Re: How to eliminate redundant constant move instructions

From	George Neuner <gneuner2@comcast.net>
Newsgroups	comp.compilers
Subject	Re: How to eliminate redundant constant move instructions
Date	2011-11-02 12:38 -0400
Organization	A noiseless patient Spider
Message-ID	<11-11-014@comp.compilers> (permalink)
References	<11-10-019@comp.compilers> <11-11-004@comp.compilers> <11-11-005@comp.compilers>

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On Tue, 1 Nov 2011 22:35:04 +0000 (UTC), glen herrmannsfeldt
<gah@ugcs.caltech.edu> wrote:

>George Neuner <gneuner2@comcast.net> wrote:
>
>(big snip on optimization and register loads)
>
>> You might be asking "if the value already is in a register, why not
>> just use it rather than load a second register?"  The answer to that
>> likely is a scheduling issue which depends on the use of the first
>> register.  You have to remember that many CPUs can execute multiple
>> instructions in parallel, and those parallel instruction streams may
>> be executed out of order with respect to a program listing.
>
>That is what register renaming is for.  Usually using more than
>the architecturally specified number of registers, the CPU
>internally remaps the registers such that it can keep one value
>in a register while an instruction is being executed out of order.

Yes.  But the compiler can't count on register renaming ... it can see
only the architectural named registers.  If the code in question had
copied Rx-> Ry then renaming would have been possible, but instead the
code performed a constant load to each register.  No possibility of
rename sharing there.

George

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Thread

How to eliminate redundant constant move instructions "Amker.Cheng" <amker.cheng@gmail.com> - 2011-10-31 17:53 +0800
  Re: How to eliminate redundant constant move instructions Kaz Kylheku <kaz@kylheku.com> - 2011-10-31 17:08 +0000
    Re: How to eliminate redundant constant move instructions amker <can.finner@gmail.com> - 2011-11-01 19:01 -0700
    Re: How to eliminate redundant constant move instructions mac <acolvin@efunct.com> - 2011-11-03 02:20 +0000
  Re: How to eliminate redundant constant move instructions George Neuner <gneuner2@comcast.net> - 2011-11-01 14:32 -0400
    Re: How to eliminate redundant constant move instructions glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2011-11-01 22:35 +0000
      Re: How to eliminate redundant constant move instructions amker <can.finner@gmail.com> - 2011-11-01 19:35 -0700
      Re: How to eliminate redundant constant move instructions amker <amker.cheng@gmail.com> - 2011-11-01 21:04 -0700
      Re: How to eliminate redundant constant move instructions George Neuner <gneuner2@comcast.net> - 2011-11-02 12:38 -0400
        Re: How to eliminate redundant constant move instructions Kaz Kylheku <kaz@kylheku.com> - 2011-11-03 03:20 +0000
          Re: How to eliminate redundant constant move instructions George Neuner <gneuner2@comcast.net> - 2011-11-04 13:27 -0400
            Re: How to eliminate redundant constant move instructions glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2011-11-04 21:19 +0000
        Re: How to eliminate redundant constant move instructions glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2011-11-03 03:32 +0000
    Re: How to eliminate redundant constant move instructions amker <can.finner@gmail.com> - 2011-11-01 19:21 -0700
      Re: How to eliminate redundant constant move instructions George Neuner <gneuner2@comcast.net> - 2011-11-04 17:26 -0400
        Re: How to eliminate redundant constant move instructions amker <amker.cheng@gmail.com> - 2011-11-07 17:33 -0800
          Re: How to eliminate redundant constant move instructions Wei-Jen Chen <chenwj@cs.NCTU.edu.tw> - 2011-11-10 08:04 +0000
          Re: How to eliminate redundant constant move instructions George Neuner <gneuner2@comcast.net> - 2011-11-10 18:18 -0500
    Re: How to eliminate redundant constant move instructions amker <amker.cheng@gmail.com> - 2011-11-01 20:58 -0700

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