Re: pthread-spin: add error checking

[Bruno Haible]

A small improvement over the last patch. For some architectures, an atomic
operation on a 32-bit word is simpler than an atomic operation on a single
byte. You see this when looking at the "gcc -O2 -S" output of the file.


2020-07-01  Bruno Haible  <br...@clisp.org>

        pthread-spin: Optimize fallback for GCC versions >= 4.7.
        * lib/pthread-spin.c (pthread_spin_init, pthread_spin_lock,
        pthread_spin_trylock, pthread_spin_unlock): Use a lock word instead of a
        lock byte.

diff --git a/lib/pthread-spin.c b/lib/pthread-spin.c
index 73eb9f3..1f73f13 100644
--- a/lib/pthread-spin.c
+++ b/lib/pthread-spin.c
@@ -69,8 +69,8 @@ pthread_spin_destroy (pthread_spinlock_t *lock)
    require to link with -latomic.  */
 
 # if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 7)
-/* Use GCC built-ins (available in GCC >= 4.7) that operate on the first byte
-   of the lock.
+/* Use GCC built-ins (available in GCC >= 4.7) that operate on the first 32-bit
+   word of the lock.
    Documentation:
    
<https://gcc.gnu.org/onlinedocs/gcc-4.7.0/gcc/_005f_005fatomic-Builtins.html>  
*/
 
@@ -81,7 +81,7 @@ int
 pthread_spin_init (pthread_spinlock_t *lock,
                    int shared_across_processes _GL_UNUSED)
 {
-  __atomic_store_n ((char *) lock, 0, __ATOMIC_SEQ_CST);
+  __atomic_store_n ((unsigned int *) lock, 0, __ATOMIC_SEQ_CST);
   return 0;
 }
 
@@ -89,9 +89,9 @@ int
 pthread_spin_lock (pthread_spinlock_t *lock)
 {
   /* Wait until *lock becomes 0, then replace it with 1.  */
-  asyncsafe_spinlock_t zero;
+  unsigned int zero;
   while (!(zero = 0,
-           __atomic_compare_exchange_n ((char *) lock, &zero, 1, false,
+           __atomic_compare_exchange_n ((unsigned int *) lock, &zero, 1, false,
                                         __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST)))
     ;
   return 0;
@@ -100,9 +100,9 @@ pthread_spin_lock (pthread_spinlock_t *lock)
 int
 pthread_spin_trylock (pthread_spinlock_t *lock)
 {
-  asyncsafe_spinlock_t zero;
+  unsigned int zero;
   if (!(zero = 0,
-        __atomic_compare_exchange_n ((char *) lock, &zero, 1, false,
+        __atomic_compare_exchange_n ((unsigned int *) lock, &zero, 1, false,
                                      __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST)))
     return EBUSY;
   return 0;
@@ -112,8 +112,8 @@ int
 pthread_spin_unlock (pthread_spinlock_t *lock)
 {
   /* If *lock is 1, then replace it with 0.  */
-  asyncsafe_spinlock_t one = 1;
-  if (!__atomic_compare_exchange_n ((char *) lock, &one, 0, false,
+  unsigned int one = 1;
+  if (!__atomic_compare_exchange_n ((unsigned int *) lock, &one, 0, false,
                                     __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST))
     abort ();
   return 0;