Are you sure you have re-compiled the math plugin?  The log code was
changed recently.

Can you add print statements to gnm_log2 in mathfunc.c to see the values
of x and e?

I added

 printf ("x = %a, e = %d\n", x, e);

to gnm_log2() just before the return... line and recompiled

Now, when computing log2(1), I get on stdout 

x = 0x1p-1, e = 1

which is what it should be (if I'm understanding the source code correctly), but gnumeric still computes the same answer as before (-3.35018471298021E-17).

                        -Andrew Warshall

Well, in hindsight that was obvious.  The whole idea was to get x==1 in there,
but we get 0.5

Please try adding this after the gnm_frexp call:

	if (x == 0.5)
		x = 1.0, e--;

This problem has been fixed in our software repository. The fix will go into the next software release. Thank you for your bug report.