Code review for palloc0 patch --- avoid dangerous and unnecessary

practice of evaluating MemSet's arguments multiple times, except for
the special case of newNode(), where we can assume the argument is
a constant sizeof() operator.
Also, add GetMemoryChunkContext() to mcxt.c's API, in preparation for
fixing recent GEQO breakage.

src/backend/nodes/nodes.c

@@ -9,7 +9,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/nodes/nodes.c,v 1.18 2002/11/10 02:17:25 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/nodes/nodes.c,v 1.19 2002/12/16 16:22:46 tgl Exp $
  *
  * HISTORY
  *	  Andrew Yu			Oct 20, 1994	file creation
@@ -17,16 +17,11 @@
  *-------------------------------------------------------------------------
  */
 #include "postgres.h"
+
 #include "nodes/nodes.h"
 
 /*
- * newNode -
- *	  create a new node of the specified size and tag the node with the
- *	  specified tag.
- *
- * !WARNING!: Avoid using newNode directly. You should be using the
- *	  macro makeNode. eg. to create a Resdom node, use makeNode(Resdom)
- *
+ * Support for newNode() macro
  */
-Node *newNodeMacroHolder;
 
+Node *newNodeMacroHolder;

src/backend/utils/mmgr/mcxt.c

@@ -14,7 +14,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/utils/mmgr/mcxt.c,v 1.37 2002/12/15 21:01:34 tgl Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/utils/mmgr/mcxt.c,v 1.38 2002/12/16 16:22:46 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -257,6 +257,35 @@ GetMemoryChunkSpace(void *pointer)
 														 pointer);
 }
 
+/*
+ * GetMemoryChunkContext
+ *		Given a currently-allocated chunk, determine the context
+ *		it belongs to.
+ */
+MemoryContext
+GetMemoryChunkContext(void *pointer)
+{
+	StandardChunkHeader *header;
+
+	/*
+	 * Try to detect bogus pointers handed to us, poorly though we can.
+	 * Presumably, a pointer that isn't MAXALIGNED isn't pointing at an
+	 * allocated chunk.
+	 */
+	Assert(pointer != NULL);
+	Assert(pointer == (void *) MAXALIGN(pointer));
+
+	/*
+	 * OK, it's probably safe to look at the chunk header.
+	 */
+	header = (StandardChunkHeader *)
+		((char *) pointer - STANDARDCHUNKHEADERSIZE);
+
+	AssertArg(MemoryContextIsValid(header->context));
+
+	return header->context;
+}
+
 /*
  * MemoryContextStats
  *		Print statistics about the named context and all its descendants.
@@ -453,25 +482,52 @@ MemoryContextAlloc(MemoryContext context, Size size)
 }
 
 /*
- * MemoryContextAllocPalloc0
+ * MemoryContextAllocZero
  *		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.
+ *	is a very common combination, so we provide the combined operation.
  */
 void *
-MemoryContextAllocPalloc0(MemoryContext context, Size size)
+MemoryContextAllocZero(MemoryContext context, Size size)
 {
 	void *ret;
 
 	AssertArg(MemoryContextIsValid(context));
 
 	if (!AllocSizeIsValid(size))
-		elog(ERROR, "MemoryContextAllocZero: invalid request size %lu",
+		elog(ERROR, "MemoryContextAlloc: invalid request size %lu",
 			 (unsigned long) size);
 
 	ret = (*context->methods->alloc) (context, size);
+
+	MemSetAligned(ret, 0, size);
+
+	return ret;
+}
+
+/*
+ * MemoryContextAllocZeroAligned
+ *		MemoryContextAllocZero where length is suitable for MemSetLoop
+ *
+ *	This might seem overly specialized, but it's not because newNode()
+ *	is so often called with compile-time-constant sizes.
+ */
+void *
+MemoryContextAllocZeroAligned(MemoryContext context, Size size)
+{
+	void *ret;
+
+	AssertArg(MemoryContextIsValid(context));
+
+	if (!AllocSizeIsValid(size))
+		elog(ERROR, "MemoryContextAlloc: invalid request size %lu",
+			 (unsigned long) size);
+
+	ret = (*context->methods->alloc) (context, 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 1.133 2002/12/05 04:04:48 momjian Exp $
+ * $Id: c.h,v 1.134 2002/12/16 16:22:46 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -579,38 +579,78 @@ 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); \
+\
+		if ((((long) _start) & INT_ALIGN_MASK) == 0 && \
+			(_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)
+
+#define MEMSET_LOOP_LIMIT  1024
+
+/*
+ * MemSetAligned is the same as MemSet except it omits the test to see if
+ * "start" is word-aligned.  This is okay to use if the caller knows a-priori
+ * that the pointer is suitably aligned (typically, because he just got it
+ * from palloc(), which always delivers a max-aligned pointer).
+ */
+#define MemSetAligned(start, val, len) \
+	do \
+	{ \
+		int32 * _start = (int32 *) (start); \
+		int		_val = (val); \
+		Size	_len = (len); \
+\
+		if ((_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)
+
+
+/*
+ * MemSetTest/MemSetLoop are a variant version that allow all the tests in
+ * MemSet to be done at compile time in cases where "val" and "len" are
+ * constants *and* we know the "start" pointer must be word-aligned.
+ * If MemSetTest succeeds, then it is okay to use MemSetLoop, otherwise use
+ * MemSetAligned.  Beware of multiple evaluations of the arguments when using
+ * this approach.
+ */
 #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)); \
-\
-	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
+	do \
+	{ \
+		int32 * _start = (int32 *) (start); \
+		int32 * _stop = (int32 *) ((char *) _start + (Size) (len)); \
+	\
+		while (_start < _stop) \
+			*_start++ = 0; \
+	} while (0)
 
 
 /* ----------------------------------------------------------------

src/include/nodes/nodes.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2002, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Id: nodes.h,v 1.133 2002/12/14 00:17:59 tgl Exp $
+ * $Id: nodes.h,v 1.134 2002/12/16 16:22:46 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -293,9 +293,16 @@ typedef struct Node
 #define nodeTag(nodeptr)		(((Node*)(nodeptr))->type)
 
 /*
+ * newNode -
+ *	  create a new node of the specified size and tag the node with the
+ *	  specified tag.
+ *
+ * !WARNING!: Avoid using newNode directly. You should be using the
+ *	  macro makeNode.  eg. to create a Query node, use makeNode(Query)
+ *
  *	There is no way to dereference the palloc'ed pointer to assign the
- *	tag, and return the pointer itself, so we need a holder variable.
- *	Fortunately, this function isn't recursive so we just define
+ *	tag, and also return the pointer itself, so we need a holder variable.
+ *	Fortunately, this macro isn't recursive so we just define
  *	a global variable for this purpose.
  */
 extern Node *newNodeMacroHolder;
@@ -303,8 +310,7 @@ extern Node *newNodeMacroHolder;
 #define newNode(size, tag) \
 ( \
 	AssertMacro((size) >= sizeof(Node)),		/* need the tag, at least */ \
-\
-	newNodeMacroHolder = (Node *) palloc0(size), \
+	newNodeMacroHolder = (Node *) palloc0fast(size), \
 	newNodeMacroHolder->type = (tag), \
 	newNodeMacroHolder \
 )

src/include/utils/memutils.h

@@ -10,7 +10,7 @@
  * Portions Copyright (c) 1996-2002, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Id: memutils.h,v 1.49 2002/12/15 21:01:34 tgl Exp $
+ * $Id: memutils.h,v 1.50 2002/12/16 16:22:46 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -83,6 +83,7 @@ extern void MemoryContextResetChildren(MemoryContext context);
 extern void MemoryContextDeleteChildren(MemoryContext context);
 extern void MemoryContextResetAndDeleteChildren(MemoryContext context);
 extern Size GetMemoryChunkSpace(void *pointer);
+extern MemoryContext GetMemoryChunkContext(void *pointer);
 extern void MemoryContextStats(MemoryContext context);
 
 #ifdef MEMORY_CONTEXT_CHECKING

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 1.23 2002/11/13 00:37:06 momjian Exp $
+ * $Id: palloc.h,v 1.24 2002/12/16 16:22:46 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -46,15 +46,25 @@ extern DLLIMPORT MemoryContext CurrentMemoryContext;
  * Fundamental memory-allocation operations (more are in utils/memutils.h)
  */
 extern void *MemoryContextAlloc(MemoryContext context, Size size);
-extern void *MemoryContextAllocPalloc0(MemoryContext context, Size size);
+extern void *MemoryContextAllocZero(MemoryContext context, Size size);
+extern void *MemoryContextAllocZeroAligned(MemoryContext context, Size size);
 
 #define palloc(sz)	MemoryContextAlloc(CurrentMemoryContext, (sz))
 
-/* We assume palloc() is already int-aligned */
-#define palloc0(sz)	\
-	( MemSetTest(0, (sz)) ? \
-		MemoryContextAllocPalloc0(CurrentMemoryContext, (sz)) : \
-		memset(palloc(sz), 0, (sz)))
+#define palloc0(sz)	MemoryContextAllocZero(CurrentMemoryContext, (sz))
+
+/*
+ * The result of palloc() is always word-aligned, so we can skip testing
+ * alignment of the pointer when deciding which MemSet variant to use.
+ * Note that this variant does not offer any advantage, and should not be
+ * used, unless its "sz" argument is a compile-time constant; therefore, the
+ * issue that it evaluates the argument multiple times isn't a problem in
+ * practice.
+ */
+#define palloc0fast(sz)	\
+	( MemSetTest(0, sz) ? \
+		MemoryContextAllocZeroAligned(CurrentMemoryContext, sz) : \
+		MemoryContextAllocZero(CurrentMemoryContext, sz) )
 
 extern void pfree(void *pointer);