Attempt for rotating register allocation

[Chunjiang Li]

Hi, all 

So far, using the dataflow info (gen with df.c and df.h), I can find out the pseudo registers use 
and def in a one bb loop. 

Now, need to establish a struct to record the lifetime activity of each pseudo register in a 
software pipelined loop. According to this struct, we can allocate rotating registers to each 
pseudo register used and defed in swp loops. 

Although, rotating register allocation for swp in GCC may be interfered by the reg alloc process 
in current GCC, which may result in failure.
I also want to try it. During this process, I can grasp more techniques in  the back-end of GCC.

Needs advice and cooperations.  

Chunjiang Li

Creative Compiler Research Group,
National University of Defense Technology, China.