Add divmod_threshold.py script.

Author: nascheme

Objects/longobject.c

@@ -4127,7 +4127,7 @@ l_divmod(PyLongObject *v, PyLongObject *w,
           "schoolbook" division is linear-time so don't use in that case.
           These limits are empirically determined and should be slightly
           conservative so that _pylong is used in cases it is likely
-          to be faster. */
+          to be faster. See Tools/scripts/divmod_threshold.py. */
         return pylong_int_divmod(v, w, pdiv, pmod);
     }
 #endif

Tools/scripts/divmod_threshold.py

@@ -0,0 +1,56 @@
+#!/usr/bin/env python3
+#
+# Determine threshold for switching from longobject.c divmod to
+# _pylong.int_divmod().
+
+from random import randrange
+from time import perf_counter as now
+from _pylong import int_divmod as divmod_fast
+
+BITS_PER_DIGIT = 30
+
+
+def rand_digits(n):
+    top = 1 << (n * BITS_PER_DIGIT)
+    return randrange(top >> 1, top)
+
+
+def probe_den(nd):
+    den = rand_digits(nd)
+    count = 0
+    for nn in range(nd, nd + 3000):
+        num = rand_digits(nn)
+        t0 = now()
+        e1, e2 = divmod(num, den)
+        t1 = now()
+        f1, f2 = divmod_fast(num, den)
+        t2 = now()
+        s1 = t1 - t0
+        s2 = t2 - t1
+        assert e1 == f1
+        assert e2 == f2
+        if s2 < s1:
+            count += 1
+            if count >= 3:
+                print(
+                    "for",
+                    nd,
+                    "denom digits,",
+                    nn - nd,
+                    "extra num digits is enough",
+                )
+                break
+        else:
+            count = 0
+    else:
+        print("for", nd, "denom digits, no num seems big enough")
+
+
+def main():
+    for nd in range(30):
+        nd = (nd + 1) * 100
+        probe_den(nd)
+
+
+if __name__ == '__main__':
+    main()