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Re: evaluate a generic swap function

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ImpalerCore wrote:
> 
> On Sep 20, 10:33 am, Malcolm McLean <malcolm.mcle...@btinternet.com>
> wrote:
> > On Sep 20, 4:51 pm, Stanley Rice <hecong...@gmail.com> wrote:> Hello all
> >
> > > I wrote a "generic" swap function, which can swap some type, such as
> > > int, double, long. But I don't know how to evaluate this function, and
> > > don't know whether it's a good idea to use such a function. I hope you
> > > can give me some suggestions.
> >
> > The function calls malloc to allocate a temporary buffer. This means
> > that it's vulnerable to failure. It's also likely to be inefficient.
> >
> > You can write swap() as a byte swapping loop, which avoids the need
> > for a malloc(). The problem is that most calls will be to arrays of
> > ints or other larger types. You can probably get a 4x speed up if you
> > know that data is a multiple of four and aligned.
> > What this means is that swap() is really a function, like memcpy, best
> > implemented by the compiler, which can apply patches to speed up
> > common cases. E.g. swapping two basic types, which is an obvious use
> > for the function,  can be turned into a sequence of three
> > instructions, or even sometimes eliminated altogether.
> 
> One method to consider is to write macros to declare the function
> prototype and definition of a swap specialized to the type.
> 
> \code snippet
> #define C_SWAP_PROTO( NAME, TYPE ) void c_swap_##NAME( TYPE* left,
> TYPE* right )
> 
> #define C_SWAP_IMPL( NAME, TYPE )             \
> void c_swap_##NAME( TYPE* left, TYPE* right ) \
> {                                             \
>   TYPE tmp;                                   \
>                                               \
>   tmp = *left;                                \
>   *left = *right;                             \
>   *right = tmp;                               \
> }
> \endcode
> 
> Then, if one wants to swap a particular type, they declare the
> prototype and definition where needed and use the generated function
> name to swap the objects.  The advantage is that in a library setting,
> one can create a generic template for a swap without the need to
> maintain an arbitrary list of swap functions, like c_swap_int,
> c_swap_long, c_swap_my_struct, etc.
> 
> The reason for the 'NAME' attribute is that types with spaces in them
> obviously cause problems in the function prototypes.  You could use
> them like the following.
> 
> \code snippet
> #include <stdio.h>
> #include <stdlib.h>
> 
> C_SWAP_PROTO( uint, unsigned int );
> 
> int main( void )
> {
>   unsigned int u1 = 10;
>   unsigned int u2 = 20;
> 
>   c_swap_uint( &u1, &u2 );
> 
>   printf( "[u1,u2] = [%u,%u]\n", u1, u2 );
> 
>   return EXIT_SUCCESS;
> }
> 
> /* Important to *not* add the semicolon after the IMPL macro */
> C_SWAP_IMPL( uint, unsigned int )
> \endcode
> 
> I fully acknowledge that some may consider this "function template" a
> coding horror.  But at the same time, I have found this kind of setup
> very convenient from a maintenance perspective.

I use these macros:

#define SWAP(N, A, B, T)                        \
(MOV##N((T), (A)), MOV##N((A), (B)), MOV##N((B), (T)))

#define MOVi(A, B)  ((void)(*(A) = *(B)))

#define MOVa(A, B)  ((void)memcpy((A), (B), sizeof *(A)))

MOVa is for when *(A) is of array type.
MOVi is for when *(A) isn't of array type.

-- 
pete

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evaluate a generic swap function Stanley Rice <heconghui@gmail.com> - 2011-09-20 06:51 -0700
  Re: evaluate a generic swap function jacob navia <jacob@spamsink.net> - 2011-09-20 16:01 +0200
    Re: evaluate a generic swap function jt@toerring.de (Jens Thoms Toerring) - 2011-09-20 14:29 +0000
      Re: evaluate a generic swap function jacob navia <jacob@spamsink.net> - 2011-09-20 21:13 +0200
        Re: evaluate a generic swap function jt@toerring.de (Jens Thoms Toerring) - 2011-09-20 20:30 +0000
          Re: evaluate a generic swap function Keith Thompson <kst-u@mib.org> - 2011-09-20 17:05 -0700
            Re: evaluate a generic swap function gazelle@shell.xmission.com (Kenny McCormack) - 2011-09-21 15:45 +0000
              Re: evaluate a generic swap function Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-09-21 09:33 -0700
              Re: evaluate a generic swap function jacob navia <jacob@spamsink.net> - 2011-09-21 19:17 +0200
                Re: evaluate a generic swap function gazelle@shell.xmission.com (Kenny McCormack) - 2011-09-21 17:34 +0000
                Re: evaluate a generic swap function Jack Foot <foot@dev.null.invalid> - 2011-09-21 18:12 +0000
  Re: evaluate a generic swap function Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-09-20 07:33 -0700
    Re: evaluate a generic swap function ImpalerCore <jadill33@gmail.com> - 2011-09-20 08:26 -0700
      Re: evaluate a generic swap function pete <pfiland@mindspring.com> - 2011-09-20 23:10 -0400
  Re: evaluate a generic swap function "BartC" <bc@freeuk.com> - 2011-09-20 22:20 +0100
    Re: evaluate a generic swap function jacob navia <jacob@spamsink.net> - 2011-09-20 23:23 +0200
      Re: evaluate a generic swap function Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-20 23:07 +0100
  Re: evaluate a generic swap function Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-09-20 21:15 -0400
    Re: evaluate a generic swap function Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-09-21 03:17 -0700
  Re: evaluate a generic swap function Gene <gene.ressler@gmail.com> - 2011-09-21 02:40 -0700
  Re: evaluate a generic swap function Seebs <usenet-nospam@seebs.net> - 2011-09-23 01:46 +0000
    Re: evaluate a generic swap function Nelu <tandauioan@gmail.com> - 2011-09-23 10:26 -0700
      Re: evaluate a generic swap function Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-23 18:36 +0100
        Re: evaluate a generic swap function Nelu <tandauioan@gmail.com> - 2011-09-23 10:49 -0700
          Re: evaluate a generic swap function Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-23 20:18 +0100
            Re: evaluate a generic swap function Nelu <tandauioan@gmail.com> - 2011-09-23 12:54 -0700
              Re: evaluate a generic swap function James Kuyper <jameskuyper@verizon.net> - 2011-09-23 16:43 -0400
                Re: evaluate a generic swap function Nelu <tandauioan@gmail.com> - 2011-09-23 13:49 -0700
                Re: evaluate a generic swap function James Kuyper <jameskuyper@verizon.net> - 2011-09-23 17:44 -0400
                Re: evaluate a generic swap function Nelu <tandauioan@gmail.com> - 2011-09-23 18:04 -0700
                Re: evaluate a generic swap function Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-24 13:52 -0800
              Re: evaluate a generic swap function Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-23 21:46 +0100
                Re: evaluate a generic swap function Nelu <tandauioan@gmail.com> - 2011-09-23 13:59 -0700
                Re: evaluate a generic swap function Seebs <usenet-nospam@seebs.net> - 2011-09-26 22:59 +0000
          Re: evaluate a generic swap function pete <pfiland@mindspring.com> - 2011-09-23 18:19 -0400
            Re: evaluate a generic swap function Nelu <tandauioan@gmail.com> - 2011-09-23 16:36 -0700

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