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We introduce a codegen optimization pass which splits functions into hot and cold parts. This pass leverages the basic block sections feature recently introduced in LLVM from the Propeller project. The pass targets functions with profile coverage, identifies cold blocks and moves them to a separate section. The linker groups all cold blocks across functions together, decreasing fragmentation and improving icache and itlb utilization. We evaluated the Machine Function Splitter pass on clang bootstrap and SPECInt 2017. For clang bootstrap we observe a mean 2.33% runtime improvement with a ~32% reduction in itlb and stlb misses. Additionally, L1 icache misses reduced by 9.5% while L2 instruction misses reduced by 20%. For SPECInt we report the change in IntRate the C/C++ benchmarks. All benchmarks apart from mcf and x264 improve, on average by 0.6% with the max for deepsjeng at 1.6%. Benchmark % Change 500.perlbench_r 0.78 502.gcc_r 0.82 505.mcf_r -0.30 520.omnetpp_r 0.18 523.xalancbmk_r 0.37 525.x264_r -0.46 531.deepsjeng_r 1.61 541.leela_r 0.83 557.xz_r 0.15 Differential Revision: https://reviews.llvm.org/D85368