On 2015-01-26 15:35:44 -0800, Peter Geoghegan wrote: > On Mon, Jan 26, 2015 at 3:12 PM, Andrew Gierth > <and...@tao11.riddles.org.uk> wrote: > > Obvious overheads in float8 comparison include having to check for NaN, > > and the fact that DatumGetFloat8 on 64bit doesn't get inlined and forces > > a store/load to memory rather than just using a register. Looking at > > those might be more beneficial than messing with abbreviations. > > Aren't there issues with the alignment of double precision floating > point numbers on x86, too? Maybe my information there is at least > partially obsolete. But it seems we'd have to control for this to be > sure.
I think getting rid of the function call for DatumGetFloat8() would be quite the win. On x86-64 the conversion then should amount to mov %rd?,-0x8(%rsp);movsd -0x8(%rsp),%xmm0 - that's pretty cheap. Both instructions have a cycle count of 1 + L1 access latency (4) + 2 because they use the same exection port. So it's about 12 fully pipelineable cycles. 2 if the pipeline can kept busy otherwise. I doubt that'd be noticeable if the conversion were inlined. Greetings, Andres Freund -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
