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>,
        Andrew Gierth <andrew@tao11.riddles.org.uk>,
        Alexander Kuzmenkov <a.kuzmenkov@postgrespro.ru>
Subject: Re: Performance improvements for src/port/snprintf.c
In-reply-to: <20181003174513.wxjxfj7eepytnlqa@alap3.anarazel.de>
References: <20181003155207.b3lqmovuv2c5c4id@alap3.anarazel.de>
 <26510.1538582852@sss.pgh.pa.us>
 <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>
Comments: In-reply-to Andres Freund <andres@anarazel.de>
	message dated "Wed, 03 Oct 2018 10:45:13 -0700"
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="----- =_aaaaaaaaaa0"
Content-ID: <3172.1538589495.0@sss.pgh.pa.us>
Date: Wed, 03 Oct 2018 14:01:35 -0400
Message-ID: <3316.1538589695@sss.pgh.pa.us>
Precedence: bulk

------- =_aaaaaaaaaa0
Content-Type: text/plain; charset="us-ascii"
Content-ID: <3172.1538589495.1@sss.pgh.pa.us>

Andres Freund <andres@anarazel.de> writes:
> So when using pg's snprintf() to print a single floating point number
> with precision, we get nearly a 10% boost.

I just tested that using my little standalone testbed, and I failed
to replicate the result.  I do see that strfromd is slightly faster,
but it's just a few percent measuring snprintf.c in isolation --- in
the overall context of COPY, I don't see how you get to 10% net savings.

So I continue to think there's something fishy about your test case.

BTW, so far as I can tell on F28, strfromd isn't exposed without
"-D__STDC_WANT_IEC_60559_BFP_EXT__", which seems fairly scary;
what else does that affect?

			regards, tom lane


------- =_aaaaaaaaaa0
Content-Type: text/x-diff; name="hack-use-of-strfromd.patch";
	charset="us-ascii"
Content-ID: <3172.1538589495.2@sss.pgh.pa.us>
Content-Description: hack-use-of-strfromd.patch
Content-Transfer-Encoding: quoted-printable

diff --git a/src/port/snprintf.c b/src/port/snprintf.c
index b9b6add..f75369c 100644
--- a/src/port/snprintf.c
+++ b/src/port/snprintf.c
@@ -1137,17 +1137,19 @@ fmtfloat(double value, char type, int forcesign, i=
nt leftjust,
 		zeropadlen =3D precision - prec;
 		fmt[0] =3D '%';
 		fmt[1] =3D '.';
-		fmt[2] =3D '*';
-		fmt[3] =3D type;
-		fmt[4] =3D '\0';
-		vallen =3D sprintf(convert, fmt, prec, value);
+		fmt[2] =3D (prec / 100) + '0';
+		fmt[3] =3D ((prec % 100) / 10) + '0';
+		fmt[4] =3D (prec % 10) + '0';
+		fmt[5] =3D type;
+		fmt[6] =3D '\0';
+		vallen =3D strfromd(convert, sizeof(convert), fmt, value);
 	}
 	else
 	{
 		fmt[0] =3D '%';
 		fmt[1] =3D type;
 		fmt[2] =3D '\0';
-		vallen =3D sprintf(convert, fmt, value);
+		vallen =3D strfromd(convert, sizeof(convert), fmt, value);
 	}
 	if (vallen < 0)
 		goto fail;

------- =_aaaaaaaaaa0--