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Re: a = b or memset/cpy?

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On 2/7/2012 12:02, nroberts wrote:
> memset and memcpy are turning up in profiles a lot.  I'd like to speed
> things up a bit.
>

You might find that the implementation actually translates a '= { 0 
};'-style initializer into a call to 'memset'.  An experiment might 
reveal whether or not that's the case.

> Sometimes it is clear that using = to initialize a local would be
> better than memset.  I might not gain anything, but at least there's a
> chance.
>

I'm not sure how you could gain anything unless the call to 'memset' 
actually translates differently than a '= { 0 };'-style initializer.

Did you know that after all subobjects that are explicitly initialized 
(by the initializer-list) have been so, the rest are initialized to what 
they would have been had the object been declared with 'static' storage 
duration?  The whole containing object is thus "touched."

> However, can I gain performance improvements when zeroing out say some
> global element in an array like so:
>
> typedef struct x { int var0; char var1[20]; } X;
>
> X gX[30];
>
> void f(int slot)
> {
>    X init = {0};
>
>    gX[slot] = init;
>
>    ...
> }
>
> vs.
> void f(int slot)
> {
>    memset(&gX[slot], 0, sizeof(X));
>
>    ...
> }
>

Well these aren't the same.  The former initializes all sub-objects to 
the "zeroey" values that would initialize a 'static'-storage-duration 
object having the same type as the sub-object and having no explicit 
initializer.

The latter fills the object with bytes with the 'unsigned char' value 
'0', which is all-bits-zero.

In your example, the 'struct' type 'X' has an 'int' member.  The object 
representation of an 'int' can have padding bits that can be used any 
way the implementation pleases.

If filling the padding bits with zeroes results in a trap representation 
for an 'int', then you might be in for a surprise.

There are similar concerns for other types, including pointers, where a 
null pointer value might not be all-bits-zero.

That is why I believe some people consider a '= { 0 };'-style 
initializer to be more portable than 'memset'.  If portability isn't a 
concern, oh well.

> Normally I wouldn't look for a micro-optimization like this but I'm
> kind of stuck with the parameters I'm given.

Optmizing and making portable might not always be compatible.  If you 
have a particular set of implementations as your target(s), there might 
be "compiler intrinsics" that you can use which are 
implementation-defined extensions to C that could offer you speed 
advantages.

For example, some Microsoft compilers offer '__movsd':

   http://msdn.microsoft.com/en-us/library/9d196b9h.aspx

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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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