Back End Responsibilities + RTL Generation

[Frank Riese <hippo at informatik dot uni-bremen dot de>]

Hi!

I need some clearification concerning the responsibilities of Middle End, Back 
End, the generation of the control flow graph (CFG) and RTL. I looked at 
several articles about the internal structure of the GCC and looked at the 
internals documentation. However, I would like to have a second opinion about 
this matter.

One of my professors stated that a GCC Back End uses the Control Flow Graph as 
its input and that generation of RTL expressions occurs later on. What roles 
do Back and Middle End play in generation of RTL? Would you consider the CFG 
or RTL expressions as the input for a GCC Back End?

I also remembered having read the following line from the gcc internals 
documentation. However, I'm still not sure how to interpret this:

"A control flow graph (CFG) is a data structure built on top of the 
intermediate code representation (the RTL or tree instruction stream) 
abstracting the control flow behavior of a function that is being compiled"

Does that mean that a control flow graph is built after rtl has been generated 
or that information about that information about the control flow is 
incorporated into the RTL data structures? Could somebody clearify this, 
please?

Cheers,
Frank