Profile-guided optimization (PGO) is a now-common compiler technique for improving the compilation process. In PGO (sometimes pronounced “pogo”), an administrator uses the first version of the binary to collect a profile, through instrumentation or sampling, then uses that information to guide the compilation process.
Profile-guided optimization can help developers make better decisions, for instance, concerning inlining or block ordering. In some cases, it can also lead to using obsolete profile information to guide compilation. For reasons that I will explain, this feature can benefit large projects. It also puts the burden on the compiler implementation to detect and handle inconsistencies.
This article focuses on how the Clang compiler implements PGO, and specifically, how it instruments binaries. We will look at what happens when Clang instruments source code during the compilation step to collect profile information during execution. Then, I’ll introduce a real-world bug that demonstrates the pitfalls of the current approach to PGO.
Continue reading “Profile-guided optimization in Clang: Dealing with modified sources”