| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> |
| 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 15:05:38 |
| Message-ID: | 986.1550243138@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Alvaro Herrera <alvherre(at)2ndquadrant(dot)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 :-(
Is POPCNT any better in this respect?
regards, tom lane
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