Index: configure.in
===================================================================
--- configure.in	(revision 34399)
+++ configure.in	(working copy)
@@ -1292,7 +1292,12 @@
 CFLAGS="$save_CFLAGS"])
 AC_DEFINE_UNQUOTED(GC_MARK_STACKFRAME_WORD, $rb_cv_gc_mark_stackframe_word)
 
+AS_CASE(["$target_os"],
+[openbsd*], [
+    AC_DEFINE_UNQUOTED(HEAP_ALIGN_LOG, 12)
+    ])
 
+
 dnl Checks for library functions.
 AC_TYPE_GETGROUPS
 AC_TYPE_SIGNAL
Index: gc.c
===================================================================
--- gc.c	(revision 34399)
+++ gc.c	(working copy)
@@ -536,8 +536,11 @@
 }
 #endif
 
-/* tiny heap size: 16KB */
+#ifndef HEAP_ALIGN_LOG
+/* default tiny heap size: 16KB */
 #define HEAP_ALIGN_LOG 14
+#endif
+
 #define HEAP_ALIGN (1UL << HEAP_ALIGN_LOG)
 #define HEAP_ALIGN_MASK (~(~0UL << HEAP_ALIGN_LOG))
 #define REQUIRED_SIZE_BY_MALLOC (sizeof(size_t) * 5)