[gcc r16-2227] libstdc++: Fix constexpr exceptions for -fno-exceptions
Jonathan Wakely
redi@gcc.gnu.org
Mon Jul 14 11:53:47 GMT 2025
https://gcc.gnu.org/g:8aff55e2598eca1e055d4ec16cec84676ca150c2
commit r16-2227-g8aff55e2598eca1e055d4ec16cec84676ca150c2
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Fri Jul 11 12:40:14 2025 +0100
libstdc++: Fix constexpr exceptions for -fno-exceptions
The if-consteval branches in std::make_exception_ptr and
std::exception_ptr_cast use a try-catch block, which gives an error for
-fno-exceptions. Just make them return a null pointer at compile-time
when -fno-exceptions is used, because there's no way to get an active
exception with -fno-exceptions.
For both functions we have a runtime-only branch that depends on RTTI,
and a fallback using try-catch which works for runtime and consteval.
Rearrange both functions to express this logic more clearly.
Also adjust some formatting and whitespace elsewhere in the file.
libstdc++-v3/ChangeLog:
* libsupc++/exception_ptr.h (make_exception_ptr): Return null
for consteval when -fno-exceptions is used.
(exception_ptr_cast): Likewise. Allow consteval path to work
with -fno-rtti.
Reviewed-by: Jakub Jelinek <jakub@redhat.com>
Diff:
---
libstdc++-v3/libsupc++/exception_ptr.h | 113 ++++++++++++++++-----------------
1 file changed, 55 insertions(+), 58 deletions(-)
diff --git a/libstdc++-v3/libsupc++/exception_ptr.h b/libstdc++-v3/libsupc++/exception_ptr.h
index ee009155a39c..f673a3343338 100644
--- a/libstdc++-v3/libsupc++/exception_ptr.h
+++ b/libstdc++-v3/libsupc++/exception_ptr.h
@@ -83,9 +83,9 @@ namespace std _GLIBCXX_VISIBILITY(default)
#if __cpp_lib_exception_ptr_cast >= 202506L
template<typename _Ex>
- constexpr const _Ex* exception_ptr_cast(const exception_ptr&) noexcept;
+ constexpr const _Ex* exception_ptr_cast(const exception_ptr&) noexcept;
template<typename _Ex>
- void exception_ptr_cast(const exception_ptr&&) = delete;
+ void exception_ptr_cast(const exception_ptr&&) = delete;
#endif
namespace __exception_ptr
@@ -138,8 +138,8 @@ namespace std _GLIBCXX_VISIBILITY(default)
_GLIBCXX_USE_NOEXCEPT;
#if __cpp_lib_exception_ptr_cast >= 202506L
template<typename _Ex>
- friend constexpr const _Ex* std::exception_ptr_cast(const exception_ptr&)
- noexcept;
+ friend constexpr const _Ex*
+ std::exception_ptr_cast(const exception_ptr&) noexcept;
#endif
const void* _M_exception_ptr_cast(const type_info&) const
@@ -296,40 +296,41 @@ namespace std _GLIBCXX_VISIBILITY(default)
using __exception_ptr::swap; // So that std::swap(exp1, exp2) finds it.
/// Obtain an exception_ptr pointing to a copy of the supplied object.
-#if (__cplusplus >= 201103L && __cpp_rtti) || __cpp_exceptions
template<typename _Ex>
+#if !(__cplusplus >= 201103L && __cpp_rtti) && !__cpp_exceptions
+ // This is always_inline so the linker will never use a useless definition
+ // instead of a working one compiled with RTTI and/or exceptions enabled.
+ __attribute__ ((__always_inline__)) inline
+#endif
_GLIBCXX26_CONSTEXPR exception_ptr
make_exception_ptr(_Ex __ex) _GLIBCXX_USE_NOEXCEPT
{
-#if __cplusplus >= 202400L
- if consteval {
- try
- {
- throw __ex;
- }
- catch(...)
- {
- return current_exception();
- }
- }
-#endif
#if __cplusplus >= 201103L && __cpp_rtti
- using _Ex2 = typename decay<_Ex>::type;
- void* __e = __cxxabiv1::__cxa_allocate_exception(sizeof(_Ex));
- (void) __cxxabiv1::__cxa_init_primary_exception(
- __e, const_cast<std::type_info*>(&typeid(_Ex)),
- __exception_ptr::__dest_thunk<_Ex2>);
- __try
- {
- ::new (__e) _Ex2(__ex);
- return exception_ptr(__e);
- }
- __catch(...)
+ // For runtime calls with -frtti enabled we can avoid try-catch overhead.
+ // We can't use this for C++98 because it relies on std::decay.
+#ifdef __glibcxx_constexpr_exceptions
+ if ! consteval
+#endif
{
- __cxxabiv1::__cxa_free_exception(__e);
- return current_exception();
+ using _Ex2 = typename decay<_Ex>::type;
+ void* __e = __cxxabiv1::__cxa_allocate_exception(sizeof(_Ex));
+ (void) __cxxabiv1::__cxa_init_primary_exception(
+ __e, const_cast<std::type_info*>(&typeid(_Ex)),
+ __exception_ptr::__dest_thunk<_Ex2>);
+ __try
+ {
+ ::new (__e) _Ex2(__ex);
+ return exception_ptr(__e);
+ }
+ __catch(...)
+ {
+ __cxxabiv1::__cxa_free_exception(__e);
+ return current_exception();
+ }
}
-#else
+#endif
+
+#ifdef __cpp_exceptions
try
{
throw __ex;
@@ -339,21 +340,14 @@ namespace std _GLIBCXX_VISIBILITY(default)
return current_exception();
}
#endif
+ return exception_ptr();
}
-#else // no RTTI and no exceptions
- // This is always_inline so the linker will never use this useless definition
- // instead of a working one compiled with RTTI and/or exceptions enabled.
- template<typename _Ex>
- __attribute__ ((__always_inline__))
- _GLIBCXX26_CONSTEXPR inline exception_ptr
- make_exception_ptr(_Ex) _GLIBCXX_USE_NOEXCEPT
- { return exception_ptr(); }
-#endif
#if __cpp_lib_exception_ptr_cast >= 202506L
template<typename _Ex>
[[__gnu__::__always_inline__]]
- constexpr const _Ex* exception_ptr_cast(const exception_ptr& __p) noexcept
+ constexpr const _Ex*
+ exception_ptr_cast(const exception_ptr& __p) noexcept
{
static_assert(!std::is_const_v<_Ex>);
static_assert(!std::is_reference_v<_Ex>);
@@ -361,28 +355,31 @@ namespace std _GLIBCXX_VISIBILITY(default)
static_assert(!std::is_array_v<_Ex>);
static_assert(!std::is_pointer_v<_Ex>);
static_assert(!std::is_member_pointer_v<_Ex>);
+
#ifdef __cpp_rtti
- if consteval {
- if (__p._M_exception_object)
- try
- {
- std::rethrow_exception(__p);
- }
- catch (const _Ex& __exc)
- {
- return &__exc;
- }
- catch (...)
- {
- }
- return nullptr;
- } else {
+ // For runtime calls with -frtti enabled we can avoid try-catch overhead.
+ if ! consteval {
const type_info &__id = typeid(const _Ex&);
return static_cast<const _Ex*>(__p._M_exception_ptr_cast(__id));
}
-#else
- return nullptr;
#endif
+
+#ifdef __cpp_exceptions
+ if (__p._M_exception_object)
+ try
+ {
+ std::rethrow_exception(__p);
+ }
+ catch (const _Ex& __exc)
+ {
+ return &__exc;
+ }
+ catch (...)
+ {
+ }
+#endif
+
+ return nullptr;
}
#endif
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