From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>
Cc: andres@anarazel.de
Cc: thomas.munro@enterprisedb.com
Cc: andrew@tao11.riddles.org.uk
Cc: david.rowley@2ndquadrant.com
Cc: pgsql-hackers@lists.postgresql.org
Subject: Re: Using POPCNT and other advanced bit manipulation instructions
Date: Fri, 15 Feb 2019 13:04:12 -0300
Message-ID: <20190215160412.GA7354@alvherre.pgsql> (raw)
In-Reply-To: <986.1550243138@sss.pgh.pa.us>
On 2019-Feb-15, Tom Lane wrote:
> Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> > Ah, I understand it now:
> > https://stackoverflow.com/questions/25683690/confusion-about-bsr-and-lzcnt/43443701#43443701
> > 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. https://stackoverflow.com/a/28803917/242383
I did find these articles:
http://danluu.com/assembly-intrinsics/https://stackoverflow.com/questions/25078285/replacing-a-32-bit-loop-counter-with-64-bit-introduces-...
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 https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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To: pgsql-hackers@postgresql.org
Cc: alvherre@2ndquadrant.com, tgl@sss.pgh.pa.us, horiguchi.kyotaro@lab.ntt.co.jp, andres@anarazel.de, thomas.munro@enterprisedb.com, andrew@tao11.riddles.org.uk, david.rowley@2ndquadrant.com, pgsql-hackers@lists.postgresql.org
Subject: Re: Using POPCNT and other advanced bit manipulation instructions
In-Reply-To: <20190215160412.GA7354@alvherre.pgsql>
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