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.", 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).