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

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2011/8/22 Andreas Löscher <andreas.loescher@s2005.tu-chemnitz.de>:
> How would such an jump table work to behave the same liek a
> switch-case-statement?

If your switch statement uses a simple integer enum with sequential
values, then it can be done quite easily. Take this as an example:

        switch (argc)
        {
            case 0: printf("No args at all, this is weird\n"); break;
            case 1: printf("No args\n"); break;
            case 2: printf("Default for second arg\n");
            case 3: printf("Two args\n"); break;
            default: printf("Too many args\n"); break;
        }

I compiled this using Open Watcom C, looked at the disassembly, and
hereby translate it into pseudocode (I'll email/post the full 80x86
disassembly if you like):

1) Check if argc > 3 (unsigned comparison), if so jump to default case.
2) Left shift argc two places, add a constant offset, fetch a pointer
from there, and jump to it - that's the jump table. One JMP statement.
3) Code follows for each case.

Incidentally, the Open Watcom compiler actually turned several of the
cases into offset-load of the appropriate string pointer, and then a
jump to the single call to printf. The fall-through from 'case 2' to
'case 3' works fine, although it means that 'case 2' has to be
de-optimized from that one simplification.

This type of optimization works best when the case values are
sequential. (If I remove the 'case 0', the compiler decrements argc
and proceeds to continue as above.) Otherwise, the jump table has to
have a lot of copies of the "default" pointer.

Chris Angelico

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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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