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I found on net Fast Inverse Square Root on http://en.wikipedia.org/wiki/Fast_inverse_square_root . Does it work properly on x64 ? Did anyone use and serious test ?
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The algorithm was approximately four times faster than computing the square root with another method and calculating the reciprocal via floating-point division.
A single Newton-Raphson iteration is performed to calculate a more accurate approximation of the inverse square root of the input. The result of the Newton-Raphson iteration is the return value of the function. The result is extremely accurate with a maximum error of 0.175%.
The algorithm is not copyrighted, but the source code of the function is copyrighted. You could learn how the algorithm works by reading the function, and then write your own function that implements the same algorithm.
Originally Fast Inverse Square Root was written for a 32-bit float, so as long as you operate on IEEE-754 floating point representation, there is no way x64 architecture will affect the result.
Note that for "double" precision floating point (64-bit) you should use another constant:
...the "magic number" for 64 bit IEEE754 size type double ... was shown to be exactly 0x5fe6eb50c7b537a9
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