Use aggressive optimizations with stable, well-structured code. These types of optimizations give you faster code, but may change the behavior of programs. This option disables all but the most conservative optimizations at each level. Conservative optimizations do not change the behavior of code, in most cases, even if the code does not conform to standards. Use this option if longer compile times are acceptable because you want additional optimizations to be performed.
Most optimizations improve execution speed and decrease executable code size.
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.