Current machine instructions

Most of the machine instructions are capable of executing in a single processor cycle if no TLB or cache miss occurs.

In contrast, if a program branches rapidly to different areas of the program and accesses data from a large number of different areas causing high TLB and cache-miss rates, the average number of processor cycles per instruction (CPI) executed might be much greater than one. The program is said to exhibit poor locality of reference. It might be using the minimum number of instructions necessary to do its job, but it is consuming an unnecessarily large number of cycles. In part because of this poor correlation between number of instructions and number of cycles, reviewing a program listing to calculate path length no longer yields a time value directly. While a shorter path is usually faster than a longer path, the speed ratio can be very different from the path-length ratio.

The compilers rearrange code in sophisticated ways to minimize the number of cycles required for the execution of the program. The programmer seeking maximum performance must be primarily concerned with ensuring that the compiler has all of the information necessary to optimize the code effectively, rather than trying to second-guess the compiler's optimization techniques (see Effective Use of Preprocessors and the Compilers). The real measure of optimization effectiveness is the performance of an authentic workload.