C++ – Different floating point result with optimization enabled – compiler bug

cc++-faqg++optimization

The below code works on Visual Studio 2008 with and without optimization. But it only works on g++ without optimization (O0).

#include <cstdlib>
#include <iostream>
#include <cmath>

double round(double v, double digit)
{
    double pow = std::pow(10.0, digit);
    double t = v * pow;
    //std::cout << "t:" << t << std::endl;
    double r = std::floor(t + 0.5);
    //std::cout << "r:" << r << std::endl;
    return r / pow;
}

int main(int argc, char *argv[])
{
    std::cout << round(4.45, 1) << std::endl;
    std::cout << round(4.55, 1) << std::endl;
}

The output should be:

4.5
4.6

But g++ with optimization (O1 – O3) will output:

4.5
4.5

If I add the volatile keyword before t, it works, so might there be some kind of optimization bug?

Test on g++ 4.1.2, and 4.4.4.

Here is the result on ideone:
http://ideone.com/Rz937

And the option I test on g++ is simple:

g++ -O2 round.cpp

The more interesting result, even I turn on /fp:fast option on Visual Studio 2008, the result still is correct.

Further question:

I was wondering, should I always turn on the -ffloat-store option?

Because the g++ version I tested is shipped with CentOS/Red Hat Linux 5 and CentOS/Redhat 6.

I compiled many of my programs under these platforms, and I am worried it will cause unexpected bugs inside my programs. It seems a little difficult to investigate all my C++ code and used libraries whether they have such problems. Any suggestion?

Is anyone interested in why even /fp:fast turned on, Visual Studio 2008 still works? It seems like Visual Studio 2008 is more reliable at this problem than g++?

Best Answer

Intel x86 processors use 80-bit extended precision internally, whereas double is normally 64-bit wide. Different optimization levels affect how often floating point values from CPU get saved into memory and thus rounded from 80-bit precision to 64-bit precision.

Use the -ffloat-store gcc option to get the same floating point results with different optimization levels.

Alternatively, use the long double type, which is normally 80-bit wide on gcc to avoid rounding from 80-bit to 64-bit precision.

man gcc says it all:

   -ffloat-store
       Do not store floating point variables in registers, and inhibit
       other options that might change whether a floating point value is
       taken from a register or memory.

       This option prevents 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.  Similarly for the
       x86 architecture.  For most programs, the excess precision does
       only good, but a few programs rely on the precise definition of
       IEEE floating point.  Use -ffloat-store for such programs, after
       modifying them to store all pertinent intermediate computations
       into variables.

In x86_64 builds compilers use SSE registers for float and double by default, so that no extended precision is used and this issue doesn't occur.

gcc compiler option -mfpmath controls that.

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