turn off let floating

[David Sabel]

Hi,


> > Results:
> > 
> >    method          runtime (s)
> >    ---------------------------
> >    pure            0.7
> >    ffi             3.2
> >    fastMut         15
> >    ioref           23
> 
> I very strongly suspect that it is the unsafePerformIO that hurts
> performance in the fastMut case.  Otherwise this case would be around
> the same speed as the FFI example, perhaps faster.
> 
> You could try out that theory by copying the definition of
> unsafePerformIO into your code, and putting an INLINE pragma on it.  I
> think it's safe to do this in your case (it's not safe in general).

That's interesting for me, in which situations isn't it safe to inline
the definition of unsafePerformIO? 

David