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

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

garrrrr! i hate coding in gmail :)
union FloatInt
{
    unsigned int i;
    float f;
};

FloatInt x, y, z;
x.f = n.x;
y.f = n.y;
z.f = n.z;
x.i &= 0x7FFFFFFF;
y.i &= 0x7FFFFFFF;
z.i &= 0x7FFFFFFF;
return (x.i > y.i) ? (x.i > z.i) ? 0 : 2 : (y.i > z.i) ? 1 : 2;

cheers

On Sun, Mar 1, 2009 at 1:10 AM, Glenn Fiedler <ga...@ga...> wrote:

> 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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>