--- a/RELEASE_NOTES	Sun Jan 26 18:22:04 2014 +0100
+++ b/RELEASE_NOTES	Tue Jan 28 12:51:47 2014 -0800
@@ -22,7 +22,9 @@
 New user-visible features
 -------------------------
 
-- a new LTE MAC downlink scheduling algorithm named Channel and QoS
+- Enable selection of high precision int64x64_t implementation
+  at configure time, for debugging purposes.
+- A new LTE MAC downlink scheduling algorithm named Channel and QoS
   Aware (CQA) Scheduler is provided by the new
   ``ns3::CqaFfMacScheduler`` object.
   

--- a/src/core/wscript	Sun Jan 26 18:22:04 2014 +0100
+++ b/src/core/wscript	Tue Jan 28 12:51:47 2014 -0800
@@ -4,14 +4,38 @@
 from waflib import Options
 import wutils
 
+int64x64 = {
+    # implementation name: [define, env, highprec]
+    'default': ['INT64X64_USE_128',    'INT64X64_USE_128',    '128-bit integer'], 
+    'int128':  ['INT64X64_USE_128',    'INT64X64_USE_128',    '128-bit integer'], 
+    'cairo':   ['INT64X64_USE_CAIRO',  'INT64X64_USE_CAIRO',  'cairo 128-bit integer'],
+    'double':  ['INT64X64_USE_DOUBLE', 'INT64X64_USE_DOUBLE', 'long double'],
+    }
+
+default_int64x64 = 'default'
+
 def options(opt):
-    opt.add_option('--int64x64-as-double',
-                   help=('Whether to use a double floating point'
-                         ' type for int64x64 values'
-                         ' WARNING: this option only has effect '
-                         'with the configure command.'),
-                   action="store_true", default=False,
-                   dest='int64x64_as_double')
+    assert default_int64x64 in int64x64
+    opt.add_option('--int64x64',
+                   action='store',
+                   default=default_int64x64,
+                   help=("Force the choice of int64x64_t implementation "
+                         "(normally only for debugging).  "
+                         "The supported implementations use int128_t, "
+                         "cairo_int128, or long double.  "
+                         "The int128_t implementation (the preferred option) "
+                         "requires compiler support.  "
+                         "The cairo implementation fallback provides exactly "
+                         "the same numerical results, but possibly at lower "
+                         "execution speed.  The long double implementation "
+                         "may not provide the same numerical results because "
+                         "the implementation-defined numerical precision may "
+                         "be less than the other implementations.  "
+                         "[Allowed Values: %s]"
+                         % ", ".join([repr(p) for p in int64x64.keys()])),
+                   choices=int64x64.keys(),
+                   dest='int64x64_impl')
+                   
     opt.add_option('--disable-pthread',
                    help=('Whether to enable the use of POSIX threads'),
                    action="store_true", default=False,
@@ -20,23 +44,24 @@
 
 
 def configure(conf):
-    a = conf.check_nonfatal(type_name='uint128_t', define_name='HAVE_UINT128_T')
-    b = conf.check_nonfatal(type_name='__uint128_t', define_name='HAVE___UINT128_T')
-
-    if Options.options.int64x64_as_double:
-        conf.define('INT64X64_USE_DOUBLE', 1)
-        conf.env['INT64X64_USE_DOUBLE'] = 1
-        highprec = 'long double'
-    elif a or b:
-        conf.define('INT64X64_USE_128', 1)
-        conf.env['INT64X64_USE_128'] = 1
-        highprec = '128-bit integer'
-    else:
-        conf.define('INT64X64_USE_CAIRO', 1)
-        conf.env['INT64X64_USE_CAIRO'] = 1
-        highprec = 'cairo 128-bit integer'
-
-    conf.msg('Checking high precision time implementation', highprec)
+    int64x64_impl = Options.options.int64x64_impl
+    if int64x64_impl == 'default' or int64x64_impl == 'int128':
+        have_uint128  = conf.check_nonfatal(type_name='uint128_t',
+                                            define_name='HAVE_UINT128_T')
+        have__uint128 = conf.check_nonfatal(type_name='__uint128_t',
+                                            define_name='HAVE___UINT128_T')
+        if have_uint128 or have__uint128:
+            int64x64_impl = 'int128'
+        else:
+            int64x64_impl = 'cairo'
+        
+    def_flag, env_flag, highprec = int64x64[int64x64_impl]
+    # Add a tag confirming default choice
+    if Options.options.int64x64_impl == 'default':
+        highprec += ' (default)'
+    conf.define(def_flag, 1)
+    conf.env[env_flag] = 1
+    conf.msg('Checking high precision implementation', highprec)
 
     conf.check_nonfatal(header_name='stdint.h', define_name='HAVE_STDINT_H')
     conf.check_nonfatal(header_name='inttypes.h', define_name='HAVE_INTTYPES_H')