Re: Using POPCNT and other advanced bit manipulation instructions

From: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>, andres(at)anarazel(dot)de, thomas(dot)munro(at)enterprisedb(dot)com, andrew(at)tao11(dot)riddles(dot)org(dot)uk, david(dot)rowley(at)2ndquadrant(dot)com, pgsql-hackers(at)lists(dot)postgresql(dot)org
Subject: Re: Using POPCNT and other advanced bit manipulation instructions
Date: 2019-02-15 16:04:12
Message-ID: 20190215160412.GA7354@alvherre.pgsql
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On 2019-Feb-15, Tom Lane wrote:

> Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> writes:
> > Ah, I understand it now:
> >
> > if you call LZCNT/TZCNT on a CPU that doesn't support it, it won't raise
> > SIGILL or anything ... it'll just silently compute the wrong result.
> > That's certainly not what I call a fallback!
> Yeah, that's pretty nasty; it means there's no backstop for whether
> your choose function gets it right :-(

Hopefully other tests will fail in some visible way, though. My fear is
whether we have such systems in buildfarm.

> Is POPCNT any better in this respect?

I couldn't find how is POPCNT encoded.

I did find these articles:

This suggests that this all a largely pointless exercise at least on
Intel and GCC/Clang. It may be better on AMD ... but to get really
better performance we'd need to be coding the popcnt calls in assembly
rather than using the compiler intrinsics, even with -mpopcnt, because
the intrinsics suck.

Álvaro Herrera
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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