3.14 {
>
>  } CXXABI_1.3.13;
>
> +CXXABI_1.3.15 {
> +
> +  global:
> +    __cxa_call_terminate;
> +
> +} CXXABI_1.3.14;
> +
>  # Symbols in the support library (libsupc++) supporting transactional
> memory.
>  CXXABI_TM_1 {
>
>
> base-commit: 2738955004256c2e9753364d78a7be340323b74b
> --
> 2.31.1
>
>