On 6/5/23 02:09, Richard Biener wrote:
On Fri, Jun 2, 2023 at 6:57 PM Jason Merrill via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
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
I only wondered about this, whether it shouldn't say CLEANUP or TRY instead
of just TRY? Do you know of other frontends making use of MUST_NOT_THROW?
If there are only CLEANUPs within the MUST_NOT_THROW, we optimize them
away and omit the landing pad, so the region is not actually reachable.
Jason
