2020 LLVM Developers’ Meeting: “Proposal for A Framework for More Effective Loop Optimizations”

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Proposal for A Framework for More Effective Loop Optimizations - Michael Kruse

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The current LLVM data structures are intended for analysis and transformations on the instruction- and control-flow level, but are suboptimal for higher-level optimization. As a consequence, writing a loop optimization involves a lot of work including a correctness check, a custom profitability analysis, and handling many low-level issues. However, even when each individual loop optimization pass itself is has the best implementation possible, combined they are not optimal: their profitability models remain separate and, if loop versioning is necessary, each pass duplicates different aspects of the loop nest again and again. Also, phase ordering problems may inhibit optimizations that otherwise would be possible.

This motivates an intermediate representation and framework that is centered around loops and can be integrated with LLVM’s optimization pipeline. The talk will present the approach already outlined in an RFC at the beginning of this year and the state of implementing it.
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