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  Intermediary Results / C++ Operators
  Submitted by



Many times I have seen a people do a C++ class for vector math. This is A Good Thing, basically, it makes your source more readable, and of course have my own version of such a class too. It's really nice to have "operator +" to add vectors and "operator %" to do a cross product or the like.

But when speed is really important, don't use them. Even if they are "inline", they have an intrinsic speed penalty. Consider this code:

   vector a, b, c, d;
   a = d + ( b % c ); 




This is meant to build the cross-product of b and c and then add d to it and store the result in a. Due to stack based arrangement of the floating-point-registers in the Intel-FPU (and so for the AMD-FPU as well since it must be Intel compatible), any compiler is handicapped of optimising this piece of code. Intermediary results, as big as a "vector", will not stay in floating point registers. In fact, the results of "operator %" will be written back to memory, the read again by "operator +" then written to a.

So when speed is important, be sure to write out the entire calculation component by component. This way, all intermediary results can be kept in FPU-registers, only written to memory at the end of the calculation. For the example abouve, this would translate to:

   vector a, b, c, d;
   a.x = d.x + b.y * c.z - b.z * c.y;
   a.y = d.y + b.z * c.x - b.x * c.z;
   a.z = d.z + b.x * c.y - b.y * c.x; 


That's all about it. I have digged in the assembly output of some compilers, so I dare the general statement for any compiler.

-- chris


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