host/include/generic/host/atomic128: Fix compilation problem with Clang 17

When compiling QEMU with Clang 17 on a s390x, the compilation fails:

In file included from ../accel/tcg/cputlb.c:32:
In file included from /root/qemu/include/exec/helper-proto-common.h:10:
In file included from /root/qemu/include/qemu/atomic128.h:62:
/root/qemu/host/include/generic/host/atomic128-ldst.h:68:15: error:
 __sync builtin operation MUST have natural alignment (consider using __
atomic). [-Werror,-Wsync-alignment]
   68 |     } while (!__sync_bool_compare_and_swap_16(ptr_align, old, new.i));
      |               ^
In file included from ../accel/tcg/cputlb.c:32:
In file included from /root/qemu/include/exec/helper-proto-common.h:10:
In file included from /root/qemu/include/qemu/atomic128.h:61:
/root/qemu/host/include/generic/host/atomic128-cas.h:36:11: error:
 __sync builtin operation MUST have natural alignment (consider using __a
tomic). [-Werror,-Wsync-alignment]
   36 |     r.i = __sync_val_compare_and_swap_16(ptr_align, c.i, n.i);
      |           ^
2 errors generated.

It's arguably a bug in Clang since we already use __builtin_assume_aligned()
to tell the compiler that the pointer is properly aligned. But according to
https://github.com/llvm/llvm-project/issues/69146 it seems like the Clang
folks don't see an easy fix on their side and recommend to use a type
declared with __attribute__((aligned(16))) to work around this problem.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1934
Message-ID: <20231108085954.313071-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>

host/include/generic/host/atomic128-cas.h

@@ -28,7 +28,7 @@ atomic16_cmpxchg(Int128 *ptr, Int128 cmp, Int128 new)
 static inline Int128 ATTRIBUTE_ATOMIC128_OPT
 atomic16_cmpxchg(Int128 *ptr, Int128 cmp, Int128 new)
 {
-    __int128_t *ptr_align = __builtin_assume_aligned(ptr, 16);
+    Int128Aligned *ptr_align = __builtin_assume_aligned(ptr, 16);
     Int128Alias r, c, n;
 
     c.s = cmp;

host/include/generic/host/atomic128-ldst.h

@@ -58,7 +58,7 @@ atomic16_read_rw(Int128 *ptr)
 static inline void ATTRIBUTE_ATOMIC128_OPT
 atomic16_set(Int128 *ptr, Int128 val)
 {
-    __int128_t *ptr_align = __builtin_assume_aligned(ptr, 16);
+    Int128Aligned *ptr_align = __builtin_assume_aligned(ptr, 16);
     __int128_t old;
     Int128Alias new;
 

include/qemu/int128.h

@@ -10,6 +10,7 @@
  */
 #if defined(CONFIG_INT128) && !defined(CONFIG_TCG_INTERPRETER)
 typedef __int128_t Int128;
+typedef __int128_t __attribute__((aligned(16))) Int128Aligned;
 
 static inline Int128 int128_make64(uint64_t a)
 {
@@ -224,6 +225,7 @@ static inline Int128 int128_rems(Int128 a, Int128 b)
 #else /* !CONFIG_INT128 */
 
 typedef struct Int128 Int128;
+typedef struct Int128 __attribute__((aligned(16))) Int128Aligned;
 
 /*
  * We guarantee that the in-memory byte representation of an