From: Tom Lane <tgl@sss.pgh.pa.us>
To: Noah Misch <noah@leadboat.com>
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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Comments: In-reply-to Noah Misch <noah@leadboat.com>
	message dated "Wed, 07 Oct 2020 15:12:21 -0700"
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Date: Wed, 07 Oct 2020 18:22:04 -0400
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Noah Misch <noah@leadboat.com> writes:
> On Wed, Oct 07, 2020 at 06:03:16PM -0400, Tom Lane wrote:
>> 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.

Yeah, that is the theory on which the existing code is built,
specifically that the list_length fetch must occur before list_concat
runs.  What I am wondering about is a more aggressive interpretation of
"sequence point", namely that the compiler is free to disregard exactly
when list_concat's side-effects occur between this statement's sequence
points.  I'm not sure that the C spec allows that interpretation, but
I'm not sure it doesn't, either.

			regards, tom lane