libstdc++: Rewrite atomic builtin checks [PR70560]

Currently the GLIBCXX_ENABLE_ATOMIC_BUILTINS macro checks for a variety
of __atomic built-ins for bool, short and int. If all those checks pass,
then it defines _GLIBCXX_ATOMIC_BUILTINS and uses the definitions from
config/cpu/generic/atomicity_builtins/atomicity.h for the non-inline
versions of __exchange_and_add and __atomic_add that get compiled into
libsupc++.

However, the config/cpu/generic/atomicity_builtins/atomicity.h
definitions only depend on __atomic_fetch_add not on
__atomic_test_and_set or __atomic_compare_exchange. And they only
operate on a variable of type _Atomic word, which is not necessarily one
of bool, short or int (e.g. for sparcv9 _Atomic_word is 64-bit long).

This means that for a target where _Atomic_word is int but there are no
1-byte or 2-byte atomic instructions, GLIBCXX_ENABLE_ATOMIC_BUILTINS
will fail the checks for bool and short and not define the macro
_GLIBCXX_ATOMIC_BUILTINS. That means that we will use a single global
mutex for reference counting in the COW std::string and std::locale,
even though we could use __atomic_fetch_add to do it lock-free.

This commit removes most of the GLIBCXX_ENABLE_ATOMIC_BUILTINS checks,
so that it only checks __atomic_fetch_add on _Atomic_word. The macro
defined by GLIBCXX_ENABLE_ATOMIC_BUILTINS is renamed from
_GLIBCXX_ATOMIC_BUILTINS to _GLIBCXX_ATOMIC_WORD_BUILTINS to better
reflect what it really means. This will enable the inline versions of
__exchange_and_add and __atomic_add for more targets. This is not an ABI
change, because targets which didn't previously use the inline
definitions of those functions made non-inlined calls to the functions
in the library. If the definitions of those functions now start using
atomics, that doesn't change the semantics for the code calling those
functions.

On affected targets, new code compiled after this change will see the
_GLIBCXX_ATOMIC_WORD_BUILTINS macro and so will use the always-inline
versions of __exchange_and_add and __atomic_add, which use
__atomic_fetch_add directly. That is also compatible with older code
which still calls the non-inline definitions, because those non-inline
definitions now also use __atomic_fetch_add.

The only configuration where this could be an ABI change is for a target
which previously defined _GLIBCXX_ATOMIC_BUILTINS (because all the
atomic built-ins for bool, short and int are supported), but which
defines _Atomic_word to some other type for which __atomic_fetch_add is
/not/ supported. Such a target would have called the inline functions
using __atomic_fetch_add, which would actually have depended on
libatomic (which is what the configure checks were supposed to
prevent!).  After this change, that target would not define the new
macro, _GLIBCXX_ATOMIC_WORD_BUILTINS, and so would make non-inline calls
into the library where __exchange_and_add and __atomic_add would use the
global mutex. That would be an ABI break. I don't consider that a
realistic scenario, because it wouldn't have made any sense to define
_Atomic_word to a wider type than int, when doing so would have required
libatomic to make libstdc++.so work. Surely such a target would have
just used int for its _Atomic_word type.

The GLIBCXX_ENABLE_BACKTRACE macro currently uses the
glibcxx_ac_atomic_int variable defined by the checks that this commit
removes from GLIBCXX_ENABLE_ATOMIC_BUILTINS. That wasn't a good check
anyway, because libbacktrace actually depends on atomic loads+stores for
pointers as well as int, and for atomic stores for size_t. This commit
replaces the glibcxx_ac_atomic_int check with a proper test for all the
required atomic operations on all three of int, void* and size_t. This
ensures that the libbacktrace code used for std::stacktrace will either
use native atomics, or implement those loads and stores only in terms of
__sync_bool_compare_and_swap (possibly requiring that to come from
libatomic or elsewhere).

libstdc++-v3/ChangeLog:

	PR libstdc++/70560
	PR libstdc++/119667
	* acinclude.m4 (GLIBCXX_ENABLE_ATOMIC_BUILTINS): Only check for
	__atomic_fetch_add on _Atomic_word. Define new macro
	_GLIBCXX_ATOMIC_WORD_BUILTINS and stop defining macro
	_GLIBCXX_ATOMIC_BUILTINS.
	(GLIBCXX_ENABLE_BACKTRACE): Check for __atomic_load_n and
	__atomic_store_n on int, void* and size_t.
	* config.h.in: Regenerate.
	* configure: Regenerate.
	* configure.host: Fix typo in comment.
	* include/ext/atomicity.h (__exchange_and_add, __atomic_add):
	Depend on _GLIBCXX_ATOMIC_WORD_BUILTINS macro instead of old
	_GLIBCXX_ATOMIC_BUILTINS macro.

0 files changed, 0 insertions, 0 deletions