Calculation differences between Debug and Release mode

[Steven Schveighoffer]

On Sat, 13 Apr 2013 02:07:39 -0400, Jeremy DeHaan  
<dehaan.jeremiah at gmail.com> wrote:

> I have a function that will calculate a random point on a circle based  
> on a specified radius and total number of points. The only point in  
> question is the first point. I get different values when the code  
> compiles in Release and Debug mode.
>
> Here is some code:
>
> Vector2f getPoint(uint index)
> {
> 		
> 	static const(float) pi = 3.141592654f;
> 		
> 	float angle = index * 2 * pi / m_pointCount - pi / 2;
>
> 		
> 	float x = cos(angle) * m_radius;
> 	float y = sin(angle) * m_radius;
> 		
>
> 	return Vector2f(m_radius + x, m_radius + y);
> }
>
> Vector2f is simply a struct that has 2 floats.
>
> In debug mode this works as expected. Let's say the radius is 50.  
> getPoint(0) returns a vector that prints X: 50 Y: 0. For some reason,  
> the same function will return a vector that prints X: 50 Y: 4.77673e-14.  
> Now, 4.77673e-14 is a crazy small number that might as well be 0, but  
> why the difference?

I would suspect that the issue is floating point error.  On certain  
hardware, the CPU uses higher-precision 80-bit floating points.  When you  
store those back to doubles, the extra precision is truncated.

In debug mode, without optimization, the compiler will do exactly as you  
say, storing intermediate calculations and using the truncated stored data  
for the next line.  But with optimizations turned on, the compiler might  
take shortcuts which allows it to use the higher-precision data still in  
the registers in the next line.

-Steve