On Fri, 2007-03-09 at 10:33 -0800, Brian Dessent wrote: > 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 So the short answers are "No" and "No", right? ;-) My understanding of -ffloat-store is that it only works when there's actually a variable involved. Is that right? (I'm used to working with Compaq and Intel Fortran compilers, where you can set flags that say, effectively, "Use a standard, well-defined and portable floating point arithmetic model irrespective of what the CPU's trying to do.") Ciao Terry
