Add guards to bypass the test when double rounding can occur.

Author: rhettinger

Lib/test/test_math.py

@@ -803,8 +803,20 @@ def testHypot(self):
             scale = FLOAT_MIN / 2.0 ** exp
             self.assertEqual(math.hypot(4*scale, 3*scale), 5*scale)
 
+    @requires_IEEE_754
+    @unittest.skipIf(HAVE_DOUBLE_ROUNDING,
+                     "hypot() loses accuracy on machines with double rounding")
     def testHypotAccuracy(self):
         # Verify improved accuracy in cases that were known to be inaccurate.
+        #
+        # The new algorithm's accuracy depends on IEEE 754 arithmetic
+        # guarantees, on having the usual ROUND HALF EVEN rounding mode, on
+        # the system not having double rounding due to extended precision,
+        # and on the compiler maintaining the specified order of operations.
+        #
+        # This test is known to succeed on most of our builds.  If it fails
+        # some build, we either need to add another skipIf if the cause is
+        # identifiable; otherwise, we can remove this test entirely.
 
         hypot = math.hypot
         Decimal = decimal.Decimal
@@ -835,7 +847,7 @@ def testHypotAccuracy(self):
             ('0x1.1243a50751fd4p+29', '0x1.a5a10175622d9p+29'),
             ('0x1.57a8596e74722p+30', '0x1.42d1af9d04da9p+30'),
 
-            # Cases with 1 ulp error in version fff3c28052e6b0750d6218e00acacd2fded4991a
+            # Cases with 1 ulp error in version fff3c28052e6b0
             ('0x1.ee7dbd9565899p+29', '0x1.7ab4d6fc6e4b4p+29'),
             ('0x1.5c6bfbec5c4dcp+30', '0x1.02511184b4970p+30'),
             ('0x1.59dcebba995cap+30', '0x1.50ca7e7c38854p+29'),