Inline the easy cases in MakeExpandedObjectReadOnly().

This attempts to buy back some of whatever performance we lost from fixing
bug #14174 by inlining the initial checks in MakeExpandedObjectReadOnly()
into the callers.  We can do that in a macro without creating multiple-
evaluation hazards, so it's pretty much free notationally; and the amount
of code added to callers should be minimal as well.  (Testing a value can't
take many more instructions than passing it to a subroutine.)

Might as well inline DatumIsReadWriteExpandedObject() while we're at it.

This is an ABI break for callers, so it doesn't seem safe to put into 9.5,
but I see no reason not to do it in HEAD.

src/backend/utils/adt/expandeddatum.c

@@ -84,36 +84,18 @@ EOH_flatten_into(ExpandedObjectHeader *eohptr,
 	(*eohptr->eoh_methods->flatten_into) (eohptr, result, allocated_size);
 }
 
-/*
- * Does the Datum represent a writable expanded object?
- */
-bool
-DatumIsReadWriteExpandedObject(Datum d, bool isnull, int16 typlen)
-{
-	/* Reject if it's NULL or not a varlena type */
-	if (isnull || typlen != -1)
-		return false;
-
-	/* Reject if not a read-write expanded-object pointer */
-	if (!VARATT_IS_EXTERNAL_EXPANDED_RW(DatumGetPointer(d)))
-		return false;
-
-	return true;
-}
-
 /*
  * If the Datum represents a R/W expanded object, change it to R/O.
  * Otherwise return the original Datum.
+ *
+ * Caller must ensure that the datum is a non-null varlena value.  Typically
+ * this is invoked via MakeExpandedObjectReadOnly(), which checks that.
  */
 Datum
-MakeExpandedObjectReadOnly(Datum d, bool isnull, int16 typlen)
+MakeExpandedObjectReadOnlyInternal(Datum d)
 {
 	ExpandedObjectHeader *eohptr;
 
-	/* Nothing to do if it's NULL or not a varlena type */
-	if (isnull || typlen != -1)
-		return d;
-
 	/* Nothing to do if not a read-write expanded-object pointer */
 	if (!VARATT_IS_EXTERNAL_EXPANDED_RW(DatumGetPointer(d)))
 		return d;

src/include/utils/expandeddatum.h

@@ -136,6 +136,15 @@ struct ExpandedObjectHeader
 #define EOHPGetRWDatum(eohptr)	PointerGetDatum((eohptr)->eoh_rw_ptr)
 #define EOHPGetRODatum(eohptr)	PointerGetDatum((eohptr)->eoh_ro_ptr)
 
+/* Does the Datum represent a writable expanded object? */
+#define DatumIsReadWriteExpandedObject(d, isnull, typlen) \
+	(((isnull) || (typlen) != -1) ? false : \
+	 VARATT_IS_EXTERNAL_EXPANDED_RW(DatumGetPointer(d)))
+
+#define MakeExpandedObjectReadOnly(d, isnull, typlen) \
+	(((isnull) || (typlen) != -1) ? (d) : \
+	 MakeExpandedObjectReadOnlyInternal(d))
+
 extern ExpandedObjectHeader *DatumGetEOHP(Datum d);
 extern void EOH_init_header(ExpandedObjectHeader *eohptr,
 				const ExpandedObjectMethods *methods,
@@ -143,8 +152,7 @@ extern void EOH_init_header(ExpandedObjectHeader *eohptr,
 extern Size EOH_get_flat_size(ExpandedObjectHeader *eohptr);
 extern void EOH_flatten_into(ExpandedObjectHeader *eohptr,
 				 void *result, Size allocated_size);
-extern bool DatumIsReadWriteExpandedObject(Datum d, bool isnull, int16 typlen);
-extern Datum MakeExpandedObjectReadOnly(Datum d, bool isnull, int16 typlen);
+extern Datum MakeExpandedObjectReadOnlyInternal(Datum d);
 extern Datum TransferExpandedObject(Datum d, MemoryContext new_parent);
 extern void DeleteExpandedObject(Datum d);