Re: Major gcc 4.1.1 and up security issue

[Valdis.Kletnieks () vt edu]

On Wed, 17 Jan 2007 19:07:19 +0100, Marcus Graf said:
> off topic thoughts:
>
> > (twice 1.15 will likely be a bit or two different than 3.30).  As Fortran
> > geeks have known for over half a century, a better way to code this is:
> >
> >     if (abs(a-b) < n*epsilon*a)
> >
> > where 'epsilon' is the hardware constant defining the smallest number such
> > that 1+a is different from 1, and 'n' controls how many significant digits
> > we require in the test.

> Hm...
>
> assume a = b = -1.0
>
> abs(a-b) is zero or maybe a little bit above
> n*epsilon*a is negative (n>0, epsilon>0, a=-1)
>
> Test fails :-(

D'Oh!  n*epsilon*(abs(a)).  :)   (And I *don't* want to hear from the propeller
beany crowd that points out that (1+epsilon)*a isn't *exactly* the smallest
floating point distinguishable from a.  :)

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