This change adds an option to basic block sections to allow cold
clusters to be assigned a custom text prefix. With a custom prefix such
as ".text.split." (D87840), lld can place them in a separate output section.
The benefits are -
* Empirically shown to improve icache and itlb metrics by 3-5%
(absolute) compared to placing split parts in .text.unlikely.
* Mitigates against poor profiles, eg samplePGO profiles used with the
machine function splitter. Optimizations such as hugepage remapping can
make different decisions at the section granularity.
* Enables section granularity hotness monitoring (checking on the
decisions made during compilation vs sample data from production).
Differential Revision: https://reviews.llvm.org/D87813
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
