4 - better GC for IORef/IOArray and lots of threads
5 - loosen rules on instance decls
12 - -split-objs now works with --make and Cabal
13 - relaxed dependency analysis (with -fglasgow-exts)
19 Internals/Source tree:
21 - unknown calls take args in registers, if available
22 - x86_64: more argument regs
25 - source tree flattened