RE: GCC and Floating-Point

["Dave Korn" <dave dot korn at artimi dot com>]

----Original Message----
>From: chris jefferson
>Sent: 25 May 2005 15:23

> Vincent Lefevre wrote:
 

>> 
>> float x = 30.0;
>> int main()
>> {
>>  if ( 90.0/x != 3.0)
>>    abort();
>>  return 0;
>> }
>> 
>> fails with -ffast-math (on x86). I would not recommend it, unless
>> the user knows all the consequences.
>> 
>> 
>> 
> On the other hand, in general using != and == on floating point numbers
> is always dangerous if you do not know all the consequences. For
> example, on your above program if I use 30.1 and 90.3, the program fails
> without -ffast-math.

  I second that.  It's simply not valid to use != or == to compare floating
point numbers; the standard idiom is and always has been to subtract them
and then see if the delta is less than or greater than some suitably-chosen
epsilon.  You have to regard floating point numbers as if they were
more-or-less random in the lowest few bits after any amount of computation
on them; the chance of getting an exact match is small.

  So never mind -ffast-math; I wouldn't recommend using floating point *at
all* unless the user is well aware of the consequences!


    cheers,
      DaveK
-- 
Can't think of a witty .sigline today....