From beginner: What is the workload of writing a RTL expander for a new IR?

[Chunhua Liao]

  I notice that there are several projects aiming at improve the 
optimization infrastructure of GCC. And it seems all of them involve 
introducing new IR(s) between the original tree representation and RTL, thus 
new RTL expanders are necessary for them.
  My questions are:
1. Is RTL a complete representation for whole translation-unit(a complete 
input sourcefile contains) which can be used for code generator without the 
help from tree representation?
   If yes, why did I found some tree references even at the very back end?
2. What is the workload(in terms of n persons m months) of writing such new 
RTL expander from a new IR? My current understanding is that it is 
non-trival, since GCC calls several levels of functions to expand a function 
in tree to a sequence of RTL instruction nodes and most of them contain 
parameters in tree type. We may need rewrite them with the  parameters of 
new IR type?
  Thanks for the view of my post!

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