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	Fri, 15 Feb 2019 10:05:38 -0500
From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
cc: Kyotaro HORIGUCHI <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: <20190215144123.GA31671@alvherre.pgsql>
References: <20190215144123.GA31671@alvherre.pgsql>
Comments: In-reply-to Alvaro Herrera <alvherre@2ndquadrant.com>
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Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> Ah, I understand it now:
> https://stackoverflow.com/questions/25683690/confusion-about-bsr-and-lzc=
nt/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 :-(

Is POPCNT any better in this respect?

			regards, tom lane