Split MemSet into three parts to constant comparisons can be optimized

away by the compiler;  used by palloc0.

src/backend/utils/mmgr/mcxt.c

@@ -14,7 +14,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/utils/mmgr/mcxt.c,v 1.35 2002/11/10 02:17:25 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/utils/mmgr/mcxt.c,v 1.36 2002/11/13 00:37:06 momjian Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -453,14 +453,14 @@ MemoryContextAlloc(MemoryContext context, Size size)
 }
 
 /*
- * MemoryContextAllocZero
+ * MemoryContextAllocPalloc0
  *		Like MemoryContextAlloc, but clears allocated memory
  *
  *	We could just call MemoryContextAlloc then clear the memory, but this
  *	function is called too many times, so we have a separate version.
  */
 void *
-MemoryContextAllocZero(MemoryContext context, Size size)
+MemoryContextAllocPalloc0(MemoryContext context, Size size)
 {
 	void *ret;
 
@@ -471,7 +471,7 @@ MemoryContextAllocZero(MemoryContext context, Size size)
 			 (unsigned long) size);
 
 	ret = (*context->methods->alloc) (context, size);
-	MemSet(ret, 0, size);
+	MemSetLoop(ret, 0, size);
 	return ret;
 }
 

src/include/c.h

@@ -12,7 +12,7 @@
  * Portions Copyright (c) 1996-2002, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Id: c.h,v 1.131 2002/11/11 03:02:19 momjian Exp $
+ * $Id: c.h,v 1.132 2002/11/13 00:37:06 momjian Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -577,26 +577,36 @@ typedef NameData *Name;
  *	memset() functions.  More research needs to be done, perhaps with
  *	platform-specific MEMSET_LOOP_LIMIT values or tests in configure.
  *
+ *	MemSet has been split into two parts so MemSetTest can be optimized
+ *	away for constant 'val' and 'len'.  This is used by palloc0().
+ *
+ *	Note, arguments are evaluated more than once.
+ *
  *	bjm 2002-10-08
  */
-#define MemSet(start, val, len) \
-	do \
-	{ \
-		int32 * _start = (int32 *) (start); \
-		int		_val = (val); \
-		Size	_len = (len); \
+#define MemSetTest(val, len) \
+	( ((len) & INT_ALIGN_MASK) == 0 && \
+	(len) <= MEMSET_LOOP_LIMIT && \
+	(val) == 0 )
+
+#define MemSetLoop(start, val, len) \
+do \
+{ \
+	int32 * _start = (int32 *) (start); \
+	int32 * _stop = (int32 *) ((char *) _start + (len)); \
 \
-		if ((( ((long) _start) | _len) & INT_ALIGN_MASK) == 0 && \
-			_val == 0 && \
-			_len <= MEMSET_LOOP_LIMIT) \
-		{ \
-			int32 * _stop = (int32 *) ((char *) _start + _len); \
-			while (_start < _stop) \
-				*_start++ = 0; \
-		} \
-		else \
-			memset((char *) _start, _val, _len); \
-	} while (0)
+	while (_start < _stop) \
+		*_start++ = 0; \
+} while (0)
+
+#define MemSet(start, val, len) \
+do \
+{ \
+	if (MemSetTest(val, len) && ((long)(start) & INT_ALIGN_MASK) == 0 ) \
+		MemSetLoop(start, val, len); \
+	else \
+		memset((char *)(start), (val), (len)); \
+} while (0)
 
 #define MEMSET_LOOP_LIMIT  1024
 

src/include/utils/palloc.h

@@ -21,7 +21,7 @@
  * Portions Copyright (c) 1996-2002, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Id: palloc.h,v 1.22 2002/11/10 02:17:25 momjian Exp $
+ * $Id: palloc.h,v 1.23 2002/11/13 00:37:06 momjian Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -46,11 +46,15 @@ extern DLLIMPORT MemoryContext CurrentMemoryContext;
  * Fundamental memory-allocation operations (more are in utils/memutils.h)
  */
 extern void *MemoryContextAlloc(MemoryContext context, Size size);
-extern void *MemoryContextAllocZero(MemoryContext context, Size size);
+extern void *MemoryContextAllocPalloc0(MemoryContext context, Size size);
 
 #define palloc(sz)	MemoryContextAlloc(CurrentMemoryContext, (sz))
 
-#define palloc0(sz)	MemoryContextAllocZero(CurrentMemoryContext, (sz))
+/* We assume palloc() is already int-aligned */
+#define palloc0(sz)	\
+	( MemSetTest(0, (sz)) ? \
+		MemoryContextAllocPalloc0(CurrentMemoryContext, (sz)) : \
+		memset(palloc(sz), 0, (sz)))
 
 extern void pfree(void *pointer);