From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
cc: Dmitry Dolgov <9erthalion6@gmail.com>,
        Michael Paquier <michael.paquier@gmail.com>,
        Vincent Lachenal <vincent.lachenal@gmail.com>,
        PostgreSQL mailing lists <pgsql-bugs@postgresql.org>
Subject: Re: BUG #14897: Segfault on statitics SQL request
In-reply-to: <20171111174512.ikd3zrx2i5ssxpww@alap3.anarazel.de>
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Comments: In-reply-to Andres Freund <andres@anarazel.de>
	message dated "Sat, 11 Nov 2017 09:45:12 -0800"
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Date: Sat, 11 Nov 2017 12:54:29 -0500
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Andres Freund <andres@anarazel.de> writes:
> On 2017-11-11 12:41:40 -0500, Tom Lane wrote:
>> Yeah, changing MAXALIGN is out of the question.  I'm thinking about
>> another flag bit for MemoryContextAllocExtended.  Do we need to think
>> about other use-cases besides int128?  Should we just force 16-byte
>> alignment on all architectures, or does it need to be platform-specific?

> I'm not sure we want to
> a) Rely on one alignment being enough for everybody.
> b) Additionally burden already hot code paths with a growing number of
>    alignment flag tests, and the necessary math.

Well, (a) we could have more flag bits later if there are other use-cases
that require even stricter alignment, and (b) I do not believe that
MemoryContextAllocExtended is a hot code path at present; there are not
enough call sites.

> How about a MemoryContextAllocAligned(context, size, alignto, flags)
> that passes on most flags but adds the necessary overhead to size, and
> padds the result appropriately?

This'd result in hard-wiring the alignment requirement at call sites,
which I think might not be a great idea.  For example, one plausible
future use-case is "align on cacheline boundaries".  I think that would
be better served by a flag like MCXT_ALLOC_ALIGN_CACHELINE than by
having the callers demand a specific numeric alignment value --- it'd
be a lot easier to make the alignment match actual hardware requirements
if it were being inserted at one specific place.

			regards, tom lane


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