Re: Internal representation of double variables - 3.4.6 vs 4.1.0

[Brian Dessent <brian at dessent dot net>]

Terry Frankcombe wrote:

> After reading this, I went off looking for a gcc option enforcing IEEE
> floating point behaviour, assuming gcc was like the Intel compilers and
> by default sacrificed some exactness in the floating point model for
> speed, even with no optimisation.  I could find none.  So, does gcc use
> a well-defined and reproducible floating-point model by default?  If
> not, can one turn on strict IEEE arithmetic?

That is not the problem.  The problem is that the intel x86 processor
uses an internal 80 bit representation for 'double', which normally has
only 64 bits.  This causes excess precision, which can appear as
rounding errors in the ULP, but are not really errors, just
misunderstandings of how IEE 754 works.  You can work around this with
-ffloat-store but this incurs a speed penalty as it means temporary
values can't be kept in registers but repeatedly stored and loaded from
memory.  Or you can use SSE for fp math (-mfpmath=sse) instead of 387,
if your architecture supports it.

See gcc bug 323 <http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323> for
examples of this non-bug being reported over and over again dozens of
times over the years.

Brian