Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.arch > #6589

Re: Indirect Bit

Path csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!news.glorb.com!npeer02.iad.highwinds-media.com!news.highwinds-media.com!feed-me.highwinds-media.com!nx02.iad01.newshosting.com!newshosting.com!216.196.98.144.MISMATCH!border3.nntp.dca.giganews.com!Xl.tags.giganews.com!border1.nntp.dca.giganews.com!nntp.giganews.com!local2.nntp.dca.giganews.com!news.giganews.com.POSTED!not-for-mail
NNTP-Posting-Date Sat, 21 Apr 2012 00:43:40 -0500
Message-ID <4F924908.6040908@SPAM.comp-arch.net> (permalink)
Date Fri, 20 Apr 2012 22:43:36 -0700
From "Andy (Super) Glew" <andy@SPAM.comp-arch.net>
Reply-To andy@SPAM.comp-arch.net
Organization comp-arch.net
User-Agent Mozilla/5.0 (Windows NT 6.1; WOW64; rv:11.0) Gecko/20120327 Thunderbird/11.0.1
MIME-Version 1.0
Newsgroups comp.arch
Subject Re: Indirect Bit
References <4F84FB1E.6CD1FC88@sonic.net> <1454560.470.1334600502251.JavaMail.geo-discussion-forums@ynbi17> <jmhor8$7pi$1@needham.csi.cam.ac.uk> <b992445b-d51c-4c5b-acb2-2ffcd5b4e7db@t7g2000pbp.googlegroups.com> <jmon4p$u8k$1@needham.csi.cam.ac.uk> <16603280.18.1334860198726.JavaMail.geo-discussion-forums@ynnf1> <1Bikr.12248$mL3.10431@newsfe23.iad>
In-Reply-To <1Bikr.12248$mL3.10431@newsfe23.iad>
Content-Type text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding 7bit
Lines 61
X-Usenet-Provider http://www.giganews.com
X-Trace sv3-BCxRdX7BBt4zw3c9qjOctvWPELxGWpmMGUriuD8FwCW6TlqkNiteKxaS6teSoNoYd5oZh2mAPCKkZnL!xWeykxG3xzHIP5Ocd1FG0CJwtbWf9q7di9gn5PzcK2zHIcZeiZMT757TtcFa
X-Complaints-To abuse@giganews.com
X-DMCA-Notifications http://www.giganews.com/info/dmca.html
X-Abuse-and-DMCA-Info Please be sure to forward a copy of ALL headers
X-Abuse-and-DMCA-Info Otherwise we will be unable to process your complaint properly
X-Postfilter 1.3.40
X-Original-Bytes 3680
X-Received-Bytes 3888
Xref csiph.com comp.arch:6589

Show key headers only | View raw

On 4/20/2012 12:14 PM, EricP wrote:
> MitchAlsup wrote:
>>
>> For todays lesson in FP optimizations: consider the following::
>>
>> x = 3.0/7.0
>> if( x == 3.0/7.0 ) blah();
>>
>> A) Is it legal for the optimizer to compute the 3.0/7.0 for the
>> assignment statement at compile time?
>> B) is it legal for the optimizer to compile the 3.0/7.0 for the
>> if-statement at compile time?
>> C) is it legal for the optimizer to dead-code-eliminate the if-
>> statement leaving only the call to blah() ?
>>
>> {Leaving out the falacy of EVER comparing FP numbers for exactness}
>>
>> Answers:
>> A) yes
>> B) no
>> C) no
>>
>> This snippet of code is designed to determine if arithmetic operations
>> are carried out with more precision in registers than is store in memory.
>> For machines that have more precision in regsiters than in memory, blah
>> is never called.
>>
>> Mitch
>
> Isn't this occurring because of silent type conversions,
> and perhaps indicative of a compiler bug?
>
> It seems to me that if the compiler supported and
> x was declared long double (80 bit float),
> and 3.0 and 7.0 were converted to long double type,
> and any intermediate values that had to be spilled
> from registers were saved as long doubles,
> that it would all work as expected (A=yes, B=yes, C=yes).
>
> The weirdness is because there is a silent conversion to
> double (64 bits) on save, then back to long double on load.
>
> In other words, its not the extended precision that is the problem,
> rather an inappropriate usage of it.
>
> Or is something else afoot?
>
> Eric

The problem was that on the old x87 floating point, there were always 
silent type conversions.

Normally all operations were done in 80 bit precision, and converted 
to/from 32 or 64 only when moved to/from memory.

Now, x87 has a precision control, PC, with can be set so that the 
register operations were 32 or 64 bit rather than 80 bit.  But (1) PC is 
slow to change, and (2) PC only affected mantissa bits, not exponent. 
So, no matter what, there was a silent type conversion, to a register 
type with the same number of mantissa bits, but more exponent bits.

Back to comp.arch | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Indirect Bit Mark Thorson <nospam@sonic.net> - 2012-04-10 19:31 -0800
  Re: Indirect Bit Robert Wessel <robertwessel2@yahoo.com> - 2012-04-11 01:29 -0500
    Re: Indirect Bit sarr.blumson@alum.dartmouth.org - 2012-04-11 14:48 +0000
      Re: Indirect Bit mac <acolvin@efunct.com> - 2012-04-12 14:43 +0000
    Re: Indirect Bit Morten Reistad <first@last.name> - 2012-04-22 15:19 +0200
      Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-22 23:24 -0700
  Re: Indirect Bit Terje Mathisen <"terje.mathisen at tmsw.no"> - 2012-04-11 08:32 +0200
    Re: Indirect Bit Michael S <already5chosen@yahoo.com> - 2012-04-11 05:49 -0700
      Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-12 21:22 -0700
        Re: Indirect Bit "Paul A. Clayton" <paaronclayton@gmail.com> - 2012-04-13 08:14 -0700
          Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-13 16:13 -0700
            Re: Indirect Bit "Paul A. Clayton" <paaronclayton@gmail.com> - 2012-04-13 20:01 -0700
              Re: Indirect Bit Andrew Reilly <areilly.---@bigpond.net.au> - 2012-04-14 03:16 +0000
                Re: Indirect Bit "Paul A. Clayton" <paaronclayton@gmail.com> - 2012-04-14 08:14 -0700
        Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-13 16:05 -0700
          Re: Indirect Bit MitchAlsup <MitchAlsup@aol.com> - 2012-04-13 17:01 -0700
            Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-14 13:49 -0700
          Re: Indirect Bit Anne & Lynn Wheeler <lynn@garlic.com> - 2012-04-13 20:07 -0400
          Re: Indirect Bit Terje Mathisen <"terje.mathisen at tmsw.no"> - 2012-04-14 18:47 +0200
          Re: Indirect Bit Morten Reistad <first@last.name> - 2012-04-22 15:26 +0200
            Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-22 23:25 -0700
              Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-23 09:18 +0100
                Re: Indirect Bit EricP <ThatWouldBeTelling@thevillage.com> - 2012-04-23 19:35 -0400
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-24 08:37 +0100
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-24 09:42 +0100
                Re: Indirect Bit Michael S <already5chosen@yahoo.com> - 2012-04-24 01:58 -0700
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-24 11:26 +0100
                Re: Indirect Bit Michael S <already5chosen@yahoo.com> - 2012-04-24 05:02 -0700
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-24 13:58 +0100
                Re: Indirect Bit Michael S <already5chosen@yahoo.com> - 2012-04-24 10:07 -0700
                Re: Indirect Bit Michael S <already5chosen@yahoo.com> - 2012-04-24 04:30 -0700
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-24 13:01 +0100
  Re: Indirect Bit Stefan Monnier <monnier@iro.umontreal.ca> - 2012-04-11 10:03 -0400
  Re: Indirect Bit kenney@cix.compulink.co.uk - 2012-04-11 09:22 -0500
    Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-12 14:11 -0700
      Re: Indirect Bit Stefan Monnier <monnier@iro.umontreal.ca> - 2012-04-12 18:05 -0400
        Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-12 20:13 -0700
      Re: Indirect Bit Mark Thorson <nospam@sonic.net> - 2012-04-12 16:55 -0800
        Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-12 17:25 -0700
          Re: Indirect Bit Robert Wessel <robertwessel2@yahoo.com> - 2012-04-12 20:03 -0500
          Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-12 21:33 -0700
            Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-13 15:57 -0700
              Re: Indirect Bit MitchAlsup <MitchAlsup@aol.com> - 2012-04-13 17:03 -0700
        Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-12 20:14 -0700
  Re: Indirect Bit Anne & Lynn Wheeler <lynn@garlic.com> - 2012-04-11 10:35 -0400
  Re: Indirect Bit MitchAlsup <MitchAlsup@aol.com> - 2012-04-11 08:54 -0700
  Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-12 07:55 -0700
    Re: Indirect Bit MitchAlsup <MitchAlsup@aol.com> - 2012-04-12 10:16 -0700
      Re: Indirect Bit jacko <jackokring@gmail.com> - 2012-04-12 11:07 -0700
        Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-12 21:34 -0700
      Re: Indirect Bit Stephen Fuld <SFuld@alumni.cmu.edu.invalid> - 2012-04-12 11:35 -0700
        Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-12 21:41 -0700
          Re: Indirect Bit jacko <jackokring@gmail.com> - 2012-04-13 14:52 -0700
          Re: Indirect Bit Stephen Fuld <SFuld@alumni.cmu.edu.invalid> - 2012-04-15 09:24 -0700
            Re: Indirect Bit Terje Mathisen <"terje.mathisen at tmsw.no"> - 2012-04-15 22:45 +0200
              Re: Indirect Bit Stephen Fuld <SFuld@alumni.cmu.edu.invalid> - 2012-04-15 18:04 -0700
      Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-12 21:12 -0700
    Re: Indirect Bit timcaffrey@aol.com (Tim McCaffrey) - 2012-04-12 19:23 +0000
    Re: Indirect Bit Terje Mathisen <"terje.mathisen at tmsw.no"> - 2012-04-13 08:54 +0200
      Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-14 14:37 -0700
    Re: Indirect Bit Andrew Reilly <areilly.---@bigpond.net.au> - 2012-04-15 23:42 +0000
      Re: Indirect Bit "Paul A. Clayton" <paaronclayton@gmail.com> - 2012-04-16 08:08 -0700
        Re: Indirect Bit Andrew Reilly <areilly.---@bigpond.net.au> - 2012-04-16 23:38 +0000
          Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-16 21:29 -0700
      Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-16 21:25 -0700
  Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-12 14:05 -0700
    Re: Indirect Bit ChrisQ <blackhole@devnull.com> - 2012-04-14 10:12 +0000
      Re: Indirect Bit "Paul A. Clayton" <paaronclayton@gmail.com> - 2012-04-14 06:25 -0700
        Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-14 14:50 +0100
          Re: Indirect Bit ChrisQ <blackhole@devnull.com> - 2012-04-14 18:01 +0000
            Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-14 14:00 -0700
              Re: Indirect Bit Anne & Lynn Wheeler <lynn@garlic.com> - 2012-04-14 20:58 -0400
            Re: Indirect Bit "Paul A. Clayton" <paaronclayton@gmail.com> - 2012-04-16 09:57 -0700
            Re: Indirect Bit timcaffrey@aol.com (Tim McCaffrey) - 2012-04-20 23:19 +0000
          Re: Indirect Bit "Paul A. Clayton" <paaronclayton@gmail.com> - 2012-04-16 08:50 -0700
            Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-16 18:09 +0100
              Re: Indirect Bit Andrew Reilly <areilly.---@bigpond.net.au> - 2012-04-16 23:19 +0000
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-17 09:20 +0100
                Re: Indirect Bit Terje Mathisen <"terje.mathisen at tmsw.no"> - 2012-04-17 12:49 +0200
                Re: Indirect Bit Andrew Reilly <areilly.---@bigpond.net.au> - 2012-04-17 12:40 +0000
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-18 12:01 +0100
                Re: Indirect Bit anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2012-04-17 12:34 +0000
                Re: Indirect Bit MitchAlsup <MitchAlsup@aol.com> - 2012-04-17 09:10 -0700
                Re: Indirect Bit Andrew Reilly <areilly.---@bigpond.net.au> - 2012-04-17 22:05 +0000
                Re: Indirect Bit jgk@panix.com (Joe keane) - 2012-04-19 19:37 +0000
                Re: Indirect Bit torbenm@diku.dk (Torben Ægidius Mogensen) - 2012-04-20 11:10 +0200
                Re: Indirect Bit Terje Mathisen <"terje.mathisen at tmsw.no"> - 2012-04-20 11:24 +0200
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-20 15:01 +0100
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-20 15:03 +0100
                Re: Indirect Bit Michael S <already5chosen@yahoo.com> - 2012-04-20 08:01 -0700
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-20 16:35 +0100
                Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-18 11:10 -0700
                Re: Indirect Bit MitchAlsup <MitchAlsup@aol.com> - 2012-04-18 15:37 -0700
          Re: Indirect Bit MitchAlsup <MitchAlsup@aol.com> - 2012-04-16 11:21 -0700
            Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-16 19:36 +0100
              Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-18 11:17 -0700
                Re: Indirect Bit MitchAlsup <MitchAlsup@aol.com> - 2012-04-18 15:33 -0700
                Re: Indirect Bit Andreas Eder <andreas_eder@gmx.net> - 2012-04-19 00:29 +0200
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-19 10:50 +0100
                Re: Indirect Bit MitchAlsup <MitchAlsup@aol.com> - 2012-04-19 11:29 -0700
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-19 21:39 +0100
                Re: Indirect Bit EricP <ThatWouldBeTelling@thevillage.com> - 2012-04-20 15:14 -0400
                Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-20 22:43 -0700
                Re: Indirect Bit EricP <ThatWouldBeTelling@thevillage.com> - 2012-04-21 17:14 -0400
                Re: Indirect Bit Robert Wessel <robertwessel2@yahoo.com> - 2012-04-21 19:47 -0500
                Re: Indirect Bit Nomen Nescio <nobody@dizum.com> - 2012-04-22 10:45 +0200
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-22 09:57 +0100
                Re: Indirect Bit Terje Mathisen <"terje.mathisen at tmsw.no"> - 2012-04-22 12:54 +0200
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-22 13:00 +0100
                Re: Indirect Bit Nomen Nescio <nobody@dizum.com> - 2012-04-22 13:28 +0200
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-22 13:04 +0100
                Re: Indirect Bit Fritz Wuehler <fritz@spamexpire-201204.rodent.frell.theremailer.net> - 2012-04-22 23:00 +0200
                Re: Indirect Bit Anne & Lynn Wheeler <lynn@garlic.com> - 2012-04-22 18:05 -0400
                Re: Indirect Bit glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-04-22 23:20 +0000
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-23 08:22 +0100
                Re: Indirect Bit Michael S <already5chosen@yahoo.com> - 2012-04-22 03:56 -0700
                Re: Indirect Bit EricP <ThatWouldBeTelling@thevillage.com> - 2012-04-22 12:28 -0400
                Re: Indirect Bit Robert Wessel <robertwessel2@yahoo.com> - 2012-04-22 12:56 -0500
                Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-22 13:15 -0700
                Re: Indirect Bit Nomen Nescio <nobody@dizum.com> - 2012-04-22 22:30 +0200
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-22 21:40 +0100
                Re: Indirect Bit Robert Wessel <robertwessel2@yahoo.com> - 2012-04-22 17:15 -0500
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-23 08:26 +0100
                Re: Indirect Bit Stephen Fuld <SFuld@alumni.cmu.edu.invalid> - 2012-04-23 08:02 -0700
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-23 17:02 +0100
                Re: Indirect Bit Fritz Wuehler <fritz@spamexpire-201204.rodent.frell.theremailer.net> - 2012-04-23 20:17 +0200
                Re: Indirect Bit Robert Wessel <robertwessel2@yahoo.com> - 2012-04-23 13:07 -0500
                Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-22 13:16 -0700
                Re: Indirect Bit MitchAlsup <MitchAlsup@aol.com> - 2012-04-23 13:14 -0700
                Re: Indirect Bit Anne & Lynn Wheeler <lynn@garlic.com> - 2012-04-23 16:51 -0400
                Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-23 14:22 -0700
                Re: Indirect Bit MitchAlsup <MitchAlsup@aol.com> - 2012-04-21 11:17 -0700
        Re: Indirect Bit jacko <jackokring@gmail.com> - 2012-04-14 08:02 -0700

csiph-web