Date: Wed, 7 Oct 2020 15:12:21 -0700
From: Noah Misch <noah@leadboat.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: David Rowley <dgrowleyml@gmail.com>,
	PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Recent failures on buildfarm member hornet
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On Wed, Oct 07, 2020 at 06:03:16PM -0400, Tom Lane wrote:
> I wrote:
> > I'm tempted to propose the attached small code rearrangement, which
> > might dissuade the compiler from thinking it can get away with this.

That patch does get hornet's -O2 build to again pass "make check".  It doesn't
harm the code, so we may as well use it.

> > While I concur with your point that an old xlc version might not be
> > that exciting, there could be other compilers doing the same thing
> > in the future.
> 
> After thinking about it a bit more, I'm not even convinced that what
> xlc seems to be doing is illegal per C spec.  There are no sequence
> points within
> 
>     return list_make2(list_concat(directargs, orderedargs),
>                       makeInteger(ndirectargs));

There is, however, a sequence point between list_length(directargs) and
list_concat(), and the problem arises because xlc reorders those two.  It's
true that makeInteger() could run before or after list_concat(), but that
alone would not have been a problem.