| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Vik Fearing <vik(dot)fearing(at)2ndquadrant(dot)com>, Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>, Stephen Frost <sfrost(at)snowman(dot)net>, Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>, Chapman Flack <chap(at)anastigmatix(dot)net>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: Greatest Common Divisor |
| Date: | 2020-01-04 00:10:01 |
| Message-ID: | 20200104001001.wzkk5nrea323hwyx@alap3.anarazel.de |
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| Lists: | pgsql-hackers |
Hi,
On 2020-01-03 18:49:18 -0500, Tom Lane wrote:
> On some older RISC architectures, integer division is really slow, like
> slower than floating-point. I'm not sure if that's true on any platform
> people still care about though. In recent years, CPU architects have been
> able to throw all the transistors they needed at such problems. On a
> machine with single-cycle divide, it's likely that the extra
> compare-and-branch is a net loss.
Which architecture has single cycle division? I think it's way above
that, based on profiles I've seen. And Agner seems to back me up:
https://www.agner.org/optimize/instruction_tables.pdf
That lists a 32/64 idiv with a latency of ~26/~42-95 cycles, even on a
moder uarch like skylake-x.
Greetings,
Andres Freund
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