Re: BUG #15121: Multiple UBSAN errors

On 19 March 2018 at 19:20, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> writes:
>> On 03/19/2018 03:28 PM, Martin Liška wrote:
>>>>> Note that building postgresql with -03, I see some array tests failing.
>
>> I'm getting failures in errors, union and alter_table, but none of those
>> are related to arrays. So, which tests are failing for you and how do
>> the failures look like?
>
> I tried -O3 with gcc 7.3.1 (Fedora 26), and that passes check-world
> just fine. Then I tried -O3 with gcc 8.0.1 (prerelease Fedora 28),
> and indeed that's got some problems. It looks like array_out fails
> for multidimensional arrays, because all the diffs look about
> like this one:

I can confirm it's really caused by -O3 optimization level for the function:
array_out

If you give me couple of minutes, I will isolate why is that caused.

>
> *** 106,116 ****
> SET c[2:2] = '{"new_word"}'
> WHERE array_dims(c) is not null;
> SELECT a,b,c FROM arrtest;
> ! a | b | c
> ! ---------------+-----------------------+-------------------
> ! {16,25,3,4,5} | {{{113,142},{1,147}}} | {}
> ! {} | {3,4} | {foo,new_word}
> ! {16,25,23} | {{3,4},{4,5}} | {foobar,new_word}
> (3 rows)
>
> SELECT a[1:3],
> --- 106,116 ----
> SET c[2:2] = '{"new_word"}'
> WHERE array_dims(c) is not null;
> SELECT a,b,c FROM arrtest;
> ! a | b | c
> ! ---------------+---------------+-------------------
> ! {16,25,3,4,5} | {{ | {}
> ! {} | {3,4} | {foo,new_word}
> ! {16,25,23} | {{3,4},{4,5}} | {foobar,new_word}
> (3 rows)
>
> SELECT a[1:3],
>
> Note that 1-D and 2-D arrays print fine, it's only 3-D or deeper
> that print wrong. Very odd. Maybe it's bad code on our part,
> but I think the odds are at least as good that it's a new gcc bug.
>
> regards, tom lane