Horton

Re: WIP: Faster Expression Processing v4

Tom Lane <tgl@sss.pgh.pa.us>

View thread View on postgr.es
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>
Date: 2017-03-27T16:18:37Z
Lists: pgsql-hackers 
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

Commits

  1. Improve performance of ExecEvalWholeRowVar.

  2. Remove unreachable code in expression evaluation.

  3. Faster expression evaluation and targetlist projection.

  4. Avoid syntax error on platforms that have neither LOCALE_T nor ICU.

  5. Add configure test to see if the C compiler has gcc-style computed gotos.

  6. Improve regression test coverage for TID scanning.

  7. Improve expression evaluation test coverage.

  8. Fix two errors with nested CASE/WHEN constructs.