Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea.

From	Bernhard Schornak <schornak@web.de>
Newsgroups	alt.comp.lang.borland-delphi, alt.comp.lang.c, alt.lang.asm, comp.lang.python
Subject	Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea.
Date	2013-10-28 10:58 +0100
Organization	A noiseless patient Spider
Message-ID	<l4lcfc$824$2@dont-email.me> (permalink)
References	<6f2b7$525f2302$5419b3e4$13466@cache1.tilbu1.nb.home.nl> <l48i1p$q38$1@dont-email.me> <774fd$526a559c$5419b3e4$11247@cache1.tilbu1.nb.home.nl>

Cross-posted to 4 groups.

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Skybuck Flying wrote:

> Because it's logical.

What is logical?

> If the exit condition was placed on the top, the loop would exit immediatly.

This might be the programmer's intention?

> Instead the desired behaviour might be to execute the code inside the loop first and then exit.

It might ... but it might be something very different, too.

Let us assume a function with variable iteration count for its loop, where the
count is passed as parameter. If the loop body always was executed completely
before the final conditional test, we had to check at least the iteration count to avoid crashes if the
iteration count is used as an array index, someone passed a negative number or
zero. Moreover, we had to restore all data manipulated by instructions inside
the loop body if we only wanted to alter data whenever the final condition was
met.

> Thus seperating logic into enter and exit conditions makes sense.

No. It introduces tons of -avoidable- extra cycles, but nothing of true value.
If exit code always was placed at the bottom, and we have a loop with variable
iterations passed as one of the function's input parameters, we had to reverse
all manipulations applied within the loop, if the exit conditions were not met
at the end of the first iteration. Moreover, every erroneously passed negative
iteration count caused a crash if the passed count was used as an array index.
Hence, we had to insert input parameter tests prior to the first execution of
the loop, adding more misprediction penalties.

To get a picture: Each misprediction penalty costs an absolute minimum of ~ 20
clock cycles on recent machines. It surely is not zher best idea to add a lot
of mispredictions with the (questionable) intention to force other programmers
to obey your universal "Condition checks belong to the bottom!" law.

As assembler programmers, we can write the fastest possible code without being
bound to counter-productive rules. Compiler developers who declare conventions
just to force programmer to use delay mechanisms like "condition checks belong
to the bottom!" are a dangerous species. With compilers like that, it is quite
obvious why software became slower and slower while processor power is growing
from one hardware generation to the next.

BTW: I placed label 0 at the wrong place. The proper loop should look like this:

func:...
...
0:dec rcx
jbe 1f
...
some
code
to
perform
...
jmp 0b

p2align 5,,31
1:continue
with
something
else
...

Greetings from Augsburg

Bernhard Schornak

Thread

Possibly better loop construct, also labels+goto important and on the fly compiler idea. "Skybuck Flying" <Windows7IsOK@DreamPC2006.com> - 2013-10-17 01:36 +0200
  Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. "Skybuck Flying" <Windows7IsOK@DreamPC2006.com> - 2013-10-17 01:44 +0200
  Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Ben Finney <ben+python@benfinney.id.au> - 2013-10-17 10:47 +1100
  Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Steven D'Aprano <steve@pearwood.info> - 2013-10-17 09:06 +0000
  Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Peter Cacioppi <peter.cacioppi@gmail.com> - 2013-10-17 16:53 -0700
    Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Skip Montanaro <skip.montanaro@gmail.com> - 2013-10-17 19:39 -0500
    Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Peter Cacioppi <peter.cacioppi@gmail.com> - 2013-10-17 17:41 -0700
    Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Mark Lawrence <breamoreboy@yahoo.co.uk> - 2013-10-18 08:40 +0100
    Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Chris Angelico <rosuav@gmail.com> - 2013-10-18 18:44 +1100
    Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Mark Lawrence <breamoreboy@yahoo.co.uk> - 2013-10-18 09:11 +0100
      Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Peter Cacioppi <peter.cacioppi@gmail.com> - 2013-10-21 14:19 -0700
        Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Dave Angel <davea@davea.name> - 2013-10-22 03:34 +0000
  Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Peter Cacioppi <peter.cacioppi@gmail.com> - 2013-10-17 17:43 -0700
    Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Mark Lawrence <breamoreboy@yahoo.co.uk> - 2013-10-18 08:42 +0100
  Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Mark Lawrence <breamoreboy@yahoo.co.uk> - 2013-10-21 22:35 +0100
  Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Bernhard Schornak <schornak@web.de> - 2013-10-23 15:13 +0200
    Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. "Skybuck Flying" <Windows7IsOK@DreamPC2006.com> - 2013-10-24 22:02 +0200
      Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Mark Lawrence <breamoreboy@yahoo.co.uk> - 2013-10-25 15:13 +0100
      Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Bernhard Schornak <schornak@web.de> - 2013-10-28 10:58 +0100
      Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Bernhard Schornak <schornak@web.de> - 2013-10-28 11:49 +0100
        Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. "Skybuck Flying" <Windows7IsOK@DreamPC2006.com> - 2013-10-29 12:37 +0100
          Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-10-29 14:08 +0000
            Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. rurpy@yahoo.com - 2013-10-29 13:00 -0700
              Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Steven D'Aprano <steve@pearwood.info> - 2013-10-30 10:22 +0000
                Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. rurpy@yahoo.com - 2013-10-30 19:48 -0700
                Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Steven D'Aprano <steve@pearwood.info> - 2013-10-31 08:41 +0000
                Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. rurpy@yahoo.com - 2013-10-31 21:41 -0700
                Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-11-01 05:41 +0000
                Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. rurpy@yahoo.com - 2013-11-01 18:50 -0700
                Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-11-02 03:52 +0000
                Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. rurpy@yahoo.com - 2013-11-03 09:46 -0800
                Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Joshua Landau <joshua@landau.ws> - 2013-11-02 18:22 +0000
                Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-11-03 05:17 +0000
                Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2013-11-03 10:45 +0100
                Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. rurpy@yahoo.com - 2013-11-03 09:50 -0800
                Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2013-11-03 19:49 +0100
                Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Ben Finney <ben+python@benfinney.id.au> - 2013-11-04 09:11 +1100
                Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2013-11-04 09:38 +0100
                Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Ben Finney <ben+python@benfinney.id.au> - 2013-11-04 20:07 +1100
                Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2013-11-04 10:38 +0100
                Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Mark Lawrence <breamoreboy@yahoo.co.uk> - 2013-11-02 18:36 +0000
                Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2013-11-01 13:50 +0100
                Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. rurpy@yahoo.com - 2013-11-01 18:51 -0700
                Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2013-11-02 12:15 +0100
          Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Bernhard Schornak <schornak@web.de> - 2013-11-01 00:53 +0100
            Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. "Skybuck Flying" <Windows7IsOK@DreamPC2006.com> - 2013-11-02 20:49 +0100
              Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Chris Angelico <rosuav@gmail.com> - 2013-11-03 15:17 +1100
        Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. "wolfgang kern" <nowhere@never.at> - 2013-10-29 19:08 +0100
          Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Bernhard Schornak <schornak@web.de> - 2013-11-01 00:44 +0100
  Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Peter Cacioppi <peter.cacioppi@gmail.com> - 2013-10-25 11:57 -0700
    Don't use default Google Group client (was re:....) Terry Reedy <tjreedy@udel.edu> - 2013-10-25 16:05 -0400
      Re: Don't use default Google Group client (was re:....) rurpy@yahoo.com - 2013-10-25 16:44 -0700
        Re: Don't use default Google Group client (was re:....) Mark Lawrence <breamoreboy@yahoo.co.uk> - 2013-10-26 01:19 +0100
          Re: Don't use default Google Group client (was re:....) rurpy@yahoo.com - 2013-10-26 07:58 -0700
            Re: Don't use default Google Group client (was re:....) Mark Lawrence <breamoreboy@yahoo.co.uk> - 2013-10-26 16:38 +0100
              Re: Don't use default Google Group client (was re:....) rurpy@yahoo.com - 2013-10-26 21:36 -0700
        Re: Don't use default Google Group client (was re:....) Chris Angelico <rosuav@gmail.com> - 2013-10-26 11:25 +1100
          Re: Don't use default Google Group client (was re:....) rurpy@yahoo.com - 2013-10-26 07:55 -0700
        Re: Don't use default Google Group client (was re:....) Terry Reedy <tjreedy@udel.edu> - 2013-10-25 20:35 -0400
          Re: Don't use default Google Group client (was re:....) rurpy@yahoo.com - 2013-10-26 08:00 -0700
        Re: Don't use default Google Group client (was re:....) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-10-26 02:40 +0000
          Re: Don't use default Google Group client (was re:....) rusi <rustompmody@gmail.com> - 2013-10-26 05:15 -0700
            Re: Don't use default Google Group client Ben Finney <ben+python@benfinney.id.au> - 2013-10-27 00:02 +1100
            Re: Don't use default Google Group client (was re:....) rurpy@yahoo.com - 2013-10-26 08:07 -0700
            Re: Don't use default Google Group client (was re:....) Chris Angelico <rosuav@gmail.com> - 2013-10-27 00:25 +1100
              Re: Don't use default Google Group client (was re:....) rurpy@yahoo.com - 2013-10-26 21:43 -0700
          Re: Don't use default Google Group client (was re:....) rurpy@yahoo.com - 2013-10-26 08:05 -0700
            Re: Don't use default Google Group client (was re:....) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-10-26 17:24 +0000
              Re: Don't use default Google Group client (was re:....) rurpy@yahoo.com - 2013-10-26 21:33 -0700
            Re: Don't use default Google Group client (was re:....) Chris Angelico <rosuav@gmail.com> - 2013-10-27 09:15 +1100
              Re: Don't use default Google Group client (was re:....) rurpy@yahoo.com - 2013-10-26 21:45 -0700
          Re: Don't use default Google Group client (was re:....) Grant Edwards <invalid@invalid.invalid> - 2013-10-28 14:23 +0000
    Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. "Rhodri James" <rhodri@wildebst.demon.co.uk> - 2013-10-25 22:09 +0100
  Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Peter Cacioppi <peter.cacioppi@gmail.com> - 2013-10-26 13:37 -0700
    Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. rusi <rustompmody@gmail.com> - 2013-10-26 18:45 -0700
      Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Chris Angelico <rosuav@gmail.com> - 2013-10-27 12:56 +1100
        Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. rurpy@yahoo.com - 2013-10-27 22:29 -0700
      Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. rurpy@yahoo.com - 2013-10-26 22:04 -0700
        Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. rusi <rustompmody@gmail.com> - 2013-10-27 00:59 -0700
          Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. rurpy@yahoo.com - 2013-10-27 22:40 -0700
            Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. rusi <rustompmody@gmail.com> - 2013-10-27 22:56 -0700
              Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. rusi <rustompmody@gmail.com> - 2013-10-27 23:51 -0700
                Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. rurpy@yahoo.com - 2013-10-28 21:03 -0700
                Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Chris Angelico <rosuav@gmail.com> - 2013-10-29 17:22 +1100
                Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. rurpy@yahoo.com - 2013-10-30 19:53 -0700
                OT: Hierarchies [was Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea.] Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-11-01 07:00 +0000
                Re: OT: Hierarchies [was Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea.] Chris Angelico <rosuav@gmail.com> - 2013-11-01 19:19 +1100
                Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-10-29 08:45 -0400
      Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-10-27 12:10 -0400
    Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. rusi <rustompmody@gmail.com> - 2013-10-27 03:53 -0700
  Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Peter Cacioppi <peter.cacioppi@gmail.com> - 2013-10-26 19:02 -0700
  Re: Possibly better loop construct, also labels+goto important and on the fly compiler idea. Peter Cacioppi <peter.cacioppi@gmail.com> - 2013-11-02 12:40 -0700

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