Profile-Guided Optimizations Overview

Profile-guided optimizations (PGO) tell the compiler which areas of an application are most frequently executed. By knowing these areas, the compiler is able to be more selective and specific in optimizing the application. For example, the use of PGO often enables the compiler to make better decisions about function inlining, which increases the effectiveness of interprocedural optimizations.

See Basic PGO Options and Using PGO in a Microsoft Visual C++ .NET* Project.

Instrumented Program

Profile-guided optimization creates an instrumented program from your source code and special code from the compiler. Each time this instrumented code is executed, the instrumented program generates a dynamic information file. When you compile a second time, the dynamic information files are merged into a summary file. Using the profile information in this file, the compiler attempts to optimize the execution of the most heavily traveled paths in the program.

Unlike other optimizations such as those strictly for size or speed, the results of IPO and PGO vary. This is due to each program having a different profile and different opportunities for optimizations. The guidelines provided help you determine if you can benefit by using IPO and PGO. You need to understand the principles of the optimizations and the unique aspects of your source code.

Added Performance with PGO

The Intel® compiler PGO feature provides the following benefits:

• Register allocation uses the profile information to optimize the location of spill code.
• For indirect function calls, branch prediction is improved by identifying the most likely targets.
  Both Pentium® 4 and Intel® Xeon™ processors have longer pipelines, which improves branch prediction and translates into high performance gains.
• The compiler detects and does not vectorize loops that execute only a small number of iterations, reducing the run time overhead that vectorization might otherwise add.