Groups > comp.lang.python > #11971

Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1")

From	Terry Reedy <tjreedy@udel.edu>
Subject	Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1")
Date	2011-08-21 19:38 -0400
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On 8/21/2011 7:17 PM, Andreas Löscher wrote:
> Am Sonntag, den 21.08.2011, 14:52 -0400 schrieb Roy Smith:
>> In article<mailman.282.1313951079.27778.python-list@python.org>,
>>   Christian Heimes<lists@cheimes.de>  wrote:
>>
>>> Am 21.08.2011 19:27, schrieb Andreas Lscher:
>>>> As for using Integers, the first case (line 1319 and 1535) are true and
>>>> there is no difference in Code. However, Python uses a huge switch-case
>>>> construct to execute it's opcodes and INPLACE_ADD cames after
>>>> BINARY_ADD, hence the difference in speed.
>>>
>>> I don't think that's the reason. Modern compiles turn a switch statement
>>> into a jump or branch table rather than a linear search like chained
>>> elif statements.
>>
>> This is true even for very small values of "modern".  I remember the
>> Unix v6 C compiler (circa 1977) was able to do this.
>
> What is the difference in speed between a jump table that is searched
> from top to bottom in comparison to an ordinary if-then-elif...? The
> difference can only be in the search algorithm regarding the table.
> Without optimization (linear search) both are the same. If the compiler
> applies some magic the difference can be relevant (linear complexity for
> if-then-elif... and O(1) if you would use a dictionary).

A jump or branch table is applicable when the case value values are all 
small ints, like bytes or less. For C, the table is simply an array of 
pointers (addressess, with entries for unused byte codes would be a void 
pointer). Hence, O(1) access.
https://secure.wikimedia.org/wikipedia/en/wiki/Jump_table

> Hence the executed code for integers is the same, there must be a slower
> path to the code of BINARY_ADD than to INPLACE_ADD.
>
> How would such an jump table work to behave the same liek a
> switch-case-statement? Beware, that things like
>
>         case PRINT_NEWLINE_TO:
> 1802	            w = stream = POP();
> 1803	            /* fall through to PRINT_NEWLINE */

add jump to address of the code for PRINT_NEWLINE

> 1804	
> 1805	        case PRINT_NEWLINE:
>
> must be supported.

-- 
Terry Jan Reedy

Thread

Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Andreas Löscher <andreas.loescher@s2005.tu-chemnitz.de> - 2011-08-21 19:27 +0200
  Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Laurent <laurent.payot@gmail.com> - 2011-08-21 10:48 -0700
    Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Laurent <laurent.payot@gmail.com> - 2011-08-21 11:03 -0700
  Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Christian Heimes <lists@cheimes.de> - 2011-08-21 20:24 +0200
    Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Roy Smith <roy@panix.com> - 2011-08-21 14:52 -0400
      Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Andreas Löscher <andreas.loescher@s2005.tu-chemnitz.de> - 2011-08-22 01:17 +0200
        Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Chris Angelico <rosuav@gmail.com> - 2011-08-22 00:37 +0100
        Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Terry Reedy <tjreedy@udel.edu> - 2011-08-21 19:38 -0400
          Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Andreas Löscher <andreas.loescher@s2005.tu-chemnitz.de> - 2011-08-22 02:00 +0200
        Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Seebs <usenet-nospam@seebs.net> - 2011-08-22 05:33 +0000
  Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Terry Reedy <tjreedy@udel.edu> - 2011-08-21 15:39 -0400
    Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Laurent <laurent.payot@gmail.com> - 2011-08-21 12:53 -0700
      Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Laurent <laurent.payot@gmail.com> - 2011-08-21 12:55 -0700
      Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Laurent <laurent.payot@gmail.com> - 2011-08-21 12:55 -0700
      Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-08-22 11:12 +1000
        Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") "Richard D. Moores" <rdmoores@gmail.com> - 2011-08-22 02:55 -0700
        Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Emile van Sebille <emile@fenx.com> - 2011-08-22 09:35 -0700
        Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Emile van Sebille <emile@fenx.com> - 2011-08-22 10:22 -0700
    Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Laurent <laurent.payot@gmail.com> - 2011-08-21 13:04 -0700
    Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Laurent <laurent.payot@gmail.com> - 2011-08-21 12:53 -0700
      Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Andreas Löscher <andreas.loescher@s2005.tu-chemnitz.de> - 2011-08-22 01:25 +0200
        Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Chris Angelico <rosuav@gmail.com> - 2011-08-22 00:41 +0100
          Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-08-22 11:16 +1000
        Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Christian Heimes <lists@cheimes.de> - 2011-08-22 04:04 +0200
        Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Terry Reedy <tjreedy@udel.edu> - 2011-08-21 22:11 -0400
      Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Stephen Hansen <me+list/python@ixokai.io> - 2011-08-21 19:08 -0700
        Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-08-22 14:14 +1000
          Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Stephen Hansen <me+list/python@ixokai.io> - 2011-08-21 21:37 -0700
          Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") Stephen Hansen <me+list/python@ixokai.io> - 2011-08-21 21:49 -0700
  Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1") casevh <casevh@gmail.com> - 2011-08-21 21:14 -0700

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