On 2011-12-24 12:02:24 +0100, David Brown wrote: > And (regarding other examples in your posts) if floating-point code > depends on things like the order of calculations, it is also wrong - > thus "-ffast-math" will not affect the correctness of the program, > but will sometimes greatly improve the speed. Whether you like it or not, a floating-point result does depend on the order of calculations in general. How would you write code to compute the mathematical expression a + b - c where you know that you have the property 1/2 <= a/c <= 2 on the inputs? Or code to compute a*a - b*b where a and b are close to each other? With the IEEE 754 rules, one can use formulas that give very accurate results, but if the compiler is allowed to rewrite the code (without control from the developer who wrote the code), the results may no longer be accurate (and may even be quite wrong). > I don't think I'm alone in wishing that more programmers used stricter > coding practices On the contrary, we are very strict on coding practices in order to get accurate results. -- Vincent Lefèvre <vincent@xxxxxxxxxx> - Web: <http://www.vinc17.net/> 100% accessible validated (X)HTML - Blog: <http://www.vinc17.net/blog/> Work: CR INRIA - computer arithmetic / Arénaire project (LIP, ENS-Lyon)
