I have written a code layout feedback directed optimization pass, which currently works for basic block reordering and function reordering. It very effectively improves the speedup (we could improve Python by 30%). The profiling method is window based context sensitive which is based on reference affinity (https://urresearch.rochester.edu/fileDownloadForInstitutionalItem.action?itemId=28368&itemFileId=143426)
The pass works in the IR level. Therefore, it may lose some information during the machine code optimization passes and perform imprecisely for BB reordering.
Eventually, I would like to see the improve for an interprocedural basic block reordering pass. However, with the current system there are several challenges ahead. The most important is that the CFG is not preserved during several passes including code-gen-prepare, cfg-simplify, remove-unreachable-blocks, tail-merge, and tail-duplication. So in order to keep track of the mapping between MBBs and BBs, one needs to insert code in every function that modifies the structure of BBs and MBBs.
The current block placement pass (MachineBasicBlockPlacement) works at the machine code level and with the new profiling structure (SampleProfileLoader), is effective as far as context-free profiling info is considered sufficient. However, the implementation of SampleProfileLoader itself encourages context sensitive info, which cannot efficiently be provided with the current profiling structure (<func,lineNo>).
Is there any way to incorporate information into the emitted MBBs so that we can get IR basic block level info instead of lineNo info?