[lore] C hacks

[Grahame Bowland]

By the way, I can't get that bit of Perl to run, although I haven't
looked too hard so perhaps I'm doing something newbish. I definitely
have the Perl module it wants.

On 02/04/2008, David Basden <davidb at ucc.gu.uwa.edu.au> wrote:
> Looks like I was a bit too hasty in posting. Looking at the assembly
>  of gcc -O3, it can obviously be shortened to the much less evil:
>
>         b -= ((unsigned)(b-a)>>(sizeof(-1)*8-1))*(b-a);
>
>
>  David
>
>
>
>  On Wed, Apr 02, 2008 at 03:47:40PM +0800, David Basden wrote:
>  > The third line doesn't work, but here is a 'better' version.
>  > Note the lack of conditionals, so none of that pesky branch
>  > prediction when calculating b = max(a,b).
>  >
>  > This line at least also compiles cleanly with gcc -Wall -ansi -pedantic
>  > on both IA-32 and AMD-64.
>  >
>  > b -= (((b-a)&(((unsigned)-1>>1)^-1))>>(sizeof(-1)*8-1))*(b-a);
>  >
>  > David
>  >
>  > On Fri, Mar 28, 2008 at 12:50:11AM +0900, David Basden wrote:
>  > > I was thinking that maybe people are just writing C that is either
>  > > too readable, too inefficient or both. Maybe a connect-the-equivilent-
>  > > code puzzle or something?
>  > >
>  > > Here is just a quick one 'cause I'm beyond tired, but please add to it;
>  > > I don't know if the third works, and might not be a good idea to let out
>  > > in public even if it does...
>  > >
>  > > while (*a++ == *b++);        strcpy(b,a);
>  > >
>  > > a ^= b ^= a;                        int tmp = a;
>  > >                                 a = b;
>  > >                                 b = a;
>  > >
>  > > a >> 1;                             a = a / 2;
>  > >
>  > > b = (((a - b) & (1<<(sizeof(int)-1))) && b) || a;   if (a > b) b = a;
>  > >
>  > > a << 3;                             a =  a * 8;
>  > >
>  > > 1 << a;                             (int) pow((int) a, 2);
>  > >
>  > > a & 1;                              a % 2;
>  > >
>  > > a & 7;                              a % 8;
>  > >
>  > > (a & 15) || ++b;            if (a % 16 == 0)
>  > >                                     b = b + 1;
>  > >
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