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
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 (IntRate) 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