Compare a floating point number to zero

Question

I want to check if an IEEE754 32-bit number has the value of exactly 0.0f (it will occasionally be set to it.) Accumulation errors will be nil as the data will be frequently updated from a sensor. My processor has no hardware FPU, so operations are done in a fairly fast software library. However, it's still hundreds of cycles for things like add, subtract and compare.

So I was wondering why my compiler does this:

240:                 if(p_viewer->roll != 0.0f)
 03FBC  B81160     mul.uu w2,#0,w2
 03FBE  900A2E     mov.w [w14+20],w4
 03FC0  900064     mov.w [w4+12],w0
 03FC2  9000F4     mov.w [w4+14],w1
 03FC4  07E91F     rcall __nesf2
 03FC6  E00000     cp0.w w0
 03FC8  320054     bra z, 0x004072

__nesf2 apparently compares two floating point numbers. Why doesn't it just do a compare against 0.0f in integer form, which is 0x00000000? Is there some reason it doesn't do this or is it just a missed optimisation opportunity?

My compiler is MPLAB C30, a version of GCC v3.23.

Answer 1

Because -0.0f also compares equal to 0.0f, as required by the IEEE-754 standard.

You could replace the call with a comparison against the integer representations of both -0 (0x80000000) and +0, if this is a major performance issue. (Or better still, mask the sign bit and compare against 0.0f).

Answer 2

If this is a IEEE floating point implementation it has to take into account signalling NAN's.

You may not care about that, but the compiler doesn't know that.