> No good. The compiler is still allowed to make optimizations on the  
   > accesses if you don't declare it volatile. And that might cause the  
   > code to break anyway. It's not only a cache problem.
   
   No, the compiler is not allowed to keep cached in a register a memory  
   access across a function call, precisely because it cannot know if  
   that function call modifies the memory.

don't callee vs. caller saved regs give the compiler a way around this?