Re: [PATCH 1/8] lib/sort: Heapsort implementation of sort()

[linux-os]

On Tue, 1 Feb 2005, Andreas Gruenbacher wrote:

> On Mon, 2005-01-31 at 18:30, Paulo Marques wrote:
> > Andreas Gruenbacher wrote:
> > > [...]
> > >
> > > static inline void swap(void *a, void *b, int size)
> > > {
> > >         if (size % sizeof(long)) {
> > >                 char t;
> > >                 do {
> > >                         t = *(char *)a;
> > >                         *(char *)a++ = *(char *)b;
> > >                         *(char *)b++ = t;
> > >                 } while (--size > 0);
> > >         } else {
> > >                 long t;
> > >                 do {
> > >                         t = *(long *)a;
> > >                         *(long *)a = *(long *)b;
> > >                         *(long *)b = t;
> > >                         size -= sizeof(long);
> > >                 } while (size > sizeof(long));
> >
> > You forgot to increment a and b, and this should be "while (size);", no?
>
> Correct, yes.
>
> > Or better yet,
> >
> > static inline void swap(void *a, void *b, int size)
> > {
> > 	long tl;
> >          char t;
> >
> > 	while (size >= sizeof(long)) {
> >                  tl = *(long *)a;
> >                  *(long *)a = *(long *)b;
> >                  *(long *)b = tl;
> > 		a += sizeof(long);
> > 		b += sizeof(long);
> >                  size -= sizeof(long);
> > 	}
> > 	while (size) {
> >                  t = *(char *)a;
> >                  *(char *)a++ = *(char *)b;
> >                  *(char *)b++ = t;
> > 		size--;
> >          }
> > }
> >
> > This works better if the size is not a multiple of sizeof(long), but is
> > bigger than a long.
>
> In case size is not a multiple of sizeof(long) here, you'll have
> unaligned memory accesses most of the time anyway, so it probably
> doesn't really matter.
>
> Thanks,
> --
> Andreas Gruenbacher <agruen@xxxxxxx>
> SUSE Labs, SUSE LINUX GMBH

This uses an GNU-ISM where you are doing pointer arithmetic
on a pointer-to-void. NotGood(tm) this is an example of
where there is absolutely no rationale whatsoever for
writing such code.


Cheers,
Dick Johnson
Penguin : Linux version 2.6.10 on an i686 machine (5537.79 BogoMips).
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