How to do fast floating-point multiplication by 1, 0, and -1?
Here''s a suggestion: write nine functions (one for each possible 3D vector with component values of -1, 0, and 1). You might gain enough from the faster functions to overcome the branch overhead.
Maybe you could do something with template specializations?
:::: [ Triple Buffer V2.0 ] ::::
[edited by - ifoobar on July 29, 2003 10:49:49 AM]
template<float TNum> void DoStuff( float& f ){}template<> void DoStuff<0.0f>( float &f ){ f = 0;}template<> void DoStuff<1.0f>( float& f ){ f = f;}template<> void DoStuff<-1.0f>( float& f ){ f = -f;}:::: [ Triple Buffer V2.0 ] ::::
[edited by - ifoobar on July 29, 2003 10:49:49 AM]
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quote:Original post by Sneftel
1. Will the variables that are -1, 0, or 1 be set to the constants -1/0/1, or will mathematical calculations produce these numbers?
2. In the equations you just showed us, which are the variables that are 0/1/-1?
1. constants
2. the bla''s with even indices
davepermen: he was talking about, and I quote, "C implementations on the Intel optimizing compiler running on a Pentium 3". Sounds very much like a software implementation to me.... Nevertheless, what you''re saying does indeed make sence for me.
JohnBolton: calling the functions would probably be more expensive... (yes, I know, I know -- profile first, talk later
)IFooBar: no templates (of this sort, at least) in Delphi.
Michael K.,
Co-designer and Graphics Programmer of "The Keepers"
We come in peace... surrender or die!
quote:Original post by xMcBaiNx
that and... NEVER TEST FLOATS FOR EQUALITY. :D
Why the hell not? I know you could get problems with not being exactly that number you''re testing, but then you could resort to checking a range. An equality test is just a range test with a range size of 0. You do have to worry about the type, float constants might not mix with double variables well. What''s wrong with that?
quote:Original post by Docquote:Original post by xMcBaiNx
that and... NEVER TEST FLOATS FOR EQUALITY. :D
Why the hell not? I know you could get problems with not being exactly that number you''re testing, but then you could resort to checking a range. An equality test is just a range test with a range size of 0. You do have to worry about the type, float constants might not mix with double variables well. What''s wrong with that?
Beacause, basically 0.0 != -0.0. They are different numbers.
Si if you write if( x == 0.0f) then when you get x=-0.0f coming along your if will false which is almost certainly not desired behaviour.
Technically, taking the IF route, you''d probably want to do something like the following:
That''d do the trick, and would account for float screwups.
(Keep in mind I randomly picked the small constant value, because I don''t remember what the standard one should be. It''s a different suggested value for doubles and floats).
Josh
#define SMALL_ASS_NUMBER 0.00001inline double LameMul(double x, double y){ if(x < -SMALL_ASS_NUMBER) return -y; if(x > SMALL_ASS_NUMBER) return y; return 0;} That''d do the trick, and would account for float screwups.
(Keep in mind I randomly picked the small constant value, because I don''t remember what the standard one should be. It''s a different suggested value for doubles and floats).
Josh
That''s also two branches and a function call. That''s awful performance, even if it''s inline.
I''d just do a fmul. Chances are that more than a few bit ops would be slower than one fmul. Though, the right answer is to try different things and profile.
I like pie.
I''d just do a fmul. Chances are that more than a few bit ops would be slower than one fmul. Though, the right answer is to try different things and profile.
I like pie.
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