Profile-Guided Optimization (PGO) support is landing within LLVM's Clang C/C++ compiler in catching up to feature parity with GCC and their never-ending effort to improve performance of compiled binaries.
For those not normally excited by compilers, Profile-Guided Optimizations are a means of profiling a compiled program so that on subsequent compilations the compiler can better optimize the code for improved run-time performance. PGO is very different from traditional source-level optimizations and it's taken some time for Clang to implement the support besides the fact not many utilize this compiler optimization technique due to it being time intensive with requiring multiple compilations plus run-time profiling.
With a code commit
that hit Clang mainline last week, there's now the initial instrumentation for Profile Guided Optimizations. While the code is still in early form, proper Clang PGO support will hopefully be a feature of LLVM 3.5.