On 1/4/17 10:03 PM, David Rowley wrote:
I recall taht pow(x, 2) and x * x result usually in the same assembly
code, but pow() can never be more optimal than a simple
multiplication. So I'd think that it is wiser to avoid it in this code
path. Documentation is missing for the new replication command
SHOW_WAL_SEG. Actually, why not just having an equivalent of the SQL
command and be able to query parameter values?
This would probably be nicer written using a bitwise trick to ensure
that no lesser significant bits are set. If it's a power of 2, then
subtracting 1 should have all the lesser significant bits as 1, so
binary ANDing to that should be 0. i.e no common bits.

Something like:

/* ensure segment size is a power of 2 */
if ((wal_segment_size & (wal_segment_size - 1)) != 0)
   fprintf(stderr, _("%s: WAL segment size must be in the power of
2\n"), progname);

There's a similar trick in bitmapset.c for RIGHTMOST_ONE, so looks
like we already have assumptions about two's complement arithmetic

Well, now that there's 3 places that need to do almost the same thing, I think it'd be best to just centralize this somewhere. I realize that's not going to save any significant amount of code, but it would make it crystal clear what's going on (assuming the excellent comment above RIGHTMOST_ONE was kept).
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
Data in Trouble? Get it in Treble! http://BlueTreble.com
855-TREBLE2 (855-873-2532)

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