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Re: Are rotating register files still a bad idea?

Message-ID <4F58CF5B.8070201@SPAM.comp-arch.net> (permalink)
Date 2012-03-08 07:25 -0800
From "Andy (Super) Glew" <andy@SPAM.comp-arch.net>
Organization comp-arch.net
Newsgroups comp.arch
Subject Re: Are rotating register files still a bad idea?
References (3 earlier) <4F2EF96A.1040107@SPAM.comp-arch.net> <ggtgp-7BA6C0.05364706022012@netnews.mchsi.com> <4F30907E.4020806@SPAM.comp-arch.net> <ggtgp-F4CD07.20021128022012@netnews.mchsi.com> <ggtgp-5C3492.22351707032012@netnews.mchsi.com>

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On 3/7/2012 8:35 PM, Brett Davis wrote:
> In article<ggtgp-F4CD07.20021128022012@netnews.mchsi.com>,
>   Brett Davis<ggtgp@yahoo.com>  wrote:
>
>> In article<4F30907E.4020806@SPAM.comp-arch.net>,
>>   "Andy (Super) Glew"<andy@SPAM.comp-arch.net>  wrote:
>>
>>> On 2/6/2012 3:36 AM, Brett Davis wrote:
>>>> In article<4F2EF96A.1040107@SPAM.comp-arch.net>,
>>>>    "Andy (Super) Glew"<andy@SPAM.comp-arch.net>   wrote:
>>>
> On the new Ivy Bridge chips just announced the MOV instructions no longer
> occupy an execution port. I think AMD has been doing this for years.
>
> This would seem to negate much of the need/use for register rotation.

This question was already partially answered: MOV elimination like Intel 
does (I wasn't aware that AMD was doing it - they weren't when I was 
there) consumes instruction fetch, decode, and retirement bandwidth.

>> You still have the decode cost using up one of your four decoders,
>> but the stated reason for using register rotation is for short loops
>> with a small number of instructions. This does not sound like a
>> situation where you are decode limited.
>>
>> If you are decode limited you are generally in a big loop with a large
>> count, where register rotation does not help as much.

Umm, quite the contrary: I've usually found it to be the smallest loops 
that are the closest to being limited by decode or graduation bandwidth. 
Perhaps I have spent too much time tweaking AXPY.

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Thread

Are rotating register files still a bad idea? Brett Davis <ggtgp@yahoo.com> - 2012-02-02 02:16 -0600
  Re: Are rotating register files still a bad idea? "Paul A. Clayton" <paaronclayton@gmail.com> - 2012-02-02 07:41 -0800
  Re: Are rotating register files still a bad idea? Nomen Nescio <nobody@dizum.com> - 2012-02-02 19:04 +0100
    Re: Are rotating register files still a bad idea? Brett Davis <ggtgp@yahoo.com> - 2012-02-02 23:16 -0600
      Re: Are rotating register files still a bad idea? Fritz Wuehler <fritz@spamexpire-201202.rodent.frell.theremailer.net> - 2012-02-03 18:19 +0100
        Re: Are rotating register files still a bad idea? Anne & Lynn Wheeler <lynn@garlic.com> - 2012-02-03 13:09 -0500
          Re: Are rotating register files still a bad idea? Stephen Fuld <SFuld@alumni.cmu.edu.invalid> - 2012-02-03 12:28 -0800
          Re: Are rotating register files still a bad idea? MitchAlsup <MitchAlsup@aol.com> - 2012-02-03 13:04 -0800
        Re: Are rotating register files still a bad idea? Thomas Womack <twomack@chiark.greenend.org.uk> - 2012-02-03 19:15 +0000
          Re: Are rotating register files still a bad idea? Mark Thorson <nospam@sonic.net> - 2012-02-03 13:08 -0800
            Re: Are rotating register files still a bad idea? Thomas Womack <twomack@chiark.greenend.org.uk> - 2012-02-03 22:21 +0000
              Re: Are rotating register files still a bad idea? Mark Thorson <nospam@sonic.net> - 2012-02-03 18:42 -0800
                Re: Are rotating register files still a bad idea? Glen Overby <coreSPAMsample@charter.net> - 2012-03-01 18:18 -0600
            Re: Are rotating register files still a bad idea? anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2012-02-04 13:26 +0000
            Re: Are rotating register files still a bad idea? Fritz Wuehler <fritz@spamexpire-201202.rodent.frell.theremailer.net> - 2012-02-05 00:58 +0100
          Re: Are rotating register files still a bad idea? Nomen Nescio <nobody@dizum.com> - 2012-02-04 21:00 +0100
            Re: Are rotating register files still a bad idea? Brett Davis <ggtgp@yahoo.com> - 2012-02-05 00:35 -0600
        Re: Are rotating register files still a bad idea? Quadibloc <jsavard@ecn.ab.ca> - 2012-02-03 10:11 -0800
  Re: Are rotating register files still a bad idea? jgk@panix.com (Joe keane) - 2012-02-03 23:20 +0000
  Re: Are rotating register files still a bad idea? "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-02-03 22:54 -0800
    Re: Are rotating register files still a bad idea? Brett Davis <ggtgp@yahoo.com> - 2012-02-04 07:15 -0600
      Re: Are rotating register files still a bad idea? "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-02-05 13:49 -0800
        Re: Are rotating register files still a bad idea? Brett Davis <ggtgp@yahoo.com> - 2012-02-06 05:36 -0600
          Re: Are rotating register files still a bad idea? "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-02-06 18:46 -0800
            Re: Are rotating register files still a bad idea? Brett Davis <ggtgp@yahoo.com> - 2012-02-28 20:02 -0600
              Re: Are rotating register files still a bad idea? Brett Davis <ggtgp@yahoo.com> - 2012-03-07 22:35 -0600
                Re: Are rotating register files still a bad idea? "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-03-08 07:25 -0800
        Re: Are rotating register files still a bad idea? anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2012-02-06 12:49 +0000
      Re: Are rotating register files still a bad idea? Quadibloc <jsavard@ecn.ab.ca> - 2012-02-04 11:14 -0800
  Re: Are rotating register files still a bad idea? Michael S <already5chosen@yahoo.com> - 2012-02-03 06:04 -0800
  Re: Are rotating register files still a bad idea? MitchAlsup <MitchAlsup@aol.com> - 2012-02-03 12:20 -0800

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