A few optimizations significantly increase code size to gain execution speed. Use this option if you have limited main memory, swap space, or disk space. This combination performs aggressive optimizations with unrestricted compile time at the highest level of optimization. These types of optimizations give you the fastest code, but are riskier than the default optimizations. Compile times will be fast and link times will be slow when using PBO because code generation happens at link time.
P takes more time than linking ordinary object files.
However, compile-times will be relatively fast. This is because the compiler is only generating the intermediate code. You can compile and instrument in one step, but you will have to recompile again when optimizing. You must use the same options on both compiles, otherwise profile-based optimization cannot be done. Profile-based optimization has the greatest impact on application performance when used with level 2 or greater optimizations.