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Re: a = b or memset/cpy?

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On 02/07/2012 12:41 PM, Jens Gustedt wrote:
> Am 02/07/2012 06:02 PM, schrieb nroberts:
>> X gX[30];
>>
>> void f(int slot)
>> {
>>   X init = {0};
>>
>>   gX[slot] = init;
>>
>>   ...
>> }
> 
> make it
> 
> X const init = { 0 };
> 
> or even better use a compound literal
> 
> gX[slot] = (X const){ 0 };
> 
>> Normally I wouldn't look for a micro-optimization like this but I'm
>> kind of stuck with the parameters I'm given.
> 
> On any decent compiler the assignment version should not be worse that

This is initialization, not assignment.

> the memset version, because the compiler must be able to see that it
> is an object only filled with 0.

I've used a compiler which, given the following code:

	double array[10][1354][3] = {0};

generated the equivalent of the following:

	array[0][0][0] = 0;
	array[0][0][1] = 0;
etc.
The resulting executable was noticeably larger that I had expected it to
be. I was a little annoyed when I figured out what was going on. I
changed it to use memset(), and got a lot smaller, and executed somewhat
faster, too. The support person I talked with said that my use of {0}
was unreasonable, not their compiler's code generation.

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a = b or memset/cpy? nroberts <roberts.noah@gmail.com> - 2012-02-07 09:02 -0800
  Re: a = b or memset/cpy? Jens Gustedt <jens.gustedt@loria.fr> - 2012-02-07 18:41 +0100
    Re: a = b or memset/cpy? James Kuyper <jameskuyper@verizon.net> - 2012-02-07 13:40 -0500
      Re: a = b or memset/cpy? Jens Gustedt <jens.gustedt@loria.fr> - 2012-02-07 20:09 +0100
      Re: a = b or memset/cpy? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-02-07 11:37 -0800
        Re: a = b or memset/cpy? Shao Miller <sha0.miller@gmail.com> - 2012-02-07 18:35 -0500
    Re: a = b or memset/cpy? nroberts <roberts.noah@gmail.com> - 2012-02-07 11:07 -0800
      Re: a = b or memset/cpy? blp@cs.stanford.edu (Ben Pfaff) - 2012-02-07 11:14 -0800
        Re: a = b or memset/cpy? nroberts <roberts.noah@gmail.com> - 2012-02-07 11:41 -0800
      Re: a = b or memset/cpy? Shao Miller <sha0.miller@gmail.com> - 2012-02-07 18:39 -0500
  Re: a = b or memset/cpy? "Scott Fluhrer" <sfluhrer@ix.netcom.com> - 2012-02-07 14:14 -0500
  Re: a = b or memset/cpy? Shao Miller <sha0.miller@gmail.com> - 2012-02-07 18:58 -0500
    Re: a = b or memset/cpy? Shao Miller <sha0.miller@gmail.com> - 2012-02-07 21:58 -0500
    Re: a = b or memset/cpy? Jens Gustedt <jens.gustedt@loria.fr> - 2012-02-09 21:34 +0100
  Re: a = b or memset/cpy? Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-07 20:34 -0500
  Re: a = b or memset/cpy? Jorgen Grahn <grahn+nntp@snipabacken.se> - 2012-02-08 18:43 +0000
  Re: a = b or memset/cpy? jgk@panix.com (Joe keane) - 2012-02-08 20:05 +0000
    Re: a = b or memset/cpy? Ian Collins <ian-news@hotmail.com> - 2012-02-09 09:18 +1300
  Re: a = b or memset/cpy? Tim Prince <tim@nospamcomputer.org> - 2012-02-14 08:30 -0500

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