improve clang support (23)

[Bruno Haible]

clang also has the atomic built-ins and the asm support that GCC has.


2020-08-11  Bruno Haible  <bruno@clisp.org>

        asyncsafe-spin: Use GCC built-ins also on clang.
        * lib/asyncsafe-spin.c (asyncsafe_spin_init, do_lock, do_unlock): Use
        the newer GCC built-ins also on clang.

diff --git a/lib/asyncsafe-spin.c b/lib/asyncsafe-spin.c
index 6ea5781..98d4fad 100644
--- a/lib/asyncsafe-spin.c
+++ b/lib/asyncsafe-spin.c
@@ -67,9 +67,11 @@ asyncsafe_spin_destroy (asyncsafe_spinlock_t *lock)
    We don't use the C11 <stdatomic.h> (available in GCC >= 4.9) because it 
would
    require to link with -latomic.  */
 
-#  if (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 7)) && !defined 
__ibmxl__
-/* Use GCC built-ins (available in GCC >= 4.7) that operate on the first byte 
of
-   the lock.
+#  if (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 7) \
+       || __clang_major > 3 || (__clang_major__ == 3 && __clang_minor__ >= 1)) 
\
+      && !defined __ibmxl__
+/* Use GCC built-ins (available in GCC >= 4.7 and clang >= 3.1) that operate on
+   the first byte of the lock.
    Documentation:
    
<https://gcc.gnu.org/onlinedocs/gcc-4.7.0/gcc/_005f_005fatomic-Builtins.html>
  */
@@ -129,8 +131,10 @@ do_unlock (asyncsafe_spinlock_t *lock)
 
 #   endif
 
-#  elif (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 1)) && !defined 
__ibmxl__
-/* Use GCC built-ins (available in GCC >= 4.1).
+#  elif (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 1) \
+         || __clang_major__ >= 3) \
+        && !defined __ibmxl__
+/* Use GCC built-ins (available in GCC >= 4.1 and clang >= 3.0).
    Documentation:
    <https://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Atomic-Builtins.html>  */
 
@@ -184,16 +188,17 @@ do_unlock (asyncsafe_spinlock_t *lock)
     abort ();
 }
 
-#  elif (defined __GNUC__ || defined __SUNPRO_C) && (defined __sparc || 
defined __i386 || defined __x86_64__)
-/* For older versions of GCC, use inline assembly.
-   GCC and the Oracle Studio C 12 compiler understand GCC's extended asm 
syntax,
-   but the plain Oracle Studio C 11 compiler understands only simple asm.  */
+#  elif (defined __GNUC__ || defined __clang__ || defined __SUNPRO_C) && 
(defined __sparc || defined __i386 || defined __x86_64__)
+/* For older versions of GCC or clang, use inline assembly.
+   GCC, clang, and the Oracle Studio C 12 compiler understand GCC's extended
+   asm syntax, but the plain Oracle Studio C 11 compiler understands only
+   simple asm.  */
 /* An implementation that verifies the unlocks.  */
 
 static void
 memory_barrier (void)
 {
-#   if defined __GNUC__ || __SUNPRO_C >= 0x590
+#   if defined __GNUC__ || defined __clang__ || __SUNPRO_C >= 0x590
 #    if defined __i386 || defined __x86_64__
   asm volatile ("mfence");
 #    endif
@@ -216,7 +221,7 @@ static unsigned int
 atomic_compare_and_swap (volatile unsigned int *vp, unsigned int cmp,
                          unsigned int newval)
 {
-#   if defined __GNUC__ || __SUNPRO_C >= 0x590
+#   if defined __GNUC__ || defined __clang__ || __SUNPRO_C >= 0x590
   unsigned int oldval;
 #    if defined __i386 || defined __x86_64__
   asm volatile (" lock\n cmpxchgl %3,(%1)"