Methods of profile information correction during compilation

Abstract

Performed by compiler code optimizing transformations efficiency can be greatly increased by obtaining and using program profile information, such compiler work is called profile-guided optimization (PGO). After applying transformations, which change control flow graph, it is necessary to adjust the profile information in order to maintain its adequacy for the work of succeeding optimizations. This paper considers two ways of representing profile information and the transition between them, describes the possible causes of uprising additional information on optimized code control flow requiring profile updating. Most frequently encountered problems of profile information update are considered and following solutions are offered: algorithm of acyclic graph profile correction according to its end nodes values; algorithm of loop average iteration number correction; control flow with “controversial node” profile correction algorithm. It is proved that the suggested algorithms of acyclic region counters and the loop iterations counter correction allow to change the counters ratio with original graph counters minimally. With the use of proposed algorithms, the mechanism of correction profile and thin profile information after loop peeling is described. All described methods are implemented in optimizing compilers lcc, lccs, lfortran, lfortrans for Elbrus and Sparc architectures.

Keywords

• профиль исполнения задачи, pgo, коррекция профиля, коррекция числа итераций цикла, расширенная профильная информация, взвешенный граф