Re: [PATCH] Improve geometric types

From: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
To: robertmhaas(at)gmail(dot)com
Cc: emre(at)hasegeli(dot)com, a(dot)alekseev(at)postgrespro(dot)ru, pgsql-hackers(at)postgresql(dot)org
Subject: Re: [PATCH] Improve geometric types
Date: 2017-10-03 08:39:06
Message-ID: 20171003.173906.235933967.horiguchi.kyotaro@lab.ntt.co.jp
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At Mon, 2 Oct 2017 08:23:49 -0400, Robert Haas <robertmhaas(at)gmail(dot)com> wrote in <CA+TgmoYsgw0TcjJQ1CE_6vDOxgEhxYQkfNx93Mfwx23WOLM0NA(at)mail(dot)gmail(dot)com>
> On Mon, Oct 2, 2017 at 4:23 AM, Kyotaro HORIGUCHI
> <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
> > For other potential reviewers:
> >
> > I found the origin of the function here.
> >
> > https://www.postgresql.org/message-id/4A90BD76.7070804@netspace.net.au
> > https://www.postgresql.org/message-id/AANLkTim4cHELcGPf5w7Zd43_dQi_2RJ_b5_F_idSSbZI%40mail.gmail.com
> >
> > And the reason for pg_hypot is seen here.
> >
> > https://www.postgresql.org/message-id/407d949e0908222139t35ad3ad2q3e6b15646a27dd64@mail.gmail.com
> >
> > I think the replacement is now acceptable according to the discussion.
> > ======
>
> I think if we're going to do this it should be separated out as its
> own patch.

+1

> Also, I think someone should explain what the reasoning
> behind the change is. Do we, for example, foresee that the built-in
> code might be faster or less likely to overflow? Because we're
> clearly taking a risk -- most trivially, that the BF will break, or
> more seriously, that some machines will have versions of this function
> that don't actually behave quite the same.
>
> That brings up a related point. How good is our test case coverage
> for hypot(), especially in strange corner cases, like this one
> mentioned in pg_hypot()'s comment:
>
> * This implementation conforms to IEEE Std 1003.1 and GLIBC, in that the
> * case of hypot(inf,nan) results in INF, and not NAN.

I'm not sure how precise we practically need them to be
identical. FWIW as a rough confirmation on my box, I compared
hypot and pg_hypot for the following arbitrary choosed pairs of
parameters.

{2.2e-308, 2.2e-308},
{2.2e-308, 1.7e307},
{1.7e307, 1.7e307},
{1.7e308, 1.7e308},
{2.2e-308, DBL_MAX},
{1.7e308, DBL_MAX},
{DBL_MIN, DBL_MAX},
{DBL_MAX, DBL_MAX},
{1.7e307, INFINITY},
{2.2e-308, INFINITY},
{0, INFINITY},
{DBL_MIN, INFINITY},
{INFINITY, INFINITY},
{1, NAN},
{INFINITY, NAN},
{NAN, NAN},

Only the first pair gave slightly not-exactly-equal results but
it seems to do no harm. hypot set underflow flag.

0: hypot=3.111269837220809e-308 (== 0.0 is 0, < DBL_MIN is 0)
pg_hypot=3.11126983722081e-308 (== 0.0 is 0, < DBL_MIN is 0)
equal=0,
hypot(errno:0, inval:0, div0:0, of=0, uf=1),
pg_hypot(errno:0, inval:0, div0:0, of=0, uf=0)

But not sure how the both functions behave on other platforms.

> I'm potentially willing to commit a patch that just makes the
> pg_hypot() -> hypot() change and does nothing else, if there are not
> objections to that change, but I want to be sure that we'll know right
> away if that turns out to break.

--
Kyotaro Horiguchi
NTT Open Source Software Center

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