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Re: Re: [PATCH] unified frontend support for pg_malloc et al and palloc/pfree mulation (was xlogreader-v4)

From: Andres Freund <andres(at)2ndquadrant(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>,pgsql-hackers(at)postgresql(dot)org
Subject: Re: Re: [PATCH] unified frontend support for pg_malloc et al and palloc/pfree mulation (was xlogreader-v4)
Date: 2013-01-10 09:31:04
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On 2013-01-10 00:05:07 +0100, Andres Freund wrote:
> On 2013-01-09 17:28:35 -0500, Tom Lane wrote:
> > (We know this doesn't
> > matter, but gcc doesn't; this version of gcc apparently isn't doing much
> > with the knowledge that elog won't return.)
> Afaics one reason for that is that we don't have any such annotation for
> elog(), just for ereport. And I don't immediately see how it could be
> added to elog without relying on variadic macros. Bit of a shame, there
> probably a good number of callsites that could actually benefit from
> that knowledge.
> Is it worth making that annotation conditional on variadic macro
> support?

A quick test confirms that. With:

diff --git a/src/include/utils/elog.h b/src/include/utils/elog.h
index cbbda04..39cd809 100644
--- a/src/include/utils/elog.h
+++ b/src/include/utils/elog.h
@@ -212,7 +212,13 @@ extern int	getinternalerrposition(void);
  *		elog(ERROR, "portal \"%s\" not found", stmt->portalname);
+#ifdef __GNUC__
+#define elog(elevel, ...) elog_start(__FILE__, __LINE__, PG_FUNCNAME_MACRO), \
+		elog_finish(elevel, __VA_ARGS__),								\
+		((elevel) >= ERROR ? __builtin_unreachable() : (void) 0)
 #define elog	elog_start(__FILE__, __LINE__, PG_FUNCNAME_MACRO), elog_finish
 extern void elog_start(const char *filename, int lineno, const char *funcname);
 extern void

nearly the same code is generated for both variants (this is no
additionally saved registers and such).

Two unfinished things:
* the above only checks for gcc although all C99 compilers should
  support varargs macros
* It uses __builtin_unreachable() instead of abort() as that creates
  a smaller executable. That's gcc 4.5 only. Saves about 30kb in a
  both a stripped and unstripped binary.
* elog(ERROR) i.e. no args would require more macro ugliness, but that
  seems unneccesary

Doing the __builtin_unreachable() for ereport as well saves about 50kb.

Given that some of those elog/ereports are in performance critical code
paths it seems like a good idea to remove code from the
callsites. Adding configure check for __VA_ARGS__ and
__builtin_unreachable() should be simple enough?


Andres Freund

 Andres Freund	         
 PostgreSQL Development, 24x7 Support, Training & Services

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