Modify the float4 datatype to be pass-by-val. Along the way, remove the last

uses of the long-deprecated float32 in contrib/seg; the definitions themselves
are still there, but no longer used.  fmgr/README updated to match.

I added a CREATE FUNCTION to account for existing seg_center() code in seg.c
too, and some tests for it and the neighbor functions.  At the same time,
remove checks for NULL which are not needed (because the functions are declared
STRICT).

I had to do some adjustments to contrib's btree_gist too.  The choices for
representation there are not ideal for changing the underlying types :-(

Original patch by Zoltan Boszormenyi, with some adjustments by me.

src/backend/utils/fmgr/README

@@ -1,4 +1,4 @@
-$PostgreSQL: pgsql/src/backend/utils/fmgr/README,v 1.10 2008/03/20 17:55:15 momjian Exp $
+$PostgreSQL: pgsql/src/backend/utils/fmgr/README,v 1.11 2008/04/18 18:43:09 alvherre Exp $
 
 Function Manager
 ================
@@ -211,14 +211,13 @@ also amenable to machine processing --- for example, we could probably
 write a script that scans code like this and extracts argument and result
 type info for comparison to the pg_proc table.
 
-For the standard data types float4, float8, and int8, these macros should
+For the standard data types float8 and int8, these macros should
 hide the indirection and space allocation involved, so that the function's
 code is not explicitly aware that these types are pass-by-reference.  This
 will offer a considerable gain in readability, and it also opens up the
 opportunity to make these types be pass-by-value on machines where it's
 feasible to do so.  (For example, on an Alpha it's pretty silly to make int8
-be pass-by-ref, since Datum is going to be 64 bits anyway.  float4 could
-become pass-by-value on all machines...)
+be pass-by-ref, since Datum is going to be 64 bits anyway.)
 
 Here are the proposed macros and coding conventions:
 
@@ -248,20 +247,20 @@ which expands to
 Argument values are ordinarily fetched using code like
 	int32	name = PG_GETARG_INT32(number);
 
-For float4, float8, and int8, the PG_GETARG macros will hide the pass-by-
-reference nature of the data types; for example PG_GETARG_FLOAT4 expands to
-	(* (float4 *) DatumGetPointer(fcinfo->arg[number]))
+For float8 and int8, the PG_GETARG macros will hide the pass-by-reference
+nature of the data types; for example PG_GETARG_FLOAT8 expands to
+	(* (float8 *) DatumGetPointer(fcinfo->arg[number]))
 and would typically be called like this:
-	float4  arg = PG_GETARG_FLOAT4(0);
-Note that "float4" and "float8" are the recommended typedefs to use, not
-"float32data" and "float64data", and the macros are named accordingly.
-But 64-bit ints should be declared as "int64".
+	float8  arg = PG_GETARG_FLOAT8(0);
+Note that "float8" is the recommended typedef to use, not "float64data", and
+the macros are named accordingly.  But 64-bit ints should be declared as
+"int64".
 
 Non-null values are returned with a PG_RETURN_XXX macro of the appropriate
 type.  For example, PG_RETURN_INT32 expands to
 	return Int32GetDatum(x)
-PG_RETURN_FLOAT4, PG_RETURN_FLOAT8, and PG_RETURN_INT64 hide the pass-by-
-reference nature of their datatypes.
+PG_RETURN_FLOAT8 and PG_RETURN_INT64 hide the pass-by-reference nature of
+their datatypes.
 
 fmgr.h will provide PG_GETARG and PG_RETURN macros for all the basic data
 types.  Modules or header files that define specialized SQL datatypes
@@ -334,7 +333,7 @@ Again, this style of coding does not allow for expressing NULL inputs
 or receiving a NULL result.
 
 As with the callee-side situation, I propose adding argument conversion
-macros that hide the pass-by-reference nature of int8, float4, and
+macros that hide the pass-by-reference nature of int8, and
 float8, with an eye to making those types relatively painless to convert
 to pass-by-value.
 

src/backend/utils/fmgr/fmgr.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/fmgr/fmgr.c,v 1.115 2008/04/17 21:37:28 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/fmgr/fmgr.c,v 1.116 2008/04/18 18:43:09 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -2060,10 +2060,25 @@ Int64GetDatum(int64 X)
 Datum
 Float4GetDatum(float4 X)
 {
-	float4	   *retval = (float4 *) palloc(sizeof(float4));
+	union {
+		float4	value;
+		int32	retval;
+	} myunion;
 
-	*retval = X;
-	return PointerGetDatum(retval);
+	myunion.value = X;
+	return SET_4_BYTES(myunion.retval);
+}
+
+float4
+DatumGetFloat4(Datum X)
+{
+	union {
+		int32	value; 
+		float4	retval;
+	} myunion;
+
+	myunion.value = GET_4_BYTES(X);
+	return myunion.retval;
 }
 
 Datum