[racket] R5.3 and #lang web-server compilation

On Sun, Aug 19, 2012 at 08:14:06AM -0600, Matthew Flatt wrote:
> At Sat, 18 Aug 2012 17:29:05 -0400 (EDT), Galler wrote:
> > Under 5.3, there's been a near order-of-magnitude improvement in 
> > compilation time for the web-server language.
> > [...]
> > Could I ask what has changed?
> 
> I'm not certain, but it might have been the change in free-variable
> calculation from a quadratic-time algorithm to a linear-time one (which
> is commit 6fded4bac1 along with ee93e35260 in the git repo).

I know what free variable are.  I don't know the specifics of what 'free 
variable calculation' is.  Testing whether a partiular variable is free?  
Determining the set  of free variables?

And what, roughly speaking are these linear and quadratic-time 
algorithms?

-- hendrik