On Mon, Oct 01, 2018 at 05:11:02PM -0400, Tom Lane wrote: > Andrew Dunstan <andrew.duns...@2ndquadrant.com> writes: > > On 10/01/2018 11:58 AM, Tom Lane wrote: > >> Oooh ... apparently, on that platform, memcmp() is willing to produce > >> INT_MIN in some cases. That's not a safe value for a sort comparator > >> to produce --- we explicitly say that somewhere, IIRC. I think we > >> implement DESC by negating the comparator's result, which explains > >> why only the DESC case fails. > > > Is there a standard that forbids this, or have we just been lucky up to now? > > We've been lucky; POSIX just says the value is less than, equal to, > or greater than zero. > > In practice, a memcmp that operates byte-at-a-time would not likely > return anything outside +-255. But on a big-endian machine you could > easily optimize to use word-wide operations to compare 4 bytes at a > time, and I suspect that's what's happening here. Or maybe there's > just some weird architecture-specific reason that makes it cheap > for them to return INT_MIN rather than some other value? > as a former S3[79]x assembler programmer, they probably do it in registers or using TRT. All of which could be word wise.
> regards, tom lane > -- Larry Rosenman http://www.lerctr.org/~ler Phone: +1 214-642-9640 E-Mail: l...@lerctr.org US Mail: 5708 Sabbia Drive, Round Rock, TX 78665-2106
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