From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
cc: Ants Aasma <ants.aasma@eesti.ee>, Heikki Linnakangas <hlinnaka@iki.fi>,
        Tomas Vondra <tomas.vondra@2ndquadrant.com>,
        PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: WIP: Faster Expression Processing v4
In-reply-to: <31039.1490629925@sss.pgh.pa.us>
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Comments: In-reply-to Tom Lane <tgl@sss.pgh.pa.us>
	message dated "Mon, 27 Mar 2017 11:52:05 -0400"
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Date: Mon, 27 Mar 2017 12:18:37 -0400
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I wrote:
> As to the point of whether it actually helps or not ...
> on gcc 4.4.7 (RHEL 6), it makes things *WORSE*.  We go from about half of
> the dispatches getting routed through a common location, to *all* of them
> (except one; for some odd reason the first EEO_NEXT in EEOP_NULLIF
> survives as a separate jump).  This seems like a bug, but there it is.

So after a bit of googling, this is a very longstanding complaint:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=39284
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71785

(hm, think I know the second submitter)

I'm not sure we should be relying on a gcc bug fix that's barely four
months old.  Even assuming it fixes the issue without new regressions,
most people are not going to have it anytime soon.

My feeling at this point is that we might be better off disabling
the computed-goto case by default.  At the very least, we're going
to need a version check that restricts it to latest gcc.

			regards, tom lane


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