[sanitizer_common][AIX] Use scoped pragma to suppress atomic alignment warnings (#139272)

Have the warning suppression apply only to the code that is currently
affected. The suppression is guarded via preprocessor conditions to
cases where it is tested and known to be needed.

Issue: llvm/llvm-project#138916

Co-authored-by: Hubert Tong <[email protected]>

Author: jakeegan

compiler-rt/lib/sanitizer_common/sanitizer_atomic_clang.h

@@ -14,6 +14,18 @@
 #ifndef SANITIZER_ATOMIC_CLANG_H
 #define SANITIZER_ATOMIC_CLANG_H
 
+// Helper to suppress warnings related to 8-byte atomic accesses when the target
+// is 32-bit AIX (where such accesses use libatomic).
+#if defined(_AIX) && !defined(__powerpc64__) && defined(__clang__)
+#  define SANITIZER_IGNORE_ATOMIC_ALIGNMENT_BEGIN \
+    _Pragma("clang diagnostic push")              \
+        _Pragma("clang diagnostic ignored \"-Watomic-alignment\"")
+#  define SANITIZER_IGNORE_ATOMIC_ALIGNMENT_END _Pragma("clang diagnostic pop")
+#else
+#  define SANITIZER_IGNORE_ATOMIC_ALIGNMENT_BEGIN
+#  define SANITIZER_IGNORE_ATOMIC_ALIGNMENT_END
+#endif
+
 namespace __sanitizer {
 
 // We use the compiler builtin atomic operations for loads and stores, which
@@ -35,6 +47,7 @@ inline void proc_yield(int cnt) {
 #endif
 }
 
+SANITIZER_IGNORE_ATOMIC_ALIGNMENT_BEGIN
 template <typename T>
 inline typename T::Type atomic_load(const volatile T *a, memory_order mo) {
   DCHECK(mo == memory_order_relaxed || mo == memory_order_consume ||
@@ -92,6 +105,8 @@ inline bool atomic_compare_exchange_weak(volatile T *a, typename T::Type *cmp,
   return atomic_compare_exchange_strong(a, cmp, xchg, mo);
 }
 
+SANITIZER_IGNORE_ATOMIC_ALIGNMENT_END
+
 }  // namespace __sanitizer
 
 #undef ATOMIC_ORDER