Re: [Sbcl-devel] comment on sbcl, complex+real

[Paul K. <pv...@pv...>]

On 2009-11-13, at 8:50 AM, alexander a donkov wrote:

> Dear Paul Khuong,
> my name is alexander, i am a beginner user of sbcl (about a year).
> I noticed your blog on complex float improvements in sbcl, x86-64,  
> and that is why i am addressing this email to you.
>
> i noticed that the current 1.0.32 sbcl, at least when installed on  
> my mac, gives a different result when a real number is added to a  
> complex number with -0.0 imaginary part.
[...]
> Perhaps, the real number is now promoted to complex with a zero im  
> part, which is mostly ok, but comes into trouble when signed zeros  
> are considered.
> My understanding of the problem with dealing with signed zeros comes  
> from the Kahan's article on complex arithmetic:
> http://www.eecs.berkeley.edu/Pubs/TechRpts/1992/CSD-92-667.pdf

This is intentional. The CL specification explicitly demands that (+  
real complex) [or any other arithmetic operation] be equivalent to (+  
real (complex complex)). Since we do not have a special case for  
complex floats with an exactly zero imaginary part, signed zeros get  
absorbed. Either interpretation of real-complex arithmetic isn't hard  
to implement, but we should first decide whether we want to stray from  
the spec on that topic.

Paul Khuong