Tools for analyzing undocumented code

[john.b.medamana]

I would like to get input from those of you who work with 
large software applications (UNIX/C).

How do you approach the problem of analyzing an existing
application which has 1 million lines of undocumented
C code?  The user wants to modify the system in order to 
improve performance.  Are there CASE tools available which simplifies 
the analysis?  Are there CASE tools which can be used to help 
document the code ?  Any general comments on approaching
a problem like this would be helpful.

Please email responses; I'll post a summary.

John Medamana
att!arch3!johnm