From: Tom Lane <tgl@sss.pgh.pa.us>
To: Noah Misch <noah@leadboat.com>
Cc: David Rowley <dgrowleyml@gmail.com>
Cc: PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Recent failures on buildfarm member hornet
Date: Wed, 07 Oct 2020 18:22:04 -0400
Message-ID: <2399356.1602109324@sss.pgh.pa.us> (raw)
In-Reply-To: <20201007221221.GC1498955@rfd.leadboat.com>
References: <1830491.1601944935@sss.pgh.pa.us>
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<2040113.1602035809@sss.pgh.pa.us>
<20201007021941.GD1422897@rfd.leadboat.com>
<2233910.1602090798@sss.pgh.pa.us>
<CAApHDvovu2O+SLerPK2fCqKKP-q6NWinzACvJkMQ9zZNEiCKJg@mail.gmail.com>
<20201007212056.GB1498955@rfd.leadboat.com>
<2397925.1602107299@sss.pgh.pa.us>
<2398543.1602108196@sss.pgh.pa.us>
<20201007221221.GC1498955@rfd.leadboat.com>
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
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Cc: tgl@sss.pgh.pa.us, noah@leadboat.com, dgrowleyml@gmail.com, pgsql-hackers@lists.postgresql.org
Subject: Re: Recent failures on buildfarm member hornet
In-Reply-To: <2399356.1602109324@sss.pgh.pa.us>
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