Building with profile feedback

It is possible to use profile feedback to optimize the compiler itself. This should result in a faster compiler binary. Experiments done on x86 using gcc 3.3 showed approximately 7 percent speedup on compiling C programs. To bootstrap the compiler with profile feedback, use make profiledbootstrap.

When make profiledbootstrap is run, it will first build a stage1 compiler. This compiler is used to build a stageprofile compiler instrumented to collect execution counts of instruction and branch probabilities. Training run is done by building stagetrain compiler. Finally a stagefeedback compiler is built using the information collected.

Unlike standard bootstrap, several additional restrictions apply. The compiler used to build stage1 needs to support a 64-bit integral type. It is recommended to only use GCC for this.

On Linux/x86_64 hosts with some restrictions (no virtualization) it is also possible to do autofdo build with make autoprofiledback. This uses Linux perf to sample branches in the binary and then rebuild it with feedback derived from the profile. Linux perf and the autofdo toolkit needs to be installed for this.

Only the profile from the current build is used, so when an error occurs it is recommended to clean before restarting. Otherwise the code quality may be much worse.