Grab the low-hanging fruit from forcing sizeof(Datum) to 8.

Remove conditionally-compiled code for smaller Datum widths,
and simplify comments that describe cases no longer of interest.

I also fixed up a few more places that were not using
DatumGetIntXX where they should, and made some cosmetic
adjustments such as using sizeof(int64) not sizeof(Datum)
in places where that fit better with the surrounding code.

One thing I remembered while preparing this part is that SP-GiST
stores pass-by-value prefix keys as Datums, so that the on-disk
representation depends on sizeof(Datum).  That's even more
unfortunate than the existing commentary makes it out to be,
because now there is a hazard that the change of sizeof(Datum)
will break SP-GiST indexes on 32-bit machines.  It appears that
there are no existing SP-GiST opclasses that are actually
affected; and if there are some that I didn't find, the number
of installations that are using them on 32-bit machines is
doubtless tiny.  So I'm proceeding on the assumption that we
can get away with this, but it's something to worry about.

(gininsert.c looks like it has a similar problem, but it's okay
because the "tuples" it's constructing are just transient data
within the tuplesort step.  That's pretty poorly documented
though, so I added some comments.)

Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/1749799.1752797397@sss.pgh.pa.us

1 files changed, 10 insertions, 17 deletions

diff --git a/src/backend/utils/adt/mac.c b/src/backend/utils/adt/mac.c
index 3644e9735f5..bb38ef2f5e4 100644
--- a/src/backend/utils/adt/mac.c
+++ b/src/backend/utils/adt/mac.c
@@ -481,33 +481,26 @@ macaddr_abbrev_convert(Datum original, SortSupport ssup)
 	Datum		res;
 
 	/*
-	 * On a 64-bit machine, zero out the 8-byte datum and copy the 6 bytes of
-	 * the MAC address in. There will be two bytes of zero padding on the end
-	 * of the least significant bits.
+	 * Zero out the 8-byte Datum and copy in the 6 bytes of the MAC address.
+	 * There will be two bytes of zero padding on the end of the least
+	 * significant bits.
 	 */
-#if SIZEOF_DATUM == 8
-	memset(&res, 0, SIZEOF_DATUM);
+	StaticAssertStmt(sizeof(res) >= sizeof(macaddr),
+					 "Datum is too small for macaddr");
+	memset(&res, 0, sizeof(res));
 	memcpy(&res, authoritative, sizeof(macaddr));
-#else							/* SIZEOF_DATUM != 8 */
-	memcpy(&res, authoritative, SIZEOF_DATUM);
-#endif
 	uss->input_count += 1;
 
 	/*
-	 * Cardinality estimation. The estimate uses uint32, so on a 64-bit
-	 * architecture, XOR the two 32-bit halves together to produce slightly
-	 * more entropy. The two zeroed bytes won't have any practical impact on
-	 * this operation.
+	 * Cardinality estimation. The estimate uses uint32, so XOR the two 32-bit
+	 * halves together to produce slightly more entropy. The two zeroed bytes
+	 * won't have any practical impact on this operation.
 	 */
 	if (uss->estimating)
 	{
 		uint32		tmp;
 
-#if SIZEOF_DATUM == 8
-		tmp = (uint32) res ^ (uint32) ((uint64) res >> 32);
-#else							/* SIZEOF_DATUM != 8 */
-		tmp = (uint32) res;
-#endif
+		tmp = DatumGetUInt32(res) ^ (uint32) (DatumGetUInt64(res) >> 32);
 
 		addHyperLogLog(&uss->abbr_card, DatumGetUInt32(hash_uint32(tmp)));
 	}