How to deal with excess precision in floating-point computations?

Question

In my numerical simulation I have code similar to the following snippet

double x;
do {
  x = /* some computation */;
} while (x <= 0.0);
/* some algorithm         


        

Answer 1

Well, GCC has a flag, -fexcess-precision which causes the problem you are discussing. It also has a flag, -ffloat-store , which solves the problem you are discussing.

"Do not store floating point variables in registers. This pre-vents undesirable excess precision on machines such as the 68000 where the floating registers (of the 68881) keep more precision than a double is supposed to have."

I doubt that solution has no performance impact, but the impact is probably not overly expensive. Random googling suggests it costs about 20%. Actually, I don't think there is a solution which is both portable and has no performance impact, since forcing a chip to not use excess precision is often going to involve some non-free operation. However, this is probably the solution you want.