_PyLong_AsByteArray: Don't do the "delicate overflow" check unless it's

truly needed; usually saves a little time, but no change in semantics.

diff --git a/Objects/longobject.c b/Objects/longobject.c
index f8a912940a94415f4513c7adec38b407e076dfa9..44819701bbef8f69a92a3f56c5ecc7ea1cafb9b0 100644 (file)
--- a/Objects/longobject.c
+++ b/Objects/longobject.c
@@ -424,18 +424,27 @@ _PyLong_AsByteArray(PyLongObject* v,
                *p = (unsigned char)(accum & 0xff);
                p += pincr;
        }
-
-       /* Fill remaining bytes with copies of the sign bit. */
-       for ( ; j < n; ++j, p += pincr)
-               *p = (unsigned char)(do_twos_comp ? 0xff : 0);
-
-       /* Check for delicate overflow (not enough room for the sign bit). */
-       if (j > 0 && is_signed) {
+       else if (j == n && n > 0 && is_signed) {
+               /* The main loop filled the byte array exactly, so the code
+                  just above didn't get to ensure there's a sign bit, and the
+                  loop below wouldn't add one either.  Make sure a sign bit
+                  exists. */
                unsigned char msb = *(p - pincr);
-               int sign_bit_set = (msb & 0x80) != 0;
-               if (sign_bit_set != do_twos_comp)
+               int sign_bit_set = msb >= 0x80;
+               assert(accumbits == 0);
+               if (sign_bit_set == do_twos_comp)
+                       return 0;
+               else
                        goto Overflow;
        }
+
+       /* Fill remaining bytes with copies of the sign bit. */
+       {
+               unsigned char signbyte = do_twos_comp ? 0xffU : 0U;
+               for ( ; j < n; ++j, p += pincr)
+                       *p = signbyte;
+       }
+
        return 0;
 
 Overflow: