Index: Makefile.in
===================================================================
--- Makefile.in	(revision 6439)
+++ Makefile.in	(working copy)
@@ -34,7 +34,7 @@
 
 empty =
 OUTFLAG = @OUTFLAG@$(empty)
-CFLAGS = @CFLAGS@ @XCFLAGS@ @ARCH_FLAG@
+CFLAGS = @CFLAGS@ @XCFLAGS@ @ARCH_FLAG@ @VALGRIND_CFLAGS@
 CPPFLAGS = -I. -I$(srcdir) @CPPFLAGS@
 LDFLAGS = @STATIC@ $(CFLAGS) @LDFLAGS@
 EXTLDFLAGS = 
Index: configure.in
===================================================================
--- configure.in	(revision 6439)
+++ configure.in	(working copy)
@@ -895,6 +895,22 @@
     fi
 fi
 
+AC_ARG_ENABLE(valgrind,
+       [  --enable-valgrind       use valgrind support],
+       [want_valgrind=$enableval], [want_valgrind=auto])
+
+if test x"$want_valgrind" != xno; then
+  AC_CHECK_HEADERS(valgrind/memcheck.h, have_valgrind=yes, have_valgrind=no)
+	if test x"$have_valgrind" = xyes; then
+		AC_DEFINE(HAVE_VALGRIND)
+		VALGRIND_CFLAGS="";
+	elif test x"$want_valgrind" = xyes -a x"$have_valgrind" = xno; then
+		AC_MSG_ERROR(valgrind support requested but valgrind not found)
+	else
+	  VALGRIND_CFLAGS="";
+	fi
+fi
+
 dnl default value for $KANJI
 DEFAULT_KCODE="KCODE_NONE"
 
@@ -1504,6 +1520,7 @@
 esac
 
 AC_SUBST(XCFLAGS)dnl
+AC_SUBST(VALGRIND_CFLAGS)dnl
 AC_SUBST(XLDFLAGS)dnl
 AC_SUBST(LIBRUBY_LDSHARED)
 AC_SUBST(LIBRUBY_DLDFLAGS)
Index: eval.c
===================================================================
--- eval.c	(revision 6439)
+++ eval.c	(working copy)
@@ -28,6 +28,12 @@
 #define EXIT_FAILURE 1
 #endif
 
+#ifdef HAVE_VALGRIND
+#include <valgrind/memcheck.h>
+#else
+#define VALGRIND_MAKE_MEM_DEFINED(p, n) /* empty */
+#endif
+
 #include <stdio.h>
 
 #include "st.h"
@@ -5225,6 +5231,9 @@
     int pcall;
 {
     ruby_current_node = lhs;
+
+    VALGRIND_MAKE_MEM_DEFINED(&val, sizeof(val));
+
     if (val == Qundef) {
 	rb_warning("assigning void value");
 	val = Qnil;
Index: gc.c
===================================================================
--- gc.c	(revision 6439)
+++ gc.c	(working copy)
@@ -30,6 +30,12 @@
 #include <sys/resource.h>
 #endif
 
+#ifdef HAVE_VALGRIND
+#include <valgrind/memcheck.h>
+#else
+#define VALGRIND_MAKE_MEM_DEFINED(p, n) /* empty */
+#endif
+
 #if defined _WIN32 || defined __CYGWIN__
 #include <windows.h>
 #endif
@@ -93,6 +99,9 @@
 {
     void *mem;
 
+    VALGRIND_MAKE_MEM_DEFINED(&malloc_increase, sizeof(malloc_increase));
+    VALGRIND_MAKE_MEM_DEFINED(&malloc_limit, sizeof(malloc_limit));
+
     if (size < 0) {
 	rb_raise(rb_eNoMemError, "negative allocation size (or too big)");
     }
@@ -623,6 +632,9 @@
     register long n;
 {
     VALUE v;
+
+    VALGRIND_MAKE_MEM_DEFINED(x, sizeof(*x) * n);
+
     while (n--) {
         v = *x;
 	if (is_pointer_to_heap((void *)v)) {
@@ -713,7 +725,10 @@
 {
     register RVALUE *obj;
 
+    VALGRIND_MAKE_MEM_DEFINED(&ptr, sizeof(ptr));
     obj = RANY(ptr);
+    VALGRIND_MAKE_MEM_DEFINED(obj, sizeof(*obj));
+
     if (rb_special_const_p(ptr)) return; /* special const not marked */
     if (obj->as.basic.flags == 0) return;       /* free cell */
     if (obj->as.basic.flags & FL_MARK) return;  /* already marked */