Fix issues in e8fdbd58.

When the 64bit atomics simulation is in use, we can't necessarily
guarantee the correct alignment of the atomics due to lack of compiler
support for doing so- that's fine from a safety perspective, because
everything is protected by a lock, but we asserted the alignment in
all cases.  Weaken them.  Per complaint from Alvaro Herrera.

My #ifdefery for PG_HAVE_8BYTE_SINGLE_COPY_ATOMICITY wasn't
sufficient. Fix that.  Per complaint from Alexander Korotkov.

src/include/port/atomics.h

@@ -425,30 +425,41 @@ pg_atomic_sub_fetch_u32(volatile pg_atomic_uint32 *ptr, int32 sub_)
 static inline void
 pg_atomic_init_u64(volatile pg_atomic_uint64 *ptr, uint64 val)
 {
+	/*
+	 * Can't necessarily enforce alignment - and don't need it - when using
+	 * the spinlock based fallback implementation. Therefore only assert when
+	 * not using it.
+	 */
+#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
 	AssertPointerAlignment(ptr, 8);
-
+#endif
 	pg_atomic_init_u64_impl(ptr, val);
 }
 
 static inline uint64
 pg_atomic_read_u64(volatile pg_atomic_uint64 *ptr)
 {
+#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
 	AssertPointerAlignment(ptr, 8);
+#endif
 	return pg_atomic_read_u64_impl(ptr);
 }
 
 static inline void
 pg_atomic_write_u64(volatile pg_atomic_uint64 *ptr, uint64 val)
 {
+#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
 	AssertPointerAlignment(ptr, 8);
+#endif
 	pg_atomic_write_u64_impl(ptr, val);
 }
 
 static inline uint64
 pg_atomic_exchange_u64(volatile pg_atomic_uint64 *ptr, uint64 newval)
 {
+#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
 	AssertPointerAlignment(ptr, 8);
-
+#endif
 	return pg_atomic_exchange_u64_impl(ptr, newval);
 }
 
@@ -456,22 +467,28 @@ static inline bool
 pg_atomic_compare_exchange_u64(volatile pg_atomic_uint64 *ptr,
 							   uint64 *expected, uint64 newval)
 {
+#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
 	AssertPointerAlignment(ptr, 8);
 	AssertPointerAlignment(expected, 8);
+#endif
 	return pg_atomic_compare_exchange_u64_impl(ptr, expected, newval);
 }
 
 static inline uint64
 pg_atomic_fetch_add_u64(volatile pg_atomic_uint64 *ptr, int64 add_)
 {
+#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
 	AssertPointerAlignment(ptr, 8);
+#endif
 	return pg_atomic_fetch_add_u64_impl(ptr, add_);
 }
 
 static inline uint64
 pg_atomic_fetch_sub_u64(volatile pg_atomic_uint64 *ptr, int64 sub_)
 {
+#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
 	AssertPointerAlignment(ptr, 8);
+#endif
 	Assert(sub_ != PG_INT64_MIN);
 	return pg_atomic_fetch_sub_u64_impl(ptr, sub_);
 }
@@ -479,28 +496,36 @@ pg_atomic_fetch_sub_u64(volatile pg_atomic_uint64 *ptr, int64 sub_)
 static inline uint64
 pg_atomic_fetch_and_u64(volatile pg_atomic_uint64 *ptr, uint64 and_)
 {
+#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
 	AssertPointerAlignment(ptr, 8);
+#endif
 	return pg_atomic_fetch_and_u64_impl(ptr, and_);
 }
 
 static inline uint64
 pg_atomic_fetch_or_u64(volatile pg_atomic_uint64 *ptr, uint64 or_)
 {
+#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
 	AssertPointerAlignment(ptr, 8);
+#endif
 	return pg_atomic_fetch_or_u64_impl(ptr, or_);
 }
 
 static inline uint64
 pg_atomic_add_fetch_u64(volatile pg_atomic_uint64 *ptr, int64 add_)
 {
+#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
 	AssertPointerAlignment(ptr, 8);
+#endif
 	return pg_atomic_add_fetch_u64_impl(ptr, add_);
 }
 
 static inline uint64
 pg_atomic_sub_fetch_u64(volatile pg_atomic_uint64 *ptr, int64 sub_)
 {
+#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
 	AssertPointerAlignment(ptr, 8);
+#endif
 	Assert(sub_ != PG_INT64_MIN);
 	return pg_atomic_sub_fetch_u64_impl(ptr, sub_);
 }

src/include/port/atomics/generic.h

@@ -271,26 +271,26 @@ pg_atomic_exchange_u64_impl(volatile pg_atomic_uint64 *ptr, uint64 xchg_)
 }
 #endif
 
-#ifndef PG_HAVE_ATOMIC_READ_U64
-#define PG_HAVE_ATOMIC_READ_U64
-static inline uint64
-pg_atomic_read_u64_impl(volatile pg_atomic_uint64 *ptr)
-{
-	return *(&ptr->value);
-}
-#endif
-
 #ifndef PG_HAVE_ATOMIC_WRITE_U64
 #define PG_HAVE_ATOMIC_WRITE_U64
+
+#if defined(PG_HAVE_8BYTE_SINGLE_COPY_ATOMICITY) && \
+	!defined(PG_HAVE_ATOMIC_U64_SIMULATION)
+
 static inline void
 pg_atomic_write_u64_impl(volatile pg_atomic_uint64 *ptr, uint64 val)
 {
+	/*
+	 * On this platform aligned 64bit writes are guaranteed to be atomic,
+	 * except if using the fallback implementation, where can't guarantee the
+	 * required alignment.
+	 */
+	AssertPointerAlignment(ptr, 8);
 	ptr->value = val;
 }
-#endif
 
-#ifndef PG_HAVE_ATOMIC_WRITE_U64
-#define PG_HAVE_ATOMIC_WRITE_U64
+#else
+
 static inline void
 pg_atomic_write_u64_impl(volatile pg_atomic_uint64 *ptr, uint64 val)
 {
@@ -300,10 +300,30 @@ pg_atomic_write_u64_impl(volatile pg_atomic_uint64 *ptr, uint64 val)
 	 */
 	pg_atomic_exchange_u64_impl(ptr, val);
 }
-#endif
+
+#endif /* PG_HAVE_8BYTE_SINGLE_COPY_ATOMICITY && !PG_HAVE_ATOMIC_U64_SIMULATION */
+#endif /* PG_HAVE_ATOMIC_WRITE_U64 */
 
 #ifndef PG_HAVE_ATOMIC_READ_U64
 #define PG_HAVE_ATOMIC_READ_U64
+
+#if defined(PG_HAVE_8BYTE_SINGLE_COPY_ATOMICITY) && \
+	!defined(PG_HAVE_ATOMIC_U64_SIMULATION)
+
+static inline uint64
+pg_atomic_read_u64_impl(volatile pg_atomic_uint64 *ptr)
+{
+	/*
+	 * On this platform aligned 64bit reads are guaranteed to be atomic,
+	 * except if using the fallback implementation, where can't guarantee the
+	 * required alignment.
+	 */
+	AssertPointerAlignment(ptr, 8);
+	return *(&ptr->value);
+}
+
+#else
+
 static inline uint64
 pg_atomic_read_u64_impl(volatile pg_atomic_uint64 *ptr)
 {
@@ -319,7 +339,8 @@ pg_atomic_read_u64_impl(volatile pg_atomic_uint64 *ptr)
 
 	return old;
 }
-#endif
+#endif /* PG_HAVE_8BYTE_SINGLE_COPY_ATOMICITY && !PG_HAVE_ATOMIC_U64_SIMULATION */
+#endif /* PG_HAVE_ATOMIC_READ_U64 */
 
 #ifndef PG_HAVE_ATOMIC_INIT_U64
 #define PG_HAVE_ATOMIC_INIT_U64