qemu/atomic: Add aligned_{int64,uint64}_t types

Use it to avoid some clang-12 -Watomic-alignment errors,
forcing some structures to be aligned and as a pointer when
we have ensured that the address is aligned.

Tested-by: Cole Robinson <crobinso@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

include/qemu/atomic.h

@@ -271,7 +271,19 @@
     _oldn;                                                              \
 })
 
-/* Abstractions to access atomically (i.e. "once") i64/u64 variables */
+/*
+ * Abstractions to access atomically (i.e. "once") i64/u64 variables.
+ *
+ * The i386 abi is odd in that by default members are only aligned to
+ * 4 bytes, which means that 8-byte types can wind up mis-aligned.
+ * Clang will then warn about this, and emit a call into libatomic.
+ *
+ * Use of these types in structures when they will be used with atomic
+ * operations can avoid this.
+ */
+typedef int64_t aligned_int64_t __attribute__((aligned(8)));
+typedef uint64_t aligned_uint64_t __attribute__((aligned(8)));
+
 #ifdef CONFIG_ATOMIC64
 /* Use __nocheck because sizeof(void *) might be < sizeof(u64) */
 #define qatomic_read_i64(P) \

include/qemu/stats64.h

@@ -21,7 +21,7 @@
 
 typedef struct Stat64 {
 #ifdef CONFIG_ATOMIC64
-    uint64_t value;
+    aligned_uint64_t value;
 #else
     uint32_t low, high;
     uint32_t lock;