I read some topics over optimization and it is mentioned that global variables can not be stored in registers and hence if we need to optimize we use register variable to store the global data and modify that register variable. Is this applies to static variables too?
For auto storage, what if we store auto variables in register variables? Won't it faster the access from register instead of stack?
Both global variables and static variables exist in the data segment, which includes the data, BSS, and heap sections. If the static variable is initialized to 0 or not initialized to anything, it goes in the BSS section. If it is given a non-zero initialization value, then it is in the "data" section. See:
http://en.wikipedia.org/wiki/Data_segment
As for auto vs. register variables: register does not guarantee that the variable will be stored in a register, it is more providing a hint from the programmer. See:
http://www.lix.polytechnique.fr/~liberti/public/computing/prog/c/C/CONCEPT/storage_class.html
Yes, it is (much) faster to access a register than to access the stack memory, but this optimization nowadays is left up to the compiler (the problem of register allocation) as well as the CPU architecture (which has a great many optimizations too complex to explain here).
Unless you're programming for a really simple or old architecture and/or using a really outdated compiler, you probably should not worry about this kind of optimization.