Re: [Algorithms] How to get 3dvector largest coordinate index?

[Glenn F. <ga...@ga...>]

if you would like this on GCC with -O2 or greater, you'll need to use unions
instead of aliasing pointers, like this:
union FloatInt
{
    unsigned int i;
    float f;
};

FloatInt nx, ny, nz;
x.f = n.x;
y.f = n.y;
z.f = n.z;
return (x.i > y.i) ? (x.i > z.i) ? 0 : 2 : (y.i > z.i) ? 1 : 2;

just fyi, you are going to have very VERY difficult optimizer bugs to track
down otherwise - and yes, unions are the only way to guarantee that GCC
generates the correct code in this case

cheers

> I did some profiling on MSVC 2009, and integer methods are ~10%
> > faster:
> >  unsigned int x = 0x7FFFFFFF & *(unsigned int *)&n.x;
> >  unsigned int y = 0x7FFFFFFF & *(unsigned int *)&n.y;
> >  unsigned int z = 0x7FFFFFFF & *(unsigned int *)&n.z;
> >
> >  return (x > y) ? (x > z) ? 0 : 2 : (y > z) ? 1 : 2;
> >
> > For some odd reason only when using integers can it simplify the
> > two inner
> > ternary methods using bit 'magic'. If we use floats or use SSE the
> > integermethods are about ~5% faster because this compiler generates
> > a bunch of
> > jumps. The method above (as well as mine) generates 1 jump.
> >
> > I actually eliminated all jumps but since I use one too many
> > registers it
> > slows it down. I think the CPU is smart enough to look ahead if you
> > get down
> > to one conditional ..so yeah I think if you use the stack it can
> > slow things
> > down slightly but you do see a gain if you use integers. I assume
> > conditional jumps would be the best way, assuming the compiler can
> > do it
> > depth first because you need 3 conditionals - the inner two, and
> > then a
> > conditional move on the results of those two
> >
> > Asm:
> >
> >
> > ; _n$ = edx
> >
> > ; 61   :   unsigned int x = 0x7FFFFFFF & *(unsigned int *)&n.x;
> >
> > mov eax, DWORD PTR [edx]
> >
> > ; 62   :   unsigned int y = 0x7FFFFFFF & *(unsigned int *)&n.y;
> >
> > mov ecx, DWORD PTR [edx+4]
> >
> > ; 63   :   unsigned int z = 0x7FFFFFFF & *(unsigned int *)&n.z;
> >
> > mov edx, DWORD PTR [edx+8]
> > and eax, 2147483647    ; 7fffffffH
> > and ecx, 2147483647    ; 7fffffffH
> > and edx, 2147483647    ; 7fffffffH
> >
> > ; 69   :   return (x > y) ? (x > z) ? 0 : 2 : (y > z) ? 1 : 2;
> >
> > cmp eax, ecx
> > jbe SHORT $LN3@coord_inde
> > cmp edx, eax
> > sbb eax, eax
> > and eax, -2     ; fffffffeH
> > add eax, 2
> >
> > ; 70   : }
> >
> > ret 0
> > $LN3@coord_inde:
> >
> > ; 69   :   return (x > y) ? (x > z) ? 0 : 2 : (y > z) ? 1 : 2;
> >
> > cmp edx, ecx
> > sbb eax, eax
> > add eax, 2
> >
> > ; 70   : }
> >
> > ret
> >
>
>
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