Groups | Search | Server Info | Login | Register

Groups > comp.lang.c.moderated > #384

Re: which one is faster?

Show all headers | View raw

On Wed, 4 Apr 2012 18:05:42 -0500 (CDT), echo ma <fatmck@gmail.com>
wrote:

>on a 32bit system.
>
>#pragma pack(4)
>struct TestStruct
>{
>	unsigned short a;
>	unsigned short b;
>        unsigned int c;
>};
>#pragma pack()
>struct TestStruct t;
>t.a = 0;      //step 1
>t.b = 1;      //step 2
>t.c = 2;      //step 3
>
>Questing is :  Are these 3 steps having the same perfomance time?
>
>Sorry for my poor english.

It will depend on your system (hardware, compiler, and options).  

There is no guarantee that
    t.a = 0;
and
    t.a = 1;
will have the same performance.  (Some systems have methods of zeroing
memory that are more efficient than storing an arbitrary value.)

For that matter, there is no guarantee that
    t.a = 0;
and 
    t.b = 0;
have the same performance.  (On some systems, access performance is
different for different alignments.)

And then step three accesses "more" memory than the first two so why
should its performance be the same.

The only way to answer these types of questions is to benchmark your
code and even that is questionable unless you disable cache and your
code is the ONLY process executing on the system, something pretty
rare on most user systems these days.

-- 
Remove del for email
-- 
comp.lang.c.moderated - moderation address: clcm@plethora.net -- you must
have an appropriate newsgroups line in your header for your mail to be seen,
or the newsgroup name in square brackets in the subject line.  Sorry.

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

Thread

which one is faster? echo ma <fatmck@gmail.com> - 2012-04-04 18:05 -0500
  Re: which one is faster? James Kuyper <jameskuyper@verizon.net> - 2012-04-23 08:31 -0500
  Re: which one is faster? "Joel C. Salomon" <joelcsalomon@gmail.com> - 2012-04-23 08:33 -0500
  Re: which one is faster? David Brown <david@westcontrol.removethisbit.com> - 2012-04-23 08:33 -0500
  Re: which one is faster? Thomas Richter <thor@math.tu-berlin.de> - 2012-04-23 08:31 -0500
    Re: which one is faster? Keith Thompson <kst-u@mib.org> - 2012-04-30 21:57 -0500
  Re: which one is faster? Barry Schwarz <schwarzb@dqel.com> - 2012-04-23 08:32 -0500
  Re: which one is faster? Keith Thompson <kst-u@mib.org> - 2012-04-23 08:32 -0500
    Re: which one is faster? George Neuner <gneuner2@comcast.net> - 2012-04-30 21:58 -0500

csiph-web