RE: Infinite loop with GCC 4.3

[Carl D. Sorensen]

> -----Original Message-----
> From: Joe Neeman [mailto:address@hidden
> Sent: Monday, May 26, 2008 1:22 AM
> To: John Mandereau
> Cc: lily-devel
> Subject: Re: Infinite loop with GCC 4.3
>
> On Mon, 2008-05-26 at 07:53 +0200, John Mandereau wrote:
> > On 2008/05/26, Joe Neeman wrote:
> > > 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').
> >
> > I agree.
>
> Well, it seems that I should have started by reading the gcc
> bug tracker. This is a very popular bug:
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
>
> The summary is that you cannot expect sane behaviour with the
> double data type using gcc (and apparently most other C
> compilers) on x86. In particular, doubles may be truncated
> from 80 bits to 64 bits at any time. Therefore, you can
> compare two (80-bit) doubles and find that they are not equal
> only for them to become equal on the very next line.

Have you experimented with the code on comment 60 of the gcc bug tracker?  It 
apparently sets the fpu internal precision to 64 bits.  I can't test it on my 
current machine, but it seemed like it might work for the current case.

Carl