Re: Infinite loop with GCC 4.3

[Mats Bengtsson]

As is taught in any course on numerical methods, you should always be 
very careful with
comparing numerical values. For example you should use tests of the form
if (abs(x-y) < epsilon)
instead of
if (x==y)
where epsilon is a fixed value somewhat larger than machine precisions.

Following the same line of thought, you should use
if (x >= y - epsilon)
instead of
if (x >= y )
(though this is probably not mentioned as often in the course text books).
and analogously it follows that you numerically cannot distinguish between
if (x > y)
and
if (x >= y)

Here you show an example where you cannot rely on
if (x > y)
but have to do something more or less equivalent to
if ( x > y - epsilon)
to guarantee numerical stability.

I haven't read the corresponding thread in the GCC bug tracker, but I'm 
sure that these
aspects have been discussed in length there as well.

   /Mats

Joe Neeman wrote:
> On Sun, 2008-05-25 at 13:41 +0200, John Mandereau wrote:
>   
> > On 2008/05/23, Joe Neeman wrote:
> >     
> > > Does re-building without optimisations help?
> > >       
> > Yes, it does!  Building with GCC 4.3.0 and without -O2 flag makes
> > LilyPond produce the same output [*] as with GCC 4.1.2 with
> > optimisations, but it is also (not very surpisingly) much slower, so
> > I'll go on building Lily with GCC 4.1.2, unless Fedora 9 provides an
> > upgrade to a newer and fixed version.
> >     
>
> Attached is the smallest example that I could find that reproduces the
> problem. With g++ 4.3.0 on Fedora 9, I get an infinite loop with -O1 but
> immediate termination with -O0. With g++ 4.2.3 on Debian, I get
> termination regardless of the optimisation setting.
>
> I think it is also easy to argue that the program should terminate,
> regardless of rounding (the value of 'i' should be the same every time
> the loop executes. So on the second time through, 'x' should be at least
> as large as 'i').
>
> Can you reproduce? If so, I'll go bug the gcc people.
>
> Joe
>
>   
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--
=============================================
        Mats Bengtsson
        Signal Processing
        School of Electrical Engineering
        Royal Institute of Technology (KTH)
        SE-100 44  STOCKHOLM
        Sweden
        Phone: (+46) 8 790 8463                         
       Fax:   (+46) 8 790 7260
        Email: address@hidden
        WWW: http://www.s3.kth.se/~mabe
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