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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
Cc: pgsql-hackers@lists.postgresql.org, Thomas Munro <thomas.munro@enterprisedb.com>
Cc: Andrew Gierth <andrew@tao11.riddles.org.uk>
Cc: Alexander Kuzmenkov <a.kuzmenkov@postgrespro.ru>
Subject: Re: Performance improvements for src/port/snprintf.c
Date: Fri, 05 Oct 2018 11:54:59 -0400
Message-ID: <30726.1538754899@sss.pgh.pa.us> (raw)
In-Reply-To: <6797.1538592539@sss.pgh.pa.us>
References: <20181003161454.ozeioucsimjg5g6m@alap3.anarazel.de>
	<27041.1538583733@sss.pgh.pa.us>
	<20181003163717.oycshpp2hnctvtfj@alap3.anarazel.de>
	<28321.1538585692@sss.pgh.pa.us>
	<20181003170043.thdyezozi343dkuc@alap3.anarazel.de>
	<2132.1538587869@sss.pgh.pa.us>
	<20181003173639.tncawup2nnkp7vbv@alap3.anarazel.de>
	<2435.1538588403@sss.pgh.pa.us>
	<20181003174513.wxjxfj7eepytnlqa@alap3.anarazel.de>
	<3316.1538589695@sss.pgh.pa.us>
	<20181003183552.xaams6fau3i7oybj@alap3.anarazel.de>
	<6797.1538592539@sss.pgh.pa.us>

Andres Freund <andres@anarazel.de> writes:
> [ let's use strfromd ]

So I'm having second thoughts about this, based on the fact that
strfromd() in't strictly a glibc-ism but is defined in an ISO/IEC
standard.  That means that we can expect to see it start showing up
on other platforms (though a quick search did not find any evidence
that it has yet).  And that means that we'd better consider
quality-of-implementation issues.  We know that glibc's version is
fractionally faster than using sprintf with "%.*g", but what are
the odds that that will be true universally?  I don't have a warm
feeling about it, given that strfromd's API isn't a very good impedance
match to what we really need.

I really think that what we ought to do is apply the float[48]out hack
I showed in <30551.1538517271@sss.pgh.pa.us> and call it good, at least
till such time as somebody wants to propose a full-on reimplementation of
float output.  I don't want to buy back into having platform dependencies
in this area after having just expended a lot of sweat to get rid of them.

			regards, tom lane




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