AutoFDO is the Automatic Feedback Directed Optimizer. AutoFDO relies on the Linux kernel's perf framework for profiling with performance counters. AutoFDO interprets the perf output and attempts to use the FDO infrastructure to produce better optimized code generation. AutoFDO according to its Google engineers is said to be noticeably faster than traditional FDO for GCC.
Developers can learn more about AutoFDO via the GCC Wiki and was previously covered on Phoronix. While talked about for two years, a few hours ago it was finally merged into mainline GCC.
Earlier this year is when Google also open-sourced their AutoFDO profile toolchain. The GCC AutoFDO support is in the latest code and will premiere in next year's release of GCC 5 along with many other features.