meson: Split test for __int128_t type from __int128_t arithmetic

Older versions of clang have missing runtime functions for arithmetic
with -fsanitize=undefined (see 464e367), so we cannot use
__int128_t for implementing Int128.  But __int128_t is present,
data movement works, and it can be used for atomic128.

Probe for both CONFIG_INT128_TYPE and CONFIG_INT128, adjust
qemu/int128.h to define Int128Alias if CONFIG_INT128_TYPE,
and adjust the meson probe for atomics to use has_int128_type.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

include/qemu/int128.h

@@ -481,14 +481,14 @@ static inline void bswap128s(Int128 *s)
  * a possible structure and the native types.  Ease parameter passing
  * via use of the transparent union extension.
  */
-#ifdef CONFIG_INT128
+#ifdef CONFIG_INT128_TYPE
 typedef union {
     __uint128_t u;
     __int128_t i;
     Int128 s;
 } Int128Alias __attribute__((transparent_union));
 #else
 typedef Int128 Int128Alias;
-#endif /* CONFIG_INT128 */
+#endif /* CONFIG_INT128_TYPE */
 
 #endif /* INT128_H */

meson.build

@@ -2543,7 +2543,13 @@ config_host_data.set('CONFIG_ATOMIC64', cc.links('''
     return 0;
   }'''))
 
-has_int128 = cc.links('''
+has_int128_type = cc.compiles('''
+  __int128_t a;
+  __uint128_t b;
+  int main(void) { b = a; }''')
+config_host_data.set('CONFIG_INT128_TYPE', has_int128_type)
+
+has_int128 = has_int128_type and cc.links('''
   __int128_t a;
   __uint128_t b;
   int main (void) {
@@ -2552,10 +2558,9 @@ has_int128 = cc.links('''
     a = a * a;
     return 0;
   }''')
-
 config_host_data.set('CONFIG_INT128', has_int128)
 
-if has_int128
+if has_int128_type
   # "do we have 128-bit atomics which are handled inline and specifically not
   # via libatomic". The reason we can't use libatomic is documented in the
   # comment starting "GCC is a house divided" in include/qemu/atomic128.h.
@@ -2564,7 +2569,7 @@ if has_int128
   # __alignof(unsigned __int128) for the host.
   atomic_test_128 = '''
     int main(int ac, char **av) {
-      unsigned __int128 *p = __builtin_assume_aligned(av[ac - 1], 16);
+      __uint128_t *p = __builtin_assume_aligned(av[ac - 1], 16);
       p[1] = __atomic_load_n(&p[0], __ATOMIC_RELAXED);
       __atomic_store_n(&p[2], p[3], __ATOMIC_RELAXED);
       __atomic_compare_exchange_n(&p[4], &p[5], p[6], 0, __ATOMIC_RELAXED, __ATOMIC_RELAXED);
@@ -2586,7 +2591,7 @@ if has_int128
       config_host_data.set('CONFIG_CMPXCHG128', cc.links('''
         int main(void)
         {
-          unsigned __int128 x = 0, y = 0;
+          __uint128_t x = 0, y = 0;
           __sync_val_compare_and_swap_16(&x, y, x);
           return 0;
         }