Avoid using potentially-under-aligned page buffers.

There's a project policy against using plain "char buf[BLCKSZ]" local
or static variables as page buffers; preferred style is to palloc or
malloc each buffer to ensure it is MAXALIGN'd.  However, that policy's
been ignored in an increasing number of places.  We've apparently got
away with it so far, probably because (a) relatively few people use
platforms on which misalignment causes core dumps and/or (b) the
variables chance to be sufficiently aligned anyway.  But this is not
something to rely on.  Moreover, even if we don't get a core dump,
we might be paying a lot of cycles for misaligned accesses.

To fix, invent new union types PGAlignedBlock and PGAlignedXLogBlock
that the compiler must allocate with sufficient alignment, and use
those in place of plain char arrays.

I used these types even for variables where there's no risk of a
misaligned access, since ensuring proper alignment should make
kernel data transfers faster.  I also changed some places where
we had been palloc'ing short-lived buffers, for coding style
uniformity and to save palloc/pfree overhead.

Since this seems to be a live portability hazard (despite the lack
of field reports), back-patch to all supported versions.

Patch by me; thanks to Michael Paquier for review.

Discussion: https://postgr.es/m/1535618100.1286.3.camel@credativ.de

contrib/pg_prewarm/pg_prewarm.c

@@ -37,7 +37,7 @@ typedef enum
 	PREWARM_BUFFER
 } PrewarmType;
 
-static char blockbuffer[BLCKSZ];
+static PGAlignedBlock blockbuffer;
 
 /*
  * pg_prewarm(regclass, mode text, fork text,
@@ -179,7 +179,7 @@ pg_prewarm(PG_FUNCTION_ARGS)
 		for (block = first_block; block <= last_block; ++block)
 		{
 			CHECK_FOR_INTERRUPTS();
-			smgrread(rel->rd_smgr, forkNumber, block, blockbuffer);
+			smgrread(rel->rd_smgr, forkNumber, block, blockbuffer.data);
 			++blocks_done;
 		}
 	}

src/backend/access/gin/ginentrypage.c

@@ -633,7 +633,7 @@ entrySplitPage(GinBtree btree, Buffer origbuf,
 
 	/* these must be static so they can be returned to caller */
 	static ginxlogSplitEntry data;
-	static char tupstore[2 * BLCKSZ];
+	static PGAlignedBlock tupstore[2];
 
 	entryPreparePage(btree, lpage, off, insertData, updateblkno);
 
@@ -642,7 +642,7 @@ entrySplitPage(GinBtree btree, Buffer origbuf,
 	 * one after another in a temporary workspace.
 	 */
 	maxoff = PageGetMaxOffsetNumber(lpage);
-	ptr = tupstore;
+	ptr = tupstore[0].data;
 	for (i = FirstOffsetNumber; i <= maxoff; i++)
 	{
 		if (i == off)
@@ -667,7 +667,7 @@ entrySplitPage(GinBtree btree, Buffer origbuf,
 		ptr += size;
 		totalsize += size + sizeof(ItemIdData);
 	}
-	tupstoresize = ptr - tupstore;
+	tupstoresize = ptr - tupstore[0].data;
 
 	/*
 	 * Initialize the left and right pages, and copy all the tuples back to
@@ -676,7 +676,7 @@ entrySplitPage(GinBtree btree, Buffer origbuf,
 	GinInitPage(rpage, GinPageGetOpaque(lpage)->flags, pageSize);
 	GinInitPage(lpage, GinPageGetOpaque(rpage)->flags, pageSize);
 
-	ptr = tupstore;
+	ptr = tupstore[0].data;
 	maxoff++;
 	lsize = 0;
 
@@ -715,7 +715,7 @@ entrySplitPage(GinBtree btree, Buffer origbuf,
 	rdata[0].next = &rdata[1];
 
 	rdata[1].buffer = InvalidBuffer;
-	rdata[1].data = tupstore;
+	rdata[1].data = tupstore[0].data;
 	rdata[1].len = tupstoresize;
 	rdata[1].next = NULL;
 

src/backend/access/gin/ginfast.c

@@ -52,18 +52,15 @@ writeListPage(Relation index, Buffer buffer,
 				size = 0;
 	OffsetNumber l,
 				off;
-	char	   *workspace;
+	PGAlignedBlock workspace;
 	char	   *ptr;
 
-	/* workspace could be a local array; we use palloc for alignment */
-	workspace = palloc(BLCKSZ);
-
 	START_CRIT_SECTION();
 
 	GinInitBuffer(buffer, GIN_LIST);
 
 	off = FirstOffsetNumber;
-	ptr = workspace;
+	ptr = workspace.data;
 
 	for (i = 0; i < ntuples; i++)
 	{
@@ -120,7 +117,7 @@ writeListPage(Relation index, Buffer buffer,
 		rdata[0].next = rdata + 1;
 
 		rdata[1].buffer = InvalidBuffer;
-		rdata[1].data = workspace;
+		rdata[1].data = workspace.data;
 		rdata[1].len = size;
 		rdata[1].next = NULL;
 
@@ -135,8 +132,6 @@ writeListPage(Relation index, Buffer buffer,
 
 	END_CRIT_SECTION();
 
-	pfree(workspace);
-
 	return freesize;
 }
 

src/backend/access/hash/hashpage.c

@@ -710,7 +710,7 @@ static bool
 _hash_alloc_buckets(Relation rel, BlockNumber firstblock, uint32 nblocks)
 {
 	BlockNumber lastblock;
-	char		zerobuf[BLCKSZ];
+	PGAlignedBlock zerobuf;
 
 	lastblock = firstblock + nblocks - 1;
 
@@ -721,10 +721,10 @@ _hash_alloc_buckets(Relation rel, BlockNumber firstblock, uint32 nblocks)
 	if (lastblock < firstblock || lastblock == InvalidBlockNumber)
 		return false;
 
-	MemSet(zerobuf, 0, sizeof(zerobuf));
+	MemSet(zerobuf.data, 0, sizeof(zerobuf));
 
 	RelationOpenSmgr(rel);
-	smgrextend(rel->rd_smgr, MAIN_FORKNUM, lastblock, zerobuf, false);
+	smgrextend(rel->rd_smgr, MAIN_FORKNUM, lastblock, zerobuf.data, false);
 
 	return true;
 }

src/backend/access/heap/heapam.c

@@ -2318,7 +2318,7 @@ heap_multi_insert(Relation relation, HeapTuple *tuples, int ntuples,
 	HeapTuple  *heaptuples;
 	int			i;
 	int			ndone;
-	char	   *scratch = NULL;
+	PGAlignedBlock scratch;
 	Page		page;
 	bool		needwal;
 	Size		saveFreeSpace;
@@ -2335,14 +2335,6 @@ heap_multi_insert(Relation relation, HeapTuple *tuples, int ntuples,
 		heaptuples[i] = heap_prepare_insert(relation, tuples[i],
 											xid, cid, options);
 
-	/*
-	 * Allocate some memory to use for constructing the WAL record. Using
-	 * palloc() within a critical section is not safe, so we allocate this
-	 * beforehand.
-	 */
-	if (needwal)
-		scratch = palloc(BLCKSZ);
-
 	/*
 	 * We're about to do the actual inserts -- but check for conflict first,
 	 * to minimize the possibility of having to roll back work we've just
@@ -2427,7 +2419,7 @@ heap_multi_insert(Relation relation, HeapTuple *tuples, int ntuples,
 			uint8		info = XLOG_HEAP2_MULTI_INSERT;
 			char	   *tupledata;
 			int			totaldatalen;
-			char	   *scratchptr = scratch;
+			char	   *scratchptr = scratch.data;
 			bool		init;
 
 			/*
@@ -2494,10 +2486,10 @@ heap_multi_insert(Relation relation, HeapTuple *tuples, int ntuples,
 					log_heap_new_cid(relation, heaptup);
 			}
 			totaldatalen = scratchptr - tupledata;
-			Assert((scratchptr - scratch) < BLCKSZ);
+			Assert((scratchptr - scratch.data) < BLCKSZ);
 
 			rdata[0].data = (char *) xlrec;
-			rdata[0].len = tupledata - scratch;
+			rdata[0].len = tupledata - scratch.data;
 			rdata[0].buffer = InvalidBuffer;
 			rdata[0].next = &rdata[1];
 

src/backend/access/heap/visibilitymap.c

@@ -610,10 +610,9 @@ static void
 vm_extend(Relation rel, BlockNumber vm_nblocks)
 {
 	BlockNumber vm_nblocks_now;
-	Page		pg;
+	PGAlignedBlock pg;
 
-	pg = (Page) palloc(BLCKSZ);
+	PageInit((Page) pg.data, BLCKSZ, 0);
-	PageInit(pg, BLCKSZ, 0);
 
 	/*
 	 * We use the relation extension lock to lock out other backends trying to
@@ -644,10 +643,10 @@ vm_extend(Relation rel, BlockNumber vm_nblocks)
 	/* Now extend the file */
 	while (vm_nblocks_now < vm_nblocks)
 	{
-		PageSetChecksumInplace(pg, vm_nblocks_now);
+		PageSetChecksumInplace((Page) pg.data, vm_nblocks_now);
 
 		smgrextend(rel->rd_smgr, VISIBILITYMAP_FORKNUM, vm_nblocks_now,
-				   (char *) pg, false);
+				   pg.data, false);
 		vm_nblocks_now++;
 	}
 
@@ -664,6 +663,4 @@ vm_extend(Relation rel, BlockNumber vm_nblocks)
 	rel->rd_smgr->smgr_vm_nblocks = vm_nblocks_now;
 
 	UnlockRelationForExtension(rel, ExclusiveLock);
-
-	pfree(pg);
 }

src/backend/access/transam/xlog.c

@@ -3173,8 +3173,7 @@ XLogFileInit(XLogSegNo logsegno, bool *use_existent, bool use_lock)
 {
 	char		path[MAXPGPATH];
 	char		tmppath[MAXPGPATH];
-	char		zbuffer_raw[XLOG_BLCKSZ + MAXIMUM_ALIGNOF];
+	PGAlignedXLogBlock zbuffer;
-	char	   *zbuffer;
 	XLogSegNo	installed_segno;
 	int			max_advance;
 	int			fd;
@@ -3228,16 +3227,12 @@ XLogFileInit(XLogSegNo logsegno, bool *use_existent, bool use_lock)
 	 * fsync below) that all the indirect blocks are down on disk.  Therefore,
 	 * fdatasync(2) or O_DSYNC will be sufficient to sync future writes to the
 	 * log file.
-	 *
-	 * Note: ensure the buffer is reasonably well-aligned; this may save a few
-	 * cycles transferring data to the kernel.
 	 */
-	zbuffer = (char *) MAXALIGN(zbuffer_raw);
+	memset(zbuffer.data, 0, XLOG_BLCKSZ);
-	memset(zbuffer, 0, XLOG_BLCKSZ);
 	for (nbytes = 0; nbytes < XLogSegSize; nbytes += XLOG_BLCKSZ)
 	{
 		errno = 0;
-		if ((int) write(fd, zbuffer, XLOG_BLCKSZ) != (int) XLOG_BLCKSZ)
+		if ((int) write(fd, zbuffer.data, XLOG_BLCKSZ) != (int) XLOG_BLCKSZ)
 		{
 			int			save_errno = errno;
 
@@ -3328,7 +3323,7 @@ XLogFileCopy(XLogSegNo destsegno, TimeLineID srcTLI, XLogSegNo srcsegno)
 {
 	char		path[MAXPGPATH];
 	char		tmppath[MAXPGPATH];
-	char		buffer[XLOG_BLCKSZ];
+	PGAlignedXLogBlock buffer;
 	int			srcfd;
 	int			fd;
 	int			nbytes;
@@ -3364,7 +3359,7 @@ XLogFileCopy(XLogSegNo destsegno, TimeLineID srcTLI, XLogSegNo srcsegno)
 	for (nbytes = 0; nbytes < XLogSegSize; nbytes += sizeof(buffer))
 	{
 		errno = 0;
-		if ((int) read(srcfd, buffer, sizeof(buffer)) != (int) sizeof(buffer))
+		if ((int) read(srcfd, buffer.data, sizeof(buffer)) != (int) sizeof(buffer))
 		{
 			if (errno != 0)
 				ereport(ERROR,
@@ -3375,7 +3370,7 @@ XLogFileCopy(XLogSegNo destsegno, TimeLineID srcTLI, XLogSegNo srcsegno)
 						(errmsg("not enough data in file \"%s\"", path)));
 		}
 		errno = 0;
-		if ((int) write(fd, buffer, sizeof(buffer)) != (int) sizeof(buffer))
+		if ((int) write(fd, buffer.data, sizeof(buffer)) != (int) sizeof(buffer))
 		{
 			int			save_errno = errno;
 
@@ -9200,7 +9195,7 @@ XLogSaveBufferForHint(Buffer buffer, bool buffer_std)
 	 */
 	if (XLogCheckBuffer(rdata, false, &lsn, &bkpb))
 	{
-		char		copied_buffer[BLCKSZ];
+		PGAlignedBlock copied_buffer;
 		char	   *origdata = (char *) BufferGetBlock(buffer);
 
 		/*
@@ -9212,8 +9207,8 @@ XLogSaveBufferForHint(Buffer buffer, bool buffer_std)
 		 * and hole_offset to 0; so the following code is safe for either
 		 * case.
 		 */
-		memcpy(copied_buffer, origdata, bkpb.hole_offset);
+		memcpy(copied_buffer.data, origdata, bkpb.hole_offset);
-		memcpy(copied_buffer + bkpb.hole_offset,
+		memcpy(copied_buffer.data + bkpb.hole_offset,
 			   origdata + bkpb.hole_offset + bkpb.hole_length,
 			   BLCKSZ - bkpb.hole_offset - bkpb.hole_length);
 
@@ -9228,7 +9223,7 @@ XLogSaveBufferForHint(Buffer buffer, bool buffer_std)
 		/*
 		 * Save copy of the buffer.
 		 */
-		rdata[1].data = copied_buffer;
+		rdata[1].data = copied_buffer.data;
 		rdata[1].len = BLCKSZ - bkpb.hole_length;
 		rdata[1].buffer = InvalidBuffer;
 		rdata[1].next = NULL;

src/backend/commands/tablecmds.c

@@ -9486,21 +9486,14 @@ static void
 copy_relation_data(SMgrRelation src, SMgrRelation dst,
 				   ForkNumber forkNum, char relpersistence)
 {
-	char	   *buf;
+	PGAlignedBlock buf;
 	Page		page;
 	bool		use_wal;
 	bool		copying_initfork;
 	BlockNumber nblocks;
 	BlockNumber blkno;
 
-	/*
+	page = (Page) buf.data;
-	 * palloc the buffer so that it's MAXALIGN'd.  If it were just a local
-	 * char[] array, the compiler might align it on any byte boundary, which
-	 * can seriously hurt transfer speed to and from the kernel; not to
-	 * mention possibly making log_newpage's accesses to the page header fail.
-	 */
-	buf = (char *) palloc(BLCKSZ);
-	page = (Page) buf;
 
 	/*
 	 * The init fork for an unlogged relation in many respects has to be
@@ -9524,7 +9517,7 @@ copy_relation_data(SMgrRelation src, SMgrRelation dst,
 		/* If we got a cancel signal during the copy of the data, quit */
 		CHECK_FOR_INTERRUPTS();
 
-		smgrread(src, forkNum, blkno, buf);
+		smgrread(src, forkNum, blkno, buf.data);
 
 		if (!PageIsVerified(page, blkno))
 			ereport(ERROR,
@@ -9550,11 +9543,9 @@ copy_relation_data(SMgrRelation src, SMgrRelation dst,
 		 * rel, because there's no need for smgr to schedule an fsync for this
 		 * write; we'll do it ourselves below.
 		 */
-		smgrextend(dst, forkNum, blkno, buf, true);
+		smgrextend(dst, forkNum, blkno, buf.data, true);
 	}
 
-	pfree(buf);
-
 	/*
 	 * If the rel is WAL-logged, must fsync before commit.  We use heap_sync
 	 * to ensure that the toast table gets fsync'd too.  (For a temp or

src/backend/replication/walsender.c

@@ -494,16 +494,16 @@ SendTimeLineHistory(TimeLineHistoryCmd *cmd)
 	bytesleft = histfilelen;
 	while (bytesleft > 0)
 	{
-		char		rbuf[BLCKSZ];
+		PGAlignedBlock rbuf;
 		int			nread;
 
-		nread = read(fd, rbuf, sizeof(rbuf));
+		nread = read(fd, rbuf.data, sizeof(rbuf));
 		if (nread <= 0)
 			ereport(ERROR,
 					(errcode_for_file_access(),
 					 errmsg("could not read file \"%s\": %m",
 							path)));
-		pq_sendbytes(&buf, rbuf, nread);
+		pq_sendbytes(&buf, rbuf.data, nread);
 		bytesleft -= nread;
 	}
 	CloseTransientFile(fd);

src/backend/storage/file/buffile.c

@@ -86,7 +86,7 @@ struct BufFile
 	off_t		curOffset;		/* offset part of current pos */
 	int			pos;			/* next read/write position in buffer */
 	int			nbytes;			/* total # of valid bytes in buffer */
-	char		buffer[BLCKSZ];
+	PGAlignedBlock buffer;
 };
 
 static BufFile *makeBufFile(File firstfile);
@@ -254,7 +254,7 @@ BufFileLoadBuffer(BufFile *file)
 	/*
 	 * Read whatever we can get, up to a full bufferload.
 	 */
-	file->nbytes = FileRead(thisfile, file->buffer, sizeof(file->buffer));
+	file->nbytes = FileRead(thisfile, file->buffer.data, sizeof(file->buffer));
 	if (file->nbytes < 0)
 		file->nbytes = 0;
 	file->offsets[file->curFile] += file->nbytes;
@@ -317,7 +317,7 @@ BufFileDumpBuffer(BufFile *file)
 				return;			/* seek failed, give up */
 			file->offsets[file->curFile] = file->curOffset;
 		}
-		bytestowrite = FileWrite(thisfile, file->buffer + wpos, bytestowrite);
+		bytestowrite = FileWrite(thisfile, file->buffer.data + wpos, bytestowrite);
 		if (bytestowrite <= 0)
 			return;				/* failed to write */
 		file->offsets[file->curFile] += bytestowrite;
@@ -385,7 +385,7 @@ BufFileRead(BufFile *file, void *ptr, size_t size)
 			nthistime = size;
 		Assert(nthistime > 0);
 
-		memcpy(ptr, file->buffer + file->pos, nthistime);
+		memcpy(ptr, file->buffer.data + file->pos, nthistime);
 
 		file->pos += nthistime;
 		ptr = (void *) ((char *) ptr + nthistime);
@@ -432,7 +432,7 @@ BufFileWrite(BufFile *file, void *ptr, size_t size)
 			nthistime = size;
 		Assert(nthistime > 0);
 
-		memcpy(file->buffer + file->pos, ptr, nthistime);
+		memcpy(file->buffer.data + file->pos, ptr, nthistime);
 
 		file->dirty = true;
 		file->pos += nthistime;

src/backend/storage/freespace/freespace.c

@@ -588,10 +588,9 @@ static void
 fsm_extend(Relation rel, BlockNumber fsm_nblocks)
 {
 	BlockNumber fsm_nblocks_now;
-	Page		pg;
+	PGAlignedBlock pg;
 
-	pg = (Page) palloc(BLCKSZ);
+	PageInit((Page) pg.data, BLCKSZ, 0);
-	PageInit(pg, BLCKSZ, 0);
 
 	/*
 	 * We use the relation extension lock to lock out other backends trying to
@@ -621,10 +620,10 @@ fsm_extend(Relation rel, BlockNumber fsm_nblocks)
 
 	while (fsm_nblocks_now < fsm_nblocks)
 	{
-		PageSetChecksumInplace(pg, fsm_nblocks_now);
+		PageSetChecksumInplace((Page) pg.data, fsm_nblocks_now);
 
 		smgrextend(rel->rd_smgr, FSM_FORKNUM, fsm_nblocks_now,
-				   (char *) pg, false);
+				   pg.data, false);
 		fsm_nblocks_now++;
 	}
 
@@ -632,8 +631,6 @@ fsm_extend(Relation rel, BlockNumber fsm_nblocks)
 	rel->rd_smgr->smgr_fsm_nblocks = fsm_nblocks_now;
 
 	UnlockRelationForExtension(rel, ExclusiveLock);
-
-	pfree(pg);
 }
 
 /*

src/bin/pg_basebackup/receivelog.c

@@ -88,7 +88,7 @@ open_walfile(XLogRecPtr startpoint, uint32 timeline, char *basedir,
 	int			f;
 	char		fn[MAXPGPATH];
 	struct stat statbuf;
-	char	   *zerobuf;
+	PGAlignedXLogBlock zerobuf;
 	int			bytes;
 	XLogSegNo	segno;
 
@@ -134,11 +134,11 @@ open_walfile(XLogRecPtr startpoint, uint32 timeline, char *basedir,
 	}
 
 	/* New, empty, file. So pad it to 16Mb with zeroes */
-	zerobuf = pg_malloc0(XLOG_BLCKSZ);
+	memset(zerobuf.data, 0, XLOG_BLCKSZ);
 	for (bytes = 0; bytes < XLogSegSize; bytes += XLOG_BLCKSZ)
 	{
 		errno = 0;
-		if (write(f, zerobuf, XLOG_BLCKSZ) != XLOG_BLCKSZ)
+		if (write(f, zerobuf.data, XLOG_BLCKSZ) != XLOG_BLCKSZ)
 		{
 			/* if write didn't set errno, assume problem is no disk space */
 			if (errno == 0)
@@ -146,13 +146,11 @@ open_walfile(XLogRecPtr startpoint, uint32 timeline, char *basedir,
 			fprintf(stderr,
 					_("%s: could not pad transaction log file \"%s\": %s\n"),
 					progname, fn, strerror(errno));
-			free(zerobuf);
 			close(f);
 			unlink(fn);
 			return false;
 		}
 	}
-	free(zerobuf);
 
 	if (lseek(f, SEEK_SET, 0) != 0)
 	{

src/bin/pg_resetxlog/pg_resetxlog.c

@@ -1060,7 +1060,7 @@ KillExistingArchiveStatus(void)
 static void
 WriteEmptyXLOG(void)
 {
-	char	   *buffer;
+	PGAlignedXLogBlock buffer;
 	XLogPageHeader page;
 	XLogLongPageHeader longpage;
 	XLogRecord *record;
@@ -1069,12 +1069,10 @@ WriteEmptyXLOG(void)
 	int			fd;
 	int			nbytes;
 
-	/* Use malloc() to ensure buffer is MAXALIGNED */
+	memset(buffer.data, 0, XLOG_BLCKSZ);
-	buffer = (char *) pg_malloc(XLOG_BLCKSZ);
-	page = (XLogPageHeader) buffer;
-	memset(buffer, 0, XLOG_BLCKSZ);
 
 	/* Set up the XLOG page header */
+	page = (XLogPageHeader) buffer.data;
 	page->xlp_magic = XLOG_PAGE_MAGIC;
 	page->xlp_info = XLP_LONG_HEADER;
 	page->xlp_tli = ControlFile.checkPointCopy.ThisTimeLineID;
@@ -1116,7 +1114,7 @@ WriteEmptyXLOG(void)
 	}
 
 	errno = 0;
-	if (write(fd, buffer, XLOG_BLCKSZ) != XLOG_BLCKSZ)
+	if (write(fd, buffer.data, XLOG_BLCKSZ) != XLOG_BLCKSZ)
 	{
 		/* if write didn't set errno, assume problem is no disk space */
 		if (errno == 0)
@@ -1127,11 +1125,11 @@ WriteEmptyXLOG(void)
 	}
 
 	/* Fill the rest of the file with zeroes */
-	memset(buffer, 0, XLOG_BLCKSZ);
+	memset(buffer.data, 0, XLOG_BLCKSZ);
 	for (nbytes = XLOG_BLCKSZ; nbytes < XLogSegSize; nbytes += XLOG_BLCKSZ)
 	{
 		errno = 0;
-		if (write(fd, buffer, XLOG_BLCKSZ) != XLOG_BLCKSZ)
+		if (write(fd, buffer.data, XLOG_BLCKSZ) != XLOG_BLCKSZ)
 		{
 			if (errno == 0)
 				errno = ENOSPC;

src/include/c.h

@@ -867,6 +867,32 @@ typedef NameData *Name;
  * ----------------------------------------------------------------
  */
 
+/*
+ * 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 and not
+ * just a string of bytes.  Otherwise the variable might be under-aligned,
+ * causing problems on alignment-picky hardware.  (In some places, we use
+ * this to declare buffers even though we only pass them to read() and
+ * write(), because copying to/from aligned buffers is usually faster than
+ * using unaligned buffers.)  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).
+ */
+typedef union PGAlignedBlock
+{
+	char		data[BLCKSZ];
+	double		force_align_d;
+	int64		force_align_i64;
+} PGAlignedBlock;
+
+/* Same, but for an XLOG_BLCKSZ-sized buffer */
+typedef union PGAlignedXLogBlock
+{
+	char		data[XLOG_BLCKSZ];
+	double		force_align_d;
+	int64		force_align_i64;
+} PGAlignedXLogBlock;
+
 /* msb for char */
 #define HIGHBIT					(0x80)
 #define IS_HIGHBIT_SET(ch)		((unsigned char)(ch) & HIGHBIT)