pgsql: Fix behavior of exp() and power() for infinity inputs.

On Sun, Jun 14, 2020 at 03:00:11PM +0000, Tom Lane wrote:

Fix behavior of exp() and power() for infinity inputs.

Previously, these functions tended to throw underflow errors for
negative-infinity exponents. The correct thing per POSIX is to
return 0, so let's do that instead. (Note that the SQL standard
is silent on such issues, as it lacks the concepts of either Inf
or NaN; so our practice is to follow POSIX whenever a corresponding
C-library function exists.)

Also, add a bunch of test cases verifying that exp() and power()
actually do follow POSIX for Inf and NaN inputs. While this patch
should guarantee that exp() passes the tests, power() will not unless
the platform's pow(3) is fully POSIX-compliant. I already know that
gaur fails some of the tests, and I am suspicious that the Windows
animals will too; the extent of compliance of other old platforms
remains to be seen. We might choose to drop failing test cases, or
to work harder at overriding pow(3) for these cases, but first let's
see just how good or bad the situation is.

The Windows servers don't actually cay anything here, but sidewinder
and hoverfly complain, on top of gaur of course:
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=hoverfly&dt=2020-06-14%2015%3A05%3A33
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=sidewinder&dt=2020-06-14%2018%3A45%3A28
--
Michael

Michael Paquier <michael@paquier.xyz> writes:

On Sun, Jun 14, 2020 at 03:00:11PM +0000, Tom Lane wrote:

Also, add a bunch of test cases verifying that exp() and power()
actually do follow POSIX for Inf and NaN inputs. While this patch
should guarantee that exp() passes the tests, power() will not unless
the platform's pow(3) is fully POSIX-compliant. I already know that
gaur fails some of the tests, and I am suspicious that the Windows
animals will too; the extent of compliance of other old platforms
remains to be seen. We might choose to drop failing test cases, or
to work harder at overriding pow(3) for these cases, but first let's
see just how good or bad the situation is.

The Windows servers don't actually cay anything here, but sidewinder
and hoverfly complain, on top of gaur of course:
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=hoverfly&amp;dt=2020-06-14%2015%3A05%3A33
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=sidewinder&amp;dt=2020-06-14%2018%3A45%3A28

Yeah, and presumably Noah's other AIX critters will fail too. I'm curious
to see what damselfly will say; that's the other old-ish platform we have
in the farm. Once that reports in, I'm intending to push the attached or
something close to it. This is more or less the same thing for pow() as
the current patch did for exp(): handle Inf cases manually and then
simplify the error checks in the normal path.

regards, tom lane

On Mon, Jun 15, 2020 at 12:10:52AM -0400, Tom Lane wrote:

Yeah, and presumably Noah's other AIX critters will fail too. I'm curious
to see what damselfly will say; that's the other old-ish platform we have
in the farm. Once that reports in, I'm intending to push the attached or
something close to it. This is more or less the same thing for pow() as
the current patch did for exp(): handle Inf cases manually and then
simplify the error checks in the normal path.

Thanks. What you have here looks consistent with what POSIX says:
https://pubs.opengroup.org/onlinepubs/9699919799/functions/pow.html
--
Michael

Michael Paquier <michael@paquier.xyz> writes:

On Mon, Jun 15, 2020 at 12:10:52AM -0400, Tom Lane wrote:

Yeah, and presumably Noah's other AIX critters will fail too. I'm curious
to see what damselfly will say; that's the other old-ish platform we have
in the farm. Once that reports in, I'm intending to push the attached or
something close to it. This is more or less the same thing for pow() as
the current patch did for exp(): handle Inf cases manually and then
simplify the error checks in the normal path.

For the record, damselfly did indeed fail:

https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=damselfly&amp;dt=2020-06-15%2011%3A23%3A34

So while it's pretty debatable whether we should care about obsolete
NetBSD or HPUX versions, we do have an issue on both AIX and Illumos,
which seems like sufficient reason to add the manual infinity handling.

Thanks. What you have here looks consistent with what POSIX says:
https://pubs.opengroup.org/onlinepubs/9699919799/functions/pow.html

Right, it's based directly on the POSIX rules.

One thing worth noting is that I changed what the code does if pow()
returns EDOM. That's probably a dead code path on any modern platform,
given that we already checked for invalid input. But if it did occur
I think we should raise the same "domain error" message we use up top.
There are two objections to the way the code stands:

1. It's effectively computing the result as though y is infinity,
which (after the inf additions I propose here) we know isn't so.
I think it makes more sense to decide "y isn't an integer even
though we thought it was" than "y is infinite even though we thought
it wasn't". Integer-ness, for float values too large to have any
fractional bits, is a squishy concept, but infinite-ness isn't.

2. If we did want to conclude y is infinite, we should be prepared to
return inf or 0 on that basis. What actually happens, given the following
error checks, is either an underflow error, an overflow error, or
returning 1.0. I don't think any of those is a sensible way of reporting
EDOM.

When that stanza of code was written, we had *none* of the special case
checks that are now done before invoking pow(), so it had to consider a
wider range of possibilities than it now does. I'm a little dubious that
it was OK even then, but it's definitely off the mark now.

regards, tom lane