signed division in hash_search_with_hash_value(ENTER) has high overhead

Hi,

During several profile runs I've seen the division in
if (action == HASH_ENTER || action == HASH_ENTER_NULL)
{
/*
* Can't split if running in partitioned mode, nor if frozen, nor if
* table is the subject of any active hash_seq_search scans. Strange
* order of these tests is to try to check cheaper conditions first.
*/
if (!IS_PARTITIONED(hctl) && !hashp->frozen &&
hctl->freeList[0].nentries / (long) (hctl->max_bucket + 1) >= hctl->ffactor &&
!has_seq_scans(hashp))
(void) expand_table(hashp);
}
taking up significant amounts of time. Which is not particularly
surprising: A signed 64 integer division (which is what we're dealing
with here) takes up to ~100 cycles, is microcoded, and prevents
instruction level parallelism.

We could cast to unsigned long - which would be faster - but it seems
like it'd be better to compute a threshold in
init_htab()/expand_table(), and make that && hctl->freeList[0].nentries >= hctl.next_expansion
or somesuch.

I don't plan to do that, but I wanted to document it, should somebody
else be motivated to look into this.

Greetings,

Andres Freund