Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #16734

Re: Optimisation questions

Show all headers | View raw

On Sun, 05 Feb 2012 23:24:33 +1100, Zoltan Kocsi wrote:
> On Sun, 5 Feb 2012 10:51:57 +0000 (UTC) Raj Pashwar
> <raj121190@hotmail.NOSPAM.com> wrote:
> 
>> Thanks for all the answers, x&8 looks like a good one for efficiency.
>> 
>> I also had another idea while sleeping - amazing how that helps :)  At
>> program startup, maybe in another thread, I can build an array of all
>> x/8, x%8 for x's that might occur, then just look up in this array
>> instead of doing the division every time. I will try this today also.
> 
> Chances are, your table will be slower, especially if it is decent size.
> A table look up means an address calculation and a memory fetch, which
> even with superscalar architectures and large caches is likely to cost
> you more than a single instruction register-register AND or shift.
> 
> By the way, why would you need to build the table in a different thread?
> To waste memory and cycles?
> 
> You'd be better off with listening to what the others replied to your
> question. Unless x must be signed, change it to unsigned and the
> compiler will merrily optimise your division away.

Unfortunately x must be signed because -1 is used to denote "invalid 
value". So I cannot take full advantage of people's suggestions.

I have tried these out. I find that the program speed does not seem to 
change much, if I use x%8 or x&7 or x=-(x>>3)<<3 : disappointing! I did 
not try the table yet, but it stands to reason that it will be quicker 
just to look up a result, than to do a computation every time.

I have two other optimisation questions I am looking at.

In one loop called many times, I need to swap 2 variables. Currently I do 
this with a temporary variable:
tmp=a; a=b; b=tmp;
I think it will save overhead to avoid this extra variable. I found on 
the internet an "XOR trick", b^=a; a^=b; b^a=a; but the article said it 
can be dangerous in some cases i.e. overflow.

I was thinking of this and I invented an alternative version which I 
believe will work correctly in 100% of cases (uses SUM not XOR):
b+=a; a=b-a; b-=a;
Has anyone thought of this idea before? Would you say it will be cheaper 
than the temporary variable?

Also program start up is very slow. It must read in 10 million data 
points from a text file - access is slow (network file). There are 2 
unsigned long integers (x,y) on each line (represented by strings) that I 
read into a 10000000x2 array.

Right now I am using fgets() and atol. I don't have any good ideas to 
optimise this except maybe setting up 100 threads to read different 
sections of the file. Are there any faster ways to convert integers than 
atol?

while(fgets(strg,999,fp))
{ ary[ln][0]=atol(strtok(strg," "));
ary[ln][1]=atol(strtok((char*)NULL,"\n"));
ln++; } 

Thank You to everyone who has helped so far.

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Optimisation questions Raj Pashwar <raj121190@hotmail.NOSPAM.com> - 2012-02-04 22:30 +0000
  Re: Optimisation questions Ian Collins <ian-news@hotmail.com> - 2012-02-05 11:44 +1300
    Re: Optimisation questions Keith Thompson <kst-u@mib.org> - 2012-02-04 14:52 -0800
      Re: Optimisation questions Ian Collins <ian-news@hotmail.com> - 2012-02-05 11:55 +1300
      Re: Optimisation questions Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-02-21 18:41 +0200
  Re: Optimisation questions "BartC" <bc@freeuk.com> - 2012-02-04 22:44 +0000
    Re: Optimisation questions "BartC" <bc@freeuk.com> - 2012-02-04 23:41 +0000
  Re: Optimisation questions Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-04 17:51 -0500
  Re: Optimisation questions Keith Thompson <kst-u@mib.org> - 2012-02-04 14:52 -0800
  Re: Optimisation questions James Kuyper <jameskuyper@verizon.net> - 2012-02-04 18:44 -0500
    Re: Optimisation questions Stephen Sprunk <stephen@sprunk.org> - 2012-02-04 18:00 -0600
      Re: Optimisation questions James Kuyper <jameskuyper@verizon.net> - 2012-02-04 21:18 -0500
        Re: Optimisation questions Kaz Kylheku <kaz@kylheku.com> - 2012-02-05 03:04 +0000
          Re: Optimisation questions Dann Corbit <dcorbit@connx.com> - 2012-02-06 09:40 -0800
            Re: Optimisation questions Willem <willem@toad.stack.nl> - 2012-02-06 18:51 +0000
            Re: Optimisation questions Markus Wichmann <nullplan@gmx.net> - 2012-02-07 14:19 +0100
              Re: Optimisation questions Dann Corbit <dcorbit@connx.com> - 2012-02-07 14:25 -0800
          Re: Optimisation questions Stephen Sprunk <stephen@sprunk.org> - 2012-02-06 17:29 -0600
            Re: Optimisation questions Kaz Kylheku <kaz@kylheku.com> - 2012-02-06 23:52 +0000
            Re: Optimisation questions pete <pfiland@mindspring.com> - 2012-02-06 19:10 -0500
    Re: Optimisation questions Raj Pashwar <raj121190@hotmail.NOSPAM.com> - 2012-02-05 10:51 +0000
      Re: Optimisation questions Ian Collins <ian-news@hotmail.com> - 2012-02-06 00:08 +1300
        Re: Optimisation questions Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-05 08:13 -0500
      Re: Optimisation questions "BartC" <bc@freeuk.com> - 2012-02-05 11:33 +0000
      Re: Optimisation questions Zoltan Kocsi <zoltan@bendor.com.au> - 2012-02-05 23:24 +1100
        Re: Optimisation questions Raj Pashwar <raj121190@hotmail.NOSPAM.com> - 2012-02-05 22:28 +0000
          Re: Optimisation questions Kaz Kylheku <kaz@kylheku.com> - 2012-02-05 22:37 +0000
          Re: Optimisation questions Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-05 17:48 -0500
          Re: Optimisation questions Keith Thompson <kst-u@mib.org> - 2012-02-05 15:57 -0800
      Re: Optimisation questions "BartC" <bc@freeuk.com> - 2012-02-05 12:53 +0000
  Re: Optimisation questions Geoff <geoff@invalid.invalid> - 2012-02-04 16:07 -0800
  Re: Optimisation questions "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2012-02-04 15:37 -0800
    Re: Optimisation questions Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-02-05 04:26 -0800
      Re: Optimisation questions lawrence.jones@siemens.com - 2012-02-07 10:56 -0500
  Re: Optimisation questions Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-02-05 04:29 -0800
  Re: Optimisation questions Michael Angelo Ravera <maravera@prodigy.net> - 2012-02-05 23:24 -0800

csiph-web