The delay function does not change any global variable.
And it does not return a computed result based on your input parameters or any global variables.
A good compiler may notice that your sdelay function does not actually "produce" any work, so the function may be optimized away.

Have a look on this forum for discussions about software loops. For short loops, you should consider assembler. For longer loops, you should consider using a hardware timer (polling or with interrupt).