Casting and Optimization Flags

[Colin Law <c.law@elec.gla.ac.uk>]

Hello,

I am having trouble with a section of code that runs fine when compiled 
with no optimization flags, however when I add optimization,flags, I get 
different results.

In the middle of a for loop, and a fairly heavily recursive procedure i 
has this;

double d = (std::log10(static_cast<double>(width))) / 
(std::log10(static_cast<double>(2))) ;
unsigned int level = static_cast<unsigned int>(d) ;

Which will (should) always return me an integer value for my values of 
width (always 2^n values). When i sprinf both d and level i can see that 
the value is cast fine with no optimization, however when i compile with 
-O2 or -O1, after the cast the int doesn't always match the double, 
casting double 3 to an unsigned int =2 !!!

It appears that my double value is so close to an int that it is 
regarded as a whole number,however during the cast the decimal values 
are simpley chopped so 2.9999..... cast to unsinged int is 2. sprinf 
(%.20f) tells me the value of d is  3.0000... not 2.9999...

I am unable to reprodude this problem in a trivial case which suggests 
it is something i am missing in the main code, however, as i am 
assigning to new variables, i cannot see where this could possible be.

If I use long double instead of double during the calculation. then 
cast, I appear to get the correct results even with optimization!

Any suggestions as to what is causing this ??
I am using g++-3.2 and am compiling the above as part of a shared 
library with -fPIC -Wall -O2 if thats any help.

Regards
~Colin.