Slightly reorganize struct SnapshotData.

This squeezes out a bunch of alignment padding, reducing the size
from 72 to 56 bytes on my machine.  At least in my testing, this
didn't produce any measurable performance improvement, but the space
savings seem like enough justification.

Andres Freund

src/include/utils/snapshot.h

@@ -46,13 +46,14 @@ typedef struct SnapshotData
 	 */
 	TransactionId xmin;			/* all XID < xmin are visible to me */
 	TransactionId xmax;			/* all XID >= xmax are invisible to me */
-	uint32		xcnt;			/* # of xact ids in xip[] */
 	TransactionId *xip;			/* array of xact IDs in progress */
+	uint32		xcnt;			/* # of xact ids in xip[] */
 	/* note: all ids in xip[] satisfy xmin <= xip[i] < xmax */
 	int32		subxcnt;		/* # of xact ids in subxip[] */
 	TransactionId *subxip;		/* array of subxact IDs in progress */
 	bool		suboverflowed;	/* has the subxip array overflowed? */
 	bool		takenDuringRecovery;	/* recovery-shaped snapshot? */
+	bool		copied;			/* false if it's a static snapshot */
 
 	/*
 	 * note: all ids in subxip[] are >= xmin, but we don't bother filtering
@@ -61,7 +62,6 @@ typedef struct SnapshotData
 	CommandId	curcid;			/* in my xact, CID < curcid are visible */
 	uint32		active_count;	/* refcount on ActiveSnapshot stack */
 	uint32		regd_count;		/* refcount on RegisteredSnapshotList */
-	bool		copied;			/* false if it's a static snapshot */
 } SnapshotData;
 
 /*