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libstdc++: Remove unconditional use of atomics in Debug Mode

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The fix for PR 91910 (r10-3426-gf7a3a382279585) introduced unconditional
uses of atomics into src/c++11/debug.cc, which causes linker errors for
arm4t where GCC emits an unresolved reference to __sync_synchronize.

By making the uses of atomics depend on _GLIBCXX_HAS_GTHREADS we can
avoid those unconditional references to __sync_synchronize for targets
where the atomics are unnecessary. As a minor performance optimization
we can also check the __gnu_cxx::__is_single_threaded function to avoid
atomics for single-threaded programs even where they don't cause linker
errors.

libstdc++-v3/ChangeLog:

	* src/c++11/debug.cc (acquire_sequence_ptr_for_lock): New
	function.
	(reset_sequence_ptr): New function.
	(_Safe_iterator_base::_M_detach)
	(_Safe_local_iterator_base::_M_detach): Replace bare atomic_load
	with acquire_sequence_ptr_for_lock.
	(_Safe_iterator_base::_M_reset): Replace bare atomic_store with
	reset_sequence_ptr.
This commit is contained in:
Jonathan Wakely 2023-09-11 16:42:54 +01:00
parent c7db9000fa
commit 4a2766ed00
1 changed files with 29 additions and 3 deletions
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32
libstdc++-v3/src/c++11/debug.cc
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@ -24,6 +24,7 @@
#include <bits/move.h>
#include <bits/stl_iterator_base_types.h>
#include <ext/atomicity.h> // __is_single_threaded
#include <debug/formatter.h>
#include <debug/safe_base.h>
@ -156,6 +157,31 @@ namespace
__old->_M_reset();
}
}
void*
acquire_sequence_ptr_for_lock(__gnu_debug::_Safe_sequence_base*& seq)
{
#ifdef __GTHREADS
if (!__gnu_cxx::__is_single_threaded())
return __atomic_load_n(&seq, __ATOMIC_ACQUIRE);
#endif
return seq;
}
void
reset_sequence_ptr(__gnu_debug::_Safe_sequence_base*& seq)
{
#ifdef __GTHREADS
if (!__gnu_cxx::__is_single_threaded())
{
__atomic_store_n(&seq, (__gnu_debug::_Safe_sequence_base*)nullptr,
__ATOMIC_RELEASE);
return;
}
#endif
seq = nullptr;
}
} // anonymous namespace
namespace __gnu_debug
@ -439,7 +465,7 @@ namespace __gnu_debug
// If the sequence destructor runs between loading the pointer and
// locking the mutex, it will detach this iterator and set _M_sequence
// to null, and then _M_detach_single() will do nothing.
if (auto seq = __atomic_load_n(&_M_sequence, __ATOMIC_ACQUIRE))
if (auto seq = acquire_sequence_ptr_for_lock(_M_sequence))
{
__gnu_cxx::__scoped_lock sentry(get_safe_base_mutex(seq));
_M_detach_single();
@ -461,7 +487,7 @@ namespace __gnu_debug
_Safe_iterator_base::
_M_reset() throw ()
{
__atomic_store_n(&_M_sequence, (_Safe_sequence_base*)0, __ATOMIC_RELEASE);
reset_sequence_ptr(_M_sequence);
// Do not reset version, so that a detached iterator does not look like a
// value-initialized one.
// _M_version = 0;
@ -523,7 +549,7 @@ namespace __gnu_debug
_Safe_local_iterator_base::
_M_detach()
{
if (auto seq = __atomic_load_n(&_M_sequence, __ATOMIC_ACQUIRE))
if (auto seq = acquire_sequence_ptr_for_lock(_M_sequence))
{
__gnu_cxx::__scoped_lock sentry(get_safe_base_mutex(seq));
_M_detach_single();