When I declared N as a global variable, the compiler left it in memory and added a load instruction to the loop. When I put N as a local variable, the compiler loaded it into a register. While I do blame the compiler for this particular behavior (because I did not declare N as volatile), we have to work with what we have.
This is because global variables can be modified at runtime unless they're const. The compiler cannot guarantee it hasn't been declared extern and modified by some other file without sufficiently powerful link-time optimization.
Comments
When I declared N as a global variable, the compiler left it in memory and added a load instruction to the loop. When I put N as a local variable, the compiler loaded it into a register. While I do blame the compiler for this particular behavior (because I did not declare N as volatile), we have to work with what we have.
This is because global variables can be modified at runtime unless they're const. The compiler cannot guarantee it hasn't been declared extern and modified by some other file without sufficiently powerful link-time optimization.