>There is something that I do not understand. You said that GCC didn't know >that _num_buckets was constant. In > that case, how did GCC know not to use a divide instruction when you tried > 65536?
The original code doesn't use _num_buckets directly when calculating the hash idx, it use a non-constant variable _size, a member of KlassInfoTable, which is assigned to _num_buckets. I guess it is the reason GCC didn't do the constant propagation in this case. And _size is not changed during the life of KlassInfoTable, it only set to zero at de-construction. In my experiment, I use _num_buckets directly instead of _size to calculate hash idx. So GCC provides the optimizations. I think your suggestion of using 65537 is valuable, And I agree that it is not easy to prove that non-prime for hash is better. So how about make the change to use constant _number_buckets directly for calculating hash idx, and leave the prime not changed. In my case, speed up JMap histo helps at avoid getting killed by a timer, especially when heap is large at ~200GB. Do you think it is reasonable to have a patch? Thanks, Lin -----Original Message----- From: Andrew Haley [mailto:a...@redhat.com] Sent: Tuesday, December 11, 2018 7:10 PM To: 臧琳 <zangl...@jd.com>; serviceability-dev@openjdk.java.net Subject: Re: optimize KlassInfoTable size to power of 2 On 12/11/18 11:05 AM, Andrew Haley wrote: > unsigned mod_m(unsigned n) { > unsigned tmp = n % 65536; > tmp -= n / 65536; > if (tmp >= 65537) // overflow > tmp += 65537; > return tmp; > } NB: this assumes that unsigned int is uint64_t. -- Andrew Haley Java Platform Lead Engineer Red Hat UK Ltd. <https://www.redhat.com> EAC8 43EB D3EF DB98 CC77 2FAD A5CD 6035 332F A671
