[PATCH] atomic: Switch to builtin atomics
Adhemerval Zanella Netto
adhemerval.zanella@linaro.org
Mon Sep 8 15:43:24 GMT 2025
On 08/09/25 09:56, Wilco Dijkstra wrote:
>
> Switch to standard builtin atomics by removing the defines for !USE_ATOMIC_COMPILER_BUILTINS.
> Fix m68k to __HAVE_64B_ATOMICS set to 0.
>
> Passes buildmanyglibc, OK for commit?
Currently !USE_ATOMIC_COMPILER_BUILTINS is used for alpha, m68k, m68k-coldfire,
hppa, microblaze, powerpc, and sh.
* For alpha, microblaze, powerpc, and hs the current minimum gcc version now correctly
implements all __atomic* builtins so it should be ok to switch.
* m68k the -m68000/-m68010 generates libcalls, but afaik we never supported such
system anyway [1]. So it should be ok to make the switch and just remove all
the old atomic definitions.
* m68k-coldfire this will make sysdeps/unix/sysv/linux/m68k/coldfire/atomic-machine.h
dead-code; since libgcc __sync routines will be used instead. As for m68k,
it is ok since for cases it route to the atomic_cmpxchg_32 syscall anyway
so the extra function call should not matter (and it improves code size).
* hppa the resulting libc.so how has the __sync_* symbol as for m68k-colfire,
also making sysdeps/unix/sysv/linux/hppa/atomic-machine.h definitions unused.
And the switch should also be ok, although I am not sure about performance
implications (since HPPA uses a 'kernel atomic light weight syscalls' which
guess is something akim a vsyscall/vdso.
I think it would be safer if we disable this per ABI basis, instead of a hard
switch for all of them; and remove the unused definitions along. The m68k fix
would be better in a different patch.
[1] http://www.linux-m68k.org/
>
> ---
>
> diff --git a/include/atomic.h b/include/atomic.h
> index 44a4ce8d711f3460194b15a1bf2c1bd8f33399a7..dbfaa3f156c3673c46865c36ed915212989e7a8e 100644
> --- a/include/atomic.h
> +++ b/include/atomic.h
> @@ -198,10 +198,6 @@
> C11. Usually, a function named atomic_OP_MO(args) is equivalent to C11's
> atomic_OP_explicit(args, memory_order_MO); exceptions noted below. */
>
> -/* Each arch can request to use compiler built-ins for C11 atomics. If it
> - does, all atomics will be based on these. */
> -#if USE_ATOMIC_COMPILER_BUILTINS
> -
> /* We require 32b atomic operations; some archs also support 64b atomic
> operations. */
> void __atomic_link_error (void);
> @@ -315,167 +311,6 @@ void __atomic_link_error (void);
> ({ __atomic_check_size((mem)); \
> __atomic_fetch_xor ((mem), (operand), __ATOMIC_RELEASE); })
>
> -#else /* !USE_ATOMIC_COMPILER_BUILTINS */
> -
> -/* By default, we assume that read, write, and full barriers are equivalent
> - to acquire, release, and seq_cst barriers. Archs for which this does not
> - hold have to provide custom definitions of the fences. */
> -# ifndef atomic_thread_fence_acquire
> -# define atomic_thread_fence_acquire() atomic_read_barrier ()
> -# endif
> -# ifndef atomic_thread_fence_release
> -# define atomic_thread_fence_release() atomic_write_barrier ()
> -# endif
> -# ifndef atomic_thread_fence_seq_cst
> -# define atomic_thread_fence_seq_cst() atomic_full_barrier ()
> -# endif
> -
> -# ifndef atomic_load_relaxed
> -# define atomic_load_relaxed(mem) \
> - ({ __typeof ((__typeof (*(mem))) *(mem)) __atg100_val; \
> - __asm ("" : "=r" (__atg100_val) : "0" (*(mem))); \
> - __atg100_val; })
> -# endif
> -# ifndef atomic_load_acquire
> -# define atomic_load_acquire(mem) \
> - ({ __typeof (*(mem)) __atg101_val = atomic_load_relaxed (mem); \
> - atomic_thread_fence_acquire (); \
> - __atg101_val; })
> -# endif
> -
> -# ifndef atomic_store_relaxed
> -/* XXX Use inline asm here? */
> -# define atomic_store_relaxed(mem, val) do { *(mem) = (val); } while (0)
> -# endif
> -# ifndef atomic_store_release
> -# define atomic_store_release(mem, val) \
> - do { \
> - atomic_thread_fence_release (); \
> - atomic_store_relaxed ((mem), (val)); \
> - } while (0)
> -# endif
> -
> -/* On failure, this CAS has memory_order_relaxed semantics. */
> -/* XXX This potentially has one branch more than necessary, but archs
> - currently do not define a CAS that returns both the previous value and
> - the success flag. */
> -# ifndef atomic_compare_exchange_weak_acquire
> -# define atomic_compare_exchange_weak_acquire(mem, expected, desired) \
> - ({ typeof (*(expected)) __atg102_expected = *(expected); \
> - *(expected) = \
> - atomic_compare_and_exchange_val_acq ((mem), (desired), *(expected)); \
> - *(expected) == __atg102_expected; })
> -# endif
> -# ifndef atomic_compare_exchange_weak_relaxed
> -/* XXX Fall back to CAS with acquire MO because archs do not define a weaker
> - CAS. */
> -# define atomic_compare_exchange_weak_relaxed(mem, expected, desired) \
> - atomic_compare_exchange_weak_acquire ((mem), (expected), (desired))
> -# endif
> -# ifndef atomic_compare_exchange_weak_release
> -# define atomic_compare_exchange_weak_release(mem, expected, desired) \
> - ({ typeof (*(expected)) __atg103_expected = *(expected); \
> - *(expected) = \
> - atomic_compare_and_exchange_val_rel ((mem), (desired), *(expected)); \
> - *(expected) == __atg103_expected; })
> -# endif
> -
> -/* XXX Fall back to acquire MO because archs do not define a weaker
> - atomic_exchange. */
> -# ifndef atomic_exchange_relaxed
> -# define atomic_exchange_relaxed(mem, val) \
> - atomic_exchange_acq ((mem), (val))
> -# endif
> -# ifndef atomic_exchange_acquire
> -# define atomic_exchange_acquire(mem, val) \
> - atomic_exchange_acq ((mem), (val))
> -# endif
> -# ifndef atomic_exchange_release
> -# define atomic_exchange_release(mem, val) \
> - atomic_exchange_rel ((mem), (val))
> -# endif
> -
> -# ifndef atomic_fetch_add_acquire
> -# define atomic_fetch_add_acquire(mem, operand) \
> - atomic_exchange_and_add_acq ((mem), (operand))
> -# endif
> -# ifndef atomic_fetch_add_relaxed
> -/* XXX Fall back to acquire MO because the MO semantics of
> - atomic_exchange_and_add are not documented; the generic version falls back
> - to atomic_exchange_and_add_acq if atomic_exchange_and_add is not defined,
> - and vice versa. */
> -# define atomic_fetch_add_relaxed(mem, operand) \
> - atomic_fetch_add_acquire ((mem), (operand))
> -# endif
> -# ifndef atomic_fetch_add_release
> -# define atomic_fetch_add_release(mem, operand) \
> - atomic_exchange_and_add_rel ((mem), (operand))
> -# endif
> -# ifndef atomic_fetch_add_acq_rel
> -# define atomic_fetch_add_acq_rel(mem, operand) \
> - ({ atomic_thread_fence_release (); \
> - atomic_exchange_and_add_acq ((mem), (operand)); })
> -# endif
> -
> -/* XXX Fall back to acquire MO because archs do not define a weaker
> - atomic_and_val. */
> -# ifndef atomic_fetch_and_relaxed
> -# define atomic_fetch_and_relaxed(mem, operand) \
> - atomic_fetch_and_acquire ((mem), (operand))
> -# endif
> -/* XXX The default for atomic_and_val has acquire semantics, but this is not
> - documented. */
> -# ifndef atomic_fetch_and_acquire
> -# define atomic_fetch_and_acquire(mem, operand) \
> - atomic_and_val ((mem), (operand))
> -# endif
> -# ifndef atomic_fetch_and_release
> -/* XXX This unnecessarily has acquire MO. */
> -# define atomic_fetch_and_release(mem, operand) \
> - ({ atomic_thread_fence_release (); \
> - atomic_and_val ((mem), (operand)); })
> -# endif
> -
> -/* XXX The default for atomic_or_val has acquire semantics, but this is not
> - documented. */
> -# ifndef atomic_fetch_or_acquire
> -# define atomic_fetch_or_acquire(mem, operand) \
> - atomic_or_val ((mem), (operand))
> -# endif
> -/* XXX Fall back to acquire MO because archs do not define a weaker
> - atomic_or_val. */
> -# ifndef atomic_fetch_or_relaxed
> -# define atomic_fetch_or_relaxed(mem, operand) \
> - atomic_fetch_or_acquire ((mem), (operand))
> -# endif
> -/* XXX Contains an unnecessary acquire MO because archs do not define a weaker
> - atomic_or_val. */
> -# ifndef atomic_fetch_or_release
> -# define atomic_fetch_or_release(mem, operand) \
> - ({ atomic_thread_fence_release (); \
> - atomic_fetch_or_acquire ((mem), (operand)); })
> -# endif
> -
> -# ifndef atomic_fetch_xor_release
> -/* Failing the atomic_compare_exchange_weak_release reloads the value in
> - __atg104_expected, so we need only do the XOR again and retry. */
> -# define atomic_fetch_xor_release(mem, operand) \
> - ({ __typeof (mem) __atg104_memp = (mem); \
> - __typeof (*(mem)) __atg104_expected = (*__atg104_memp); \
> - __typeof (*(mem)) __atg104_desired; \
> - __typeof (*(mem)) __atg104_op = (operand); \
> - \
> - do \
> - __atg104_desired = __atg104_expected ^ __atg104_op; \
> - while (__glibc_unlikely \
> - (atomic_compare_exchange_weak_release ( \
> - __atg104_memp, &__atg104_expected, __atg104_desired) \
> - == 0)); \
> - __atg104_expected; })
> -#endif
> -
> -#endif /* !USE_ATOMIC_COMPILER_BUILTINS */
> -
> /* This operation does not affect synchronization semantics but can be used
> in the body of a spin loop to potentially improve its efficiency. */
> #ifndef atomic_spin_nop
> diff --git a/sysdeps/m68k/m680x0/m68020/atomic-machine.h b/sysdeps/m68k/m680x0/m68020/atomic-machine.h
> index 9d25fbc194182c9f2dc08fad33031a4de5b2c6bf..d586d06d89b85162aaaeb9ec2c5eadde433d6e2c 100644
> --- a/sysdeps/m68k/m680x0/m68020/atomic-machine.h
> +++ b/sysdeps/m68k/m680x0/m68020/atomic-machine.h
> @@ -15,7 +15,7 @@
> License along with the GNU C Library. If not, see
> <https://www.gnu.org/licenses/>. */
>
> -#define __HAVE_64B_ATOMICS 1
> +#define __HAVE_64B_ATOMICS 0
> #define USE_ATOMIC_COMPILER_BUILTINS 0
>
> /* XXX Is this actually correct? */
>
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