PING Re: [PATCH RFA (tree-eh)] c++: use __cxa_call_terminate for MUST_NOT_THROW [PR97720]

[Jason Merrill <jason@redhat.com>]

Since Jonathan approved the library change, I'm looking for middle-end 
approval for the tree-eh change, even without advice on the potential 
follow-up.

On 5/24/23 14:55, Jason Merrill 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?
> 
> -- 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