Complex sentence: Many optimizations that operate on abstract programming concepts (loops, objects, structures) are independent of the machine targeted by the compiler, but many of the most effective optimizations are those that best exploit special features of the target platform.
Split into simpler sentences: Many optimizations that operate on abstract programming concepts (loops, objects, structures) are independent of the machine targeted by the compiler. But many of the most effective optimizations are those that best exploit special features of the target platform.