Date: Sun, 12 Aug 2012 23:10:24 -0000 From: Bruce Evans <brde@optusnet.com.au> To: Stephen Montgomery-Smith <stephen@missouri.edu> Cc: Diane Bruce <db@db.net>, Bruce Evans <brde@optusnet.com.au>, John Baldwin <jhb@freebsd.org>, David Chisnall <theraven@freebsd.org>, Bruce Evans <bde@freebsd.org>, Steve Kargl <sgk@troutmask.apl.washington.edu>, David Schultz <das@freebsd.org>, Peter Jeremy <peter@rulingia.com>, Warner Losh <imp@bsdimp.com> Subject: Re: Use of C99 extra long double math functions after r236148 Message-ID: <20120721122309.R856@besplex.bde.org> Resent-Message-ID: <20120812231016.GV20453@server.rulingia.com> In-Reply-To: <5009BB46.3050001@missouri.edu> References: <20120714120432.GA70706@server.rulingia.com> <20120717084457.U3890@besplex.bde.org> <5004A5C7.1040405@missouri.edu> <5004DEA9.1050001@missouri.edu> <20120717200931.U6624@besplex.bde.org> <5006D13D.2080702@missouri.edu> <20120718205625.GA409@troutmask.apl.washington.edu> <500725F2.7060603@missouri.edu> <20120719025345.GA1376@troutmask.apl.washington.edu> <50077987.1080307@missouri.edu> <20120719032706.GA1558@troutmask.apl.washington.edu> <5007826D.7060806@missouri.edu> <5007AD41.9070000@missouri.edu> <20120719205347.T2601@besplex.bde.org> <50084322.7020401@missouri.edu> <20120720035001.W4053@besplex.bde.org> <50085441.4090305@missouri.edu> <20120720162953.N2162@besplex.bde.org> <20120720184114.B2790@besplex.bde.org> <50095CDE.4050507@missouri.edu> <20120721011112.D5008@besplex.bde.org> <5009BB46.3050001@missouri.edu>
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On Fri, 20 Jul 2012, Stephen Montgomery-Smith wrote: > On 07/20/2012 11:25 AM, Bruce Evans wrote: >> On Fri, 20 Jul 2012, Stephen Montgomery-Smith wrote: > > >> % x0 = (float)x; >> % x1 = x - x0; >> % y0 = (float)y; >> % y1 = y - y0; >> >> A good way to do the hi+lo decompositions. > > That was the way I tried first. But it didn't work for me! > > But I see you changed things further down, so that is probably why it works > for you. I didn't understand what was happening before, but think I can explain it now: - the above gives correct hi+lo decompositions. Both hi and lo are usually nonzero. The code below did't really understand hi+lo decompositions, and often increases the final error (relative to naive code). - your code often gives null but backwards hi+lo decompositions, with hi = 0 and lo = full value. The code below did't really understand hi+lo decompositions. But when hi = 0, it is especially easy to add and multiply it exactly, so the final error isn't increased so often. Bruce
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