Reduce cutoff for calling the new implementation from 3.5M to 2M.

It could probably be reduced more as-is, and more still with more
fine-tuning. This was remarkably easy: it just required inner()
to let Python convert much larger strings to ints, so large that
the existing Karatsuba int(str) usually gets invoked.

Lib/_pylong.py

@@ -269,7 +269,12 @@ def inner(a, b):
 del defaultdict
 
 def _dec_str_to_int_inner(s, *, GUARD=8):
-    BYTELIM = 512
+    # Yes, BYTELIM is "large". Large enough that CPython will usually
+    # use the Karatsuba _str_to_int_inner to convert the string. This
+    # allowed reducing the cutoff for calling _this_ function from 3.5M
+    # to 2M digits. We could almost certainly do even better by
+    # fine-tuning this and/or using a larger output base than 256.
+    BYTELIM = 100_000
     D = decimal.Decimal
     result = bytearray()
     # See notes at end of file for discussion of GUARD.
@@ -389,7 +394,7 @@ def int_from_string(s):
     # contain underscores and have trailing whitespace.
     s = s.rstrip().replace('_', '')
     func = _str_to_int_inner
-    if len(s) >= 3_500_000 and _decimal is not None:
+    if len(s) >= 2_000_000 and _decimal is not None:
         func = _dec_str_to_int_inner
     return func(s)
 

Lib/test/test_int.py

@@ -938,8 +938,8 @@ def test_whitebox_dec_str_to_int_inner_failsafe(self):
         # wrong about that. We have no input that reaches that block.
         # Here we test a contrived input that _does_ reach that block,
         # provided the number of guard digits is reduced to 1.
-        sn = "6" * (4000000 - 1)
-        n = (10**len(sn) - 1) // 9 * 6
+        sn = "9" * 2000156
+        n = 10**len(sn) - 1
         orig_spread = _pylong._spread.copy()
         _pylong._spread.clear()
         try: