Just curious, how clean is the Cycles source code?
The commit logs indicate that it’s pretty tidy compared to the mess that is the code for BI (little in the way of hacks and workarounds).
Why?
Browse the code for yourself – https://git.blender.org/gitweb/gitweb.cgi/blender.git/tree/HEAD:/intern/cycles
koff, koff … It is: “production software source-code.” 
About 7. On a scale of “there is no measure for clean code”. One man’s clean code is the other man’s mess.
I spent a night compiling and had a nasty rash the next morning. Take that for what it’s worth.