On 9/25/05, Andy Little <andy@servocomm.freeserve.co.uk> wrote:
There is neverending discussion on comp.lang.c++.mod regarding differences between runtime floats on various platforms. IMO this is an ideal opportunity to create a platform independent floating point type, IOW one with an exact length of mantissa and exponent specified and with a consistent policy on rounding etc. I think this is how it is done in Java though the only link I can find is:http://www.concentric.net/~Ttwang/tech/javafloat.htm Other question is ... What sort of rounding do you use?
For multiplication and division operations: If the 53'rd bit is 1, add 1 to the 52'nd bit. For addition/subtraction Ignore extra bits, cutoff after the 52'nd bit.
This mimics the behaviour of the VC compilers.
Note that not even this will help you, if you allow for optimizations. Just consider the evaluation of the following two expressions: x=b*c; y=a*b*c; Using common subexpression elimination, the compiler might rewrite this as x=b*c; y=a*x; The second line is now equivalent to a*(b*c), while before it was (a*b)*c, and hence the numbers could differ in the last bit(s), because of different rounding. Note that in the past this has actually prevented the Java compiler from performing such optimizations, because such roundoff discrepancies were not allowed! I am aware that at compile time you might just ignore such optimizations, but the result might still differ from the runtime result, because of possible runtime optimizations. Since, hence, now perfect agreement with runtime values will ever be possible unless optimization is completely turned off anyways, one should either, not put too much weight on equivalence to runtime computation, or just forget about compile time computations at all. Matthias