Compiler optimization in DS-5 Debugger ??


I'm debugging my projects with Altera Cyclone V SoC board. Whenever I'm using built-in function supported by the ARM compiler 6, for example, the strlen() function, I've always got the message "Variable doesn't currently have a location. The compiler has optimised the code causing the variable's location to be undefined at the current execution address". What does this mean ?? Is it not recommended to use the built-in function from the standard library and do I have to write my own function instead ?? Thanks.