Make type Datum be 8 bytes wide everywhere.

This patch makes sizeof(Datum) be 8 on all platforms including
32-bit ones.  The objective is to allow USE_FLOAT8_BYVAL to be true
everywhere, and in consequence to remove a lot of code that is
specific to pass-by-reference handling of float8, int8, etc.  The
code for abbreviated sort keys can be simplified similarly.  In this
way we can reduce the maintenance effort involved in supporting 32-bit
platforms, without going so far as to actually desupport them.  Since
Datum is strictly an in-memory concept, this has no impact on on-disk
storage, though an initdb or pg_upgrade will be needed to fix affected
catalog entries.

We have required platforms to support [u]int64 for ages, so this
breaks no supported platform.  We can expect that this change will
make 32-bit builds a bit slower and more memory-hungry, although being
able to use pass-by-value handling of 8-byte types may buy back some
of that.  But we stopped optimizing for 32-bit cases a long time ago,
and this seems like just another step on that path.

This initial patch simply forces the correct type definition and
USE_FLOAT8_BYVAL setting, and cleans up a couple of minor compiler
complaints that ensued.  This is sufficient for testing purposes.
In the wake of a bunch of Datum-conversion cleanups by Peter
Eisentraut, this now compiles cleanly with gcc on a 32-bit platform.
(I'd only tested the previous version with clang, which it turns out
is less picky than gcc about width-changing coercions.)

There is a good deal of now-dead code that I'll remove in separate
follow-up patches.

A catversion bump is required because this affects initial catalog
contents (on 32-bit machines) in two ways: pg_type.typbyval changes
for some built-in types, and Const nodes in stored views/rules will
now have 8 bytes not 4 for pass-by-value types.

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, 13 insertions, 8 deletions

diff --git a/src/include/postgres.h b/src/include/postgres.h
index 8a41a668687..e81829bfa6f 100644
--- a/src/include/postgres.h
+++ b/src/include/postgres.h
@@ -58,15 +58,22 @@
 
 /*
  * A Datum contains either a value of a pass-by-value type or a pointer to a
- * value of a pass-by-reference type.  Therefore, we require:
- *
- * sizeof(Datum) == sizeof(void *) == 4 or 8
+ * value of a pass-by-reference type.  Therefore, we must have
+ * sizeof(Datum) >= sizeof(void *).  No current or foreseeable Postgres
+ * platform has pointers wider than 8 bytes, and standardizing on Datum being
+ * exactly 8 bytes has advantages in reducing cross-platform differences.
  *
  * The functions below and the analogous functions for other types should be used to
  * convert between a Datum and the appropriate C type.
  */
 
-typedef uintptr_t Datum;
+typedef uint64_t Datum;
+
+/*
+ * This symbol is now vestigial, but we continue to define it so as not to
+ * unnecessarily break extension code.
+ */
+#define SIZEOF_DATUM 8
 
 /*
  * A NullableDatum is used in places where both a Datum and its nullness needs
@@ -83,8 +90,6 @@ typedef struct NullableDatum
 	/* due to alignment padding this could be used for flags for free */
 } NullableDatum;
 
-#define SIZEOF_DATUM SIZEOF_VOID_P
-
 /*
  * DatumGetBool
  *		Returns boolean value of a datum.
@@ -316,7 +321,7 @@ CommandIdGetDatum(CommandId X)
 static inline Pointer
 DatumGetPointer(Datum X)
 {
-	return (Pointer) X;
+	return (Pointer) (uintptr_t) X;
 }
 
 /*
@@ -326,7 +331,7 @@ DatumGetPointer(Datum X)
 static inline Datum
 PointerGetDatum(const void *X)
 {
-	return (Datum) X;
+	return (Datum) (uintptr_t) X;
 }
 
 /*