[SSA] Two ways to implement it

I have evaluated two different approaches to SSAF in SBCL compiler. Both have some advantages and drawbacks, more or less significant from different points of view. So, here they are:


1. Insert the SSA pass after find-initial-dfo call, and modify the legacy IR1 component slightly (insert phi-functions & ensure unique assignments). Come out of SSA before ir1-phases call.

+ Faster to implement
+ Will probably fix the non-linear LVARs bug
- Does not allow to perform SSA-based optimisations (because it means the IR1 functionality duplication)
- Slows down the compilation process
- Requires a good bidirectional mapping algorithm between SSA form and IR1 representation.


2. Remove all IR1 code after make-functional-from-toplevel-lambda (including this function as well), and compile Lisp forms to SSAF. Apply SSA-based optimisations, and finally convert SSAF into VOPs, to use the existing backend.

+ Count all current IR1 bugs for sure
+ Easier conversion to SSAF (no need to analyse IR1 components)
+ Advanced SSA-based optimization, loops analysis, etc.
+ Probably faster compilation process?
+ More fun and interesting than the first variant
- A really huge project
- New bugs, as usual
- Merging perspectives are unclear (I do not believe that maintainers will throw out almost the whole compiler frontend).


As for now, I would prefer the second approach. I think that it will be a hobby project for me, and it will take a year or two before it can be announced as a beta or so, if at all. From my own development experience I can say that compilers should be designed and implemented by quite large (and preferably not remote) teams, not by a single person. So, this is just my own way to have fun with Common Lisp, and I feel that it is very likely to fail eventually. I do not expect brilliant results here, but I can't stop myself from giving it a try - at least to learn some new and cool things.