NetNews Usenet Archive 1992 #26/NN_1992_26.iso/spool/comp/sys/next/programm/7054

Internet Message Format | 1992-11-06 | 1.6 KB

Path: sparky!uunet!rosie!NeXT.COM
From: [e-mail] (Mark Wagner)
Newsgroups: comp.sys.next.programmer
Subject: Re: cc problem and cure: comparison among doubles can produce wrong results
Message-ID: <[e-mail]>
Date: 6 Nov 92 00:56:19 GMT
References: <[e-mail]>
Sender: [e-mail]
Lines: 32

> >      However, given that the conversion
> >      algorithm from external to internal format is the same, I would
> argue that
> >      ALL instances of the SAME decimal value will result in the SAME
> binary
> >      floating point value internally and can therefore be compared for
> equality
> >      with impunity.
>
> Actually, it's worse than this.  Sometimes floating point values can be
> different depending on whether or not they've been in and out of
> registers, etc.  Moral:  Use something like this:
>
> #define EQU(_x,_y)  (((_x) > (_y)) ? (((_x)-(_y))<EPSILON) :
> (((_y)-(_x))<EPSILON))
> #define EPSILON .00001  // or whatever

One can use the -ffloat-store option to "cure" this problem but at some
cost in terms of performance (depending, of course, on the application).

     -ffloat-store
          Don't store floating-point variables in registers.
          This prevents undesirable excess precision due to the
          floating registers keeping more precision than a double
          is supposed to have.

          For most programs, the excess precision does no harm,
          but a few programs rely on the precise definition of
          IEEE floating point.  Use -ffloat-store for such pro-
          grams.

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