[LLVMdev] Lowering switch statements with PGO

Chad Rosier mcrosier at codeaurora.org
Mon Dec 15 09:57:48 PST 2014

About two months ago I posted a patch that hoisted the hottest case
statement from a switch statement during ISelLowering.

See: http://reviews.llvm.org/D5786

Sean was rather adamant about using a Huffman tree (and I agree this is a
more complete solution), so I'd like to put a patch together.  That being
said, I have a few questions.

The current switch lowering code sorts based on case values and is lowered
with a combination of binary trees, lookup tables, bit tests and magic. 
If we lower the switch based on branch probabilities, I think the most
reasonable approach would be to disable the lookup tables and stick with a
purely tree structure.  Does anyone object or have opinions on this

Also, is there a way to determine if branch probabilities are based on
real data or static heuristics?  I assume we'd only want to build a
Huffman tree if we have real data.  Please correct me if I'm wrong.

Comments are very welcome!


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