C11 alignas instead of unions

This changes a few union members that only existed to ensure
alignments and replaces them with the C11 alignas specifier.

This change only uses fundamental alignments (meaning approximately
alignments of basic types), which all C11 compilers must support.
There are opportunities for similar changes using extended alignments,
for example in PGIOAlignedBlock, but these are not necessarily
supported by all compilers, so they are kept as a separate change.

Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/46f05236-d4d4-4b4e-84d4-faa500f14691%40eisentraut.org

src/backend/access/common/toast_internals.c

@@ -287,11 +287,9 @@ toast_save_datum(Relation rel, Datum value,
 		bool		t_isnull[3] = {0};
 		union
 		{
-			struct varlena hdr;
+			alignas(int32) struct varlena hdr;
 			/* this is to make the union big enough for a chunk: */
 			char		data[TOAST_MAX_CHUNK_SIZE + VARHDRSZ];
-			/* ensure union is aligned well enough: */
-			int32		align_it;
 		}			chunk_data;
 		int32		chunk_size;
 

src/backend/commands/async.c

@@ -1987,14 +1987,10 @@ asyncQueueProcessPageEntries(QueuePosition *current,
 	/*
 	 * We copy the entries into a local buffer to avoid holding the SLRU lock
 	 * while we transmit them to our frontend.  The local buffer must be
-	 * adequately aligned, so use a union.
+	 * adequately aligned.
 	 */
-	union
-	{
-		char		buf[QUEUE_PAGESIZE];
-		AsyncQueueEntry align;
-	}			local_buf;
-	char	   *local_buf_end = local_buf.buf;
+	alignas(AsyncQueueEntry) char local_buf[QUEUE_PAGESIZE];
+	char	   *local_buf_end = local_buf;
 
 	slotno = SimpleLruReadPage_ReadOnly(NotifyCtl, curpage,
 										InvalidTransactionId);
@@ -2082,8 +2078,8 @@ asyncQueueProcessPageEntries(QueuePosition *current,
 	 * Now that we have let go of the SLRU bank lock, send the notifications
 	 * to our backend
 	 */
-	Assert(local_buf_end - local_buf.buf <= BLCKSZ);
-	for (char *p = local_buf.buf; p < local_buf_end;)
+	Assert(local_buf_end - local_buf <= BLCKSZ);
+	for (char *p = local_buf; p < local_buf_end;)
 	{
 		AsyncQueueEntry *qe = (AsyncQueueEntry *) p;
 

src/backend/storage/large_object/inv_api.c

@@ -556,11 +556,9 @@ inv_write(LargeObjectDesc *obj_desc, const char *buf, int nbytes)
 	bool		pfreeit;
 	union
 	{
-		bytea		hdr;
+		alignas(int32) bytea hdr;
 		/* this is to make the union big enough for a LO data chunk: */
 		char		data[LOBLKSIZE + VARHDRSZ];
-		/* ensure union is aligned well enough: */
-		int32		align_it;
 	}			workbuf = {0};
 	char	   *workb = VARDATA(&workbuf.hdr);
 	HeapTuple	newtup;
@@ -747,11 +745,9 @@ inv_truncate(LargeObjectDesc *obj_desc, int64 len)
 	Form_pg_largeobject olddata;
 	union
 	{
-		bytea		hdr;
+		alignas(int32) bytea hdr;
 		/* this is to make the union big enough for a LO data chunk: */
 		char		data[LOBLKSIZE + VARHDRSZ];
-		/* ensure union is aligned well enough: */
-		int32		align_it;
 	}			workbuf = {0};
 	char	   *workb = VARDATA(&workbuf.hdr);
 	HeapTuple	newtup;

src/include/c.h

@@ -1117,15 +1117,11 @@ pg_noreturn extern void ExceptionalCondition(const char *conditionName,
  * Use this, not "char buf[BLCKSZ]", to declare a field or local variable
  * holding a page buffer, if that page might be accessed as a page.  Otherwise
  * the variable might be under-aligned, causing problems on alignment-picky
- * hardware.  We include both "double" and "int64" in the union to ensure that
- * the compiler knows the value must be MAXALIGN'ed (cf. configure's
- * computation of MAXIMUM_ALIGNOF).
+ * hardware.
  */
-typedef union PGAlignedBlock
+typedef struct PGAlignedBlock
 {
-	char		data[BLCKSZ];
-	double		force_align_d;
-	int64		force_align_i64;
+	alignas(MAXIMUM_ALIGNOF) char data[BLCKSZ];
 } PGAlignedBlock;
 
 /*