Replace all stack pointer with frame pointer in rtl level

[Lu, Kangjie]

Hi All,

I am a beginner of gcc. 

My goal of modifying gcc is to maintain two stacks.
One is accessed by rbp, the other is accessed by rsp.
rsp is only used by call/ret. All other accesses (e.g., access parameters, access local variables) 
to stack are via rbp.

For easy implementation, I would use flags "-fomit-frame-pointer -ffixed-rbp" to
compile the target program, which prevents gcc using rbp. And in the RTL level, 
I will replace all rsp with rbp (I may also change push/pop into mov instructions).

My question is how to iterate all registers used in insns in rtl level, and replace all
rsp with rbp.



Thanks,
Ken