On Thu, 6 Apr 2006, Daniel Bratell wrote: > that it would be safe to use that flag unless I relied on extreme > precision or behaviour related to NaN or +-Inf, and that normal simple > arithmetics should still give the same result.
Unfortunately, with -ffast-math simple arithemtics no longer have precisely the same result. Converting the division into multiplication by reciprocal sometimes results in numbers that can't precisely be represented such as 1/7. Of course, in this case you are relying on "extreme precision" as you describe it above. As pointed out by Andrew the result is 0.99999999999999994448884876874217298 which should be close enough to one for most -ffast-math applications. The real problem is the source line: > if(diff_days/7.0 != (int)(diff_days/7.0)) Here you're using C's default FP->int conversion with is truncation towards zero. The fact that 0.99999999 is less than 1 means the result is zero. Instead, you should probably use "rint" or "round" or "lround" to return the nearest integer. Roger --
