diff --git a/configure.in b/configure.in
index 04c70c207..4d14fcac3 100644
--- a/configure.in
+++ b/configure.in
@@ -167,6 +167,31 @@ AC_SEARCH_LIBS([nan], [m], [AC_DEFINE([HAVE_NAN], [1])])
 AC_SEARCH_LIBS([fmin], [m], [AC_DEFINE([HAVE_FMIN], [1])])
 AC_SEARCH_LIBS([fmax], [m], [AC_DEFINE([HAVE_FMAX], [1])])
 
+# Check to see if an unsigned long and uint64_t are the same from
+# a compiler perspective. We can not just check that they are the
+# same size since unsigned long and unsigned long long are not the
+# same from an overloading perspective even though they could be
+# the same size on some 64 bit machines. The result from this test
+# is only used if inttypes.h is available, so if the test fails for
+# that reason we don't care.
+AC_LANG(C++)
+AC_MSG_CHECKING(if uint64_t and unsigned long are identical)
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include "inttypes.h"
+static bool check(unsigned long val)
+{
+  return val != 0;
+}
+static bool check(uint64_t val)
+{
+  return val != 0;
+}]], [[unsigned long ulval = 1;
+bool result = check(ulval);
+uint64_t uival = 1;
+result &= check(uival);
+return !result;]])],
+   [AC_MSG_RESULT(no)],
+   [AC_DEFINE([UINT64_T_AND_ULONG_SAME], [1]) AC_MSG_RESULT(yes)])
+
 # Linker option used when compiling the target
 AX_LD_RDYNAMIC
 
diff --git a/vvp/config.h.in b/vvp/config.h.in
index 71691a402..50b4c5450 100644
--- a/vvp/config.h.in
+++ b/vvp/config.h.in
@@ -49,6 +49,7 @@
 # undef HAVE_INTTYPES_H
 # undef HAVE_LROUND
 # undef HAVE_NAN
+# undef UINT64_T_AND_ULONG_SAME
 
 /*
  * Define this if you want to compile vvp with memory freeing and
@@ -89,7 +90,7 @@ typedef uint64_t vvp_time64_t;
 # define TIME_FMT_U PRIu64
 # define TIME_FMT_X PRIx64
 
-# if SIZEOF_UNSIGNED_LONG == 8
+# if UINT64_T_AND_ULONG_SAME
 #  define UL_AND_TIME64_SAME
 # endif