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Using LLVM build itself for PGO training is convenient and a great starting point but it also has several issues: * LLVM build implicitly depends on tools other than CMake and C/C++ compiler and if those tools aren't available in PATH, the build will fail. * LLVM build also requires standard headers and libraries which may not always be available in the default location requiring an explicit sysroot. * Building a single configuration (-DCMAKE_BUILD_TYPE=Release) only exercises the -O3 pipeline and can pesimize other configurations. * Building for the host target doesn't exercise all other targets. * Since LLVMSupport is a static library, this doesn't exercise the linker (beyond what the CMake itself does). Rather than using LLVM build, ideally we would provide a more minimal, purpose built corpus. While we're working on building such a corpus, provide a CMake option that lets vendors disable the use LLVM build for PGO training.