Index: configure.in
===================================================================
--- configure.in	(revision 17858)
+++ configure.in	(working copy)
@@ -377,6 +377,10 @@
 	: handled by ext/thread/extconf.rb
         ])
 
+AC_ARG_WITH(valgrind,
+        [  --with-valgrind         enable valgrind memcheck support.],
+        [AC_CHECK_HEADERS(valgrind/memcheck.h)])
+
 dnl Checks for libraries.
 case "$target_os" in
 nextstep*)	;;
Index: gc.c
===================================================================
--- gc.c	(revision 17858)
+++ gc.c	(working copy)
@@ -37,6 +37,19 @@
 void re_free_registers _((struct re_registers*));
 void rb_io_fptr_finalize _((struct rb_io_t*));
 
+#ifdef HAVE_VALGRIND_MEMCHECK_H
+# include <valgrind/memcheck.h>
+# ifndef VALGRIND_MAKE_MEM_DEFINED
+#  define VALGRIND_MAKE_MEM_DEFINED(p, n) VALGRIND_MAKE_READABLE(p, n)
+# endif
+# ifndef VALGRIND_MAKE_MEM_UNDEFINED
+#  define VALGRIND_MAKE_MEM_UNDEFINED(p, n) VALGRIND_MAKE_WRITABLE(p, n)
+# endif
+#else
+# define VALGRIND_MAKE_MEM_DEFINED(p, n) /* empty */
+# define VALGRIND_MAKE_MEM_UNDEFINED(p, n) /* empty */
+#endif
+
 #define rb_setjmp(env) RUBY_SETJMP(env)
 #define rb_jmp_buf rb_jmpbuf_t
 #ifdef __CYGWIN__
@@ -668,6 +681,7 @@
     VALUE v;
     while (n--) {
         v = *x;
+        VALGRIND_MAKE_MEM_DEFINED(&v, sizeof(v));
 	if (is_pointer_to_heap((void *)v)) {
 	    gc_mark(v, 0);
 	}