Various dtoa.c cleanups. 1. Despagghetify _Py_dg_strtod parsing code

and exit points.  2. Simplify bigcomp comparison loop.  3. Don't set
ERANGE on _Py_dg_strtod underflow (it was set inconsistently anyway).
4. Remove unused dsign field from BCinfo struct.

Lib/test/test_strtod.py

@@ -164,10 +164,10 @@ class StrtodTests(unittest.TestCase):
                 self.check_strtod(s)
 
     def test_bigcomp(self):
-        DIG10 = 10**50
-        for i in xrange(1000):
-            for j in xrange(TEST_SIZE):
-                digits = random.randrange(DIG10)
+        for ndigs in 5, 10, 14, 15, 16, 17, 18, 19, 20, 40, 41, 50:
+            dig10 = 10**ndigs
+            for i in xrange(100 * TEST_SIZE):
+                digits = random.randrange(dig10)
                 exponent = random.randrange(-400, 400)
                 s = '{}e{}'.format(digits, exponent)
                 self.check_strtod(s)
@@ -259,6 +259,43 @@ class StrtodTests(unittest.TestCase):
             '10000000000000000000000000000000000000000e-17',
             # issue 7632 bug 8:  the following produced 10.0
             '10.900000000000000012345678912345678912345',
+            # exercise exit conditions in bigcomp comparison loop
+            '2602129298404963083833853479113577253105939995688e2',
+            '260212929840496308383385347911357725310593999568896e0',
+            '26021292984049630838338534791135772531059399956889601e-2',
+            '260212929840496308383385347911357725310593999568895e0',
+            '260212929840496308383385347911357725310593999568897e0',
+            '260212929840496308383385347911357725310593999568996e0',
+            '260212929840496308383385347911357725310593999568866e0',
+            # 2**53
+            '9007199254740992.00',
+            # 2**1024 - 2**970:  exact overflow boundary.  All values
+            # smaller than this should round to something finite;  any value
+            # greater than or equal to this one overflows.
+            '179769313486231580793728971405303415079934132710037' #...
+            '826936173778980444968292764750946649017977587207096' #...
+            '330286416692887910946555547851940402630657488671505' #...
+            '820681908902000708383676273854845817711531764475730' #...
+            '270069855571366959622842914819860834936475292719074' #...
+            '168444365510704342711559699508093042880177904174497792',
+            # 2**1024 - 2**970 - tiny
+            '179769313486231580793728971405303415079934132710037' #...
+            '826936173778980444968292764750946649017977587207096' #...
+            '330286416692887910946555547851940402630657488671505' #...
+            '820681908902000708383676273854845817711531764475730' #...
+            '270069855571366959622842914819860834936475292719074' #...
+            '168444365510704342711559699508093042880177904174497791.999',
+            # 2**1024 - 2**970 + tiny
+            '179769313486231580793728971405303415079934132710037' #...
+            '826936173778980444968292764750946649017977587207096' #...
+            '330286416692887910946555547851940402630657488671505' #...
+            '820681908902000708383676273854845817711531764475730' #...
+            '270069855571366959622842914819860834936475292719074' #...
+            '168444365510704342711559699508093042880177904174497792.001',
+            # 1 - 2**-54, +-tiny
+            '999999999999999944488848768742172978818416595458984375e-54',
+            '9999999999999999444888487687421729788184165954589843749999999e-54',
+            '9999999999999999444888487687421729788184165954589843750000001e-54',
             ]
         for s in test_strings:
             self.check_strtod(s)