Why are doubles added incorrectly in a specific Visual Studio 2008 project?

Question

Trying to port java code to C++ I've stumbled over some weird behaviour. I can't get double addition to work (even though compiler option /fp:strict which means "correct" floating point math is set in Visual Studio 2008).

double a = 0.4;
/* a: 0.40000000000000002, correct */

double b = 0.0 + 0.4;
/* b: 0.40000000596046448, incorrect
(0 + 0.4 is the same). It's not even close to correct. */

double c = 0;  
float f = 0.4f;  
c += f;
/* c: 0.40000000596046448 too */

In a different test project I set up it works fine (/fp:strict behaves according to IEEE754).

Using Visual Studio 2008 (standard) with No optimization and FP: strict.

Any ideas? Is it really truncating to floats? This project really needs same behaviour on both java and C++ side. I got all values by reading from debug window in VC++.

Solution: _fpreset(); // Barry Kelly's idea solved it. A library was setting the FP precision to low.

Answer 1

The only thing I can think of is perhaps you are linking against a library or DLL which has modified the CPU precision via the control word.

Have you tried calling _fpreset() from float.h before the problematic computation?

Answer 2

Yes, it's certainly truncating to floats. I get the same value printing float f = 0.4 as you do in the "inaccurate" case. Try:

double b = 0.0 + (double) 0.4;

The question then is why it's truncating to floats. There's no excuse in the standard for treating 0.0 + 0.4 as a single-precision expression, since floating point literals are double-precision unless they have a suffix to say otherwise.

So something must be interfering with your settings, but I have no idea what.