WT-3380 Make 8-byte timestamps a special case (#3509)

* Change wt_timestamp_t to union of uint64 and uint8 array

* Add contents from Keith's change

* Whitespace and s_string ok addition

* Get rid of WT_GET_TIMESTAMP_PTR

* Fix the change after the merge

* Remove superfluous struct around union, simplify macros

* Remove packed attribute from WT_UPDATE. (#3523)

* Remove packed attribute from WT_UPDATE.

Add 3B of data declaration at the end of the WT_UPDATE structure. That
way we don't have to pack the structure to avoid wasting data bytes,
and we don't have to use a macro to identify the start of the data.

* Locate the timestamp in the WT_UPDATE structure depending on its
alignment, to avoid padding.

* I lost a change, the size of the WT_UPDATE structure has to reflect the
size of the timestamp.

* Change the __wt_timestamp_t union into a structure so the compiler doesn't
insert padding in the middle of the WT_UPDATE structure (the existence of
the uint64_t in the union causes the whole thing to be aligned, even if we
never access it).

Incorporate Michael's change to replace sizeof(WT_UPDATE) with WT_UPDATESIZE,
the compiler is padding the structure at the end and we need to ignore that.

* If there's no union, we can't reach into it and get the field, take
the address like we do everywhere else.

* Simplify size calculations for WT_UPDATE.

In particular, go back to having WT_UPDATE_SIZE match the size of a
WT_UPDATE excluding the payload data.  That means the declared size of
the data array in WT_UPDATE is no longer special.

* Switch from a 3-byte array to a C99 flexible array member.

There is no longer anything special about the 3 byte array: since the
timestamp can be any size, there is no guarantee it makes a WT_UPDATE
any nicely rounded size.  Compilers enforce some rules around how
flexible array members can be used: we should consider switching our
other uses of structs with trailing arrays (in a separate ticket).

* Remove some more unnecessary casts now that upd->data is typed.

1 files changed, 20 insertions, 11 deletions

diff --git a/src/include/btmem.h b/src/include/btmem.h
index 37501a484b0..b694930c8fd 100644
--- a/src/include/btmem.h
+++ b/src/include/btmem.h
@@ -885,9 +885,11 @@ struct __wt_ikey {
  * is done for an entry, WT_UPDATE structures are formed into a forward-linked
  * list.
  */
-WT_PACKED_STRUCT_BEGIN(__wt_update)
-	volatile uint64_t txnid;			/* Transaction ID */
-	WT_DECL_TIMESTAMP(timestamp)
+struct __wt_update {
+	volatile uint64_t txnid;	/* transaction ID */
+#if WT_TIMESTAMP_SIZE == 8
+	WT_DECL_TIMESTAMP(timestamp)	/* aligned uint64_t timestamp */
+#endif
 
 	WT_UPDATE *next;		/* forward-linked list */
 
@@ -898,13 +900,20 @@ WT_PACKED_STRUCT_BEGIN(__wt_update)
 #define	WT_UPDATE_RESERVED	2
 	uint8_t type;			/* type (one byte to conserve memory) */
 
-	/* The update includes a complete value. */
+	/* If the update includes a complete value. */
 #define	WT_UPDATE_DATA_VALUE(upd)					\
 	((upd)->type == WT_UPDATE_STANDARD || (upd)->type == WT_UPDATE_DELETED)
 
-	/* The untyped value immediately follows the WT_UPDATE structure. */
-#define	WT_UPDATE_DATA(upd)						\
-	((void *)((uint8_t *)(upd) + sizeof(WT_UPDATE)))
+#if WT_TIMESTAMP_SIZE != 8
+	WT_DECL_TIMESTAMP(timestamp)	/* unaligned uint8_t array timestamp */
+#endif
+
+	/*
+	 * Zero or more bytes of value (the payload) immediately follows the
+	 * WT_UPDATE structure.  We use a C99 flexible array member which has
+	 * the semantics we want.
+	 */
+	uint8_t data[];			/* start of the data */
 
 	/*
 	 * The memory size of an update: include some padding because this is
@@ -912,12 +921,12 @@ WT_PACKED_STRUCT_BEGIN(__wt_update)
 	 * cache overhead calculation.
 	 */
 #define	WT_UPDATE_MEMSIZE(upd)						\
-	WT_ALIGN(sizeof(WT_UPDATE) + (upd)->size, 32)
-WT_PACKED_STRUCT_END
+	WT_ALIGN(WT_UPDATE_SIZE + (upd)->size, 32)
+};
 
 /*
- * WT_UPDATE_SIZE is the expected structure size -- we verify the build to
- * ensure the compiler hasn't inserted padding.
+ * WT_UPDATE_SIZE is the expected structure size excluding the payload data --
+ * we verify the build to ensure the compiler hasn't inserted padding.
  */
 #define	WT_UPDATE_SIZE	(21 + WT_TIMESTAMP_SIZE)