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Re: a = b or memset/cpy?

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

     The official answer is: The definition of the C language says
nothing about which constructs are faster or slower than others.

     That said, I would expect memset() to be faster, usually, if
the wind is not unfavorable and the Moon is in the right quarter.
Argument: In the assignment version, the code must allocate the auto
variable `init', zero it, and then copy all those zeroes to `gX[slot]';
on the face of it, this sounds like more work than just zeroing
`gX[slot]' to begin with.

     It is just possible that a very smart compiler could (1) realize
that the `init' variable is not actually necessary, (2) decide to
clear `gX[slot]' directly instead of clearing `init' and copying,
and (3) clear `gX[slot]' more efficiently than memset() can, perhaps
with in-line code.  My suspicion, though, is that a compiler smart
enough for (1,2,3) would not at the same time be so dumb as to
implement memset() with an actual call to an actual external function;
you'd need a strange combination of brilliance and stupidity to get
an advantage for initialize-and-copy.

     ... and, of course, measurement is the only way to be sure.

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

     My prejudice (and I admit it's something of a prejudice) would be
to take a hard look at those memset() and memcpy() calls, with a view
toward eliminating at least some of them -- if you can eliminate a
call you get an infinite speedup, as opposed to a mere hundredfold!
Making copies of bits you've already computed usually doesn't advance
the state of the computation very much; making many duplicates of a
single byte is also not usually a great addition to the program's
"knowledge."  There are, of course, exceptions: qsort() just rearranges
bits you already own, for example, but can be useful nonetheless.
Still, if memset() and memcpy() are dominating the run time, it seems
likely that there may be a lot of needless setting and copying going
on.  See what you can jettison.

-- 
Eric Sosman
esosman@ieee-dot-org.invalid

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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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