Re: Odd 9.4, 9.3 buildfarm failure on s390x

On Mon, Oct 01, 2018 at 05:11:02PM -0400, Tom Lane wrote:
> Andrew Dunstan <andrew(dot)dunstan(at)2ndquadrant(dot)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
>

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